Correction Appended

First-quarter fundraising reports due Friday are the first stretch of the 2012 Senate horse race, offering an early indication of who is jumping into the lead and, in general, how the 2012 Senate landscape is shaping up when control of the chamber is on the line.

Senate candidates such as Rep. Christopher Murphy (D-Conn.) and former Dallas Mayor Tom Leppert (R) could not wait to announce raising an eye-popping $1.1 million each in the first three months of the year, giving them instant edges on their competitors and putting strong initial stamps on their races. Vulnerable Republican incumbents such as Republican Sens. Dick Lugar (Ind.) and Scott Brown (Mass.) are bolstering their campaign coffers with strong fundraising as well.

As the Senate landscape begins to form more than three months into the cycle, just one competitive Senate race is set in stone.

Montana, where Democratic Sen. Jon Tester will face Republican Rep. Denny Rehberg in a marquee matchup, is the lone state in which both parties’ preferred candidates are in the race and no primary is expected, so they are virtually assured of facing off in the general.

Beyond the Big Sky State, the Senate field remains largely in flux. A few races are beginning to gel, though all but Montana could look drastically different based on who wins the primary. Or, as is the case in many more states, it is still too early to project who the major players will be.

Democrats have 23 seats to defend this cycle as the party clings to a three-seat Senate majority. Five Democrats, including Sen. Joe Lieberman (I-Conn.), have already announced retirements, with at least four of those seats possible Republican pickup opportunities. And it is not a foregone conclusion that two more Democratic Senators, Ben Nelson (Neb.) and Herb Kohl (Wis.), will run.

Roll Call Politics has eight Senate races listed as Tossups. Six are Democrat-held seats, including the open-seat races in New Mexico and Virginia, as well as the re-election bids of Tester in Montana and Sen. Claire McCaskill in Missouri, both in their first terms.

The numbers are so lopsided against Democrats thanks to the party’s exceptional success in the 2006 cycle, including knocking off Republican incumbents in states such as Missouri, Ohio, Pennsylvania and Montana. At this time in 2005, political handicappers had yet to believe the GOP’s recently increased edge in the Senate was truly in jeopardy, and Montana was not on anyone’s map of potentially competitive races.

Rehberg’s February announcement put the early spotlight squarely on the Big Sky State, where in 2006 Tester defeated an ethics-plagued Republican incumbent. This time he is up against a six-term Congressman and Appropriations cardinal who is among Montana’s best-known politicians in part because as an at-large Member he is elected statewide.

Montana has a history of electing Democratic Senators, but it has voted for a Democratic presidential nominee just once since 1964. And a mid-March independent poll showing the race tied crystallized its place among the most competitive races of the cycle.

“On the whole, it’s a positive, particularly in the fundraising aspect,” Erik Iverson, a Rehberg adviser and former longtime chief of staff, said of the early formation of the race. “The disadvantage is that you have the target on your back for 19 months, but on the whole, it’s better. You can begin fundraising, and both sides can begin to hone their message.”

Here’s a look at the rest of the field, broken down by Roll Call Politics’ race ratings.

Tossup

Florida

Republicans are beginning to line up for the chance to challenge Sen. Bill Nelson, the only Democrat in Florida currently holding statewide office.

State Senate President Mike Haridopolos launched his campaign last month. The Republican has reported a successful fundraising operation to date, despite having to wade through a rash of negative press recently.

Haridopolos was joined this month by former Sen. George LeMieux, whose entrance was largely expected and ensures a GOP primary. The primary will likely include at least one more contender, former state House Majority Leader Adam Hasner, a favorite of grass-roots conservatives who could prove to be a strong candidate, especially in a GOP primary where only registered Republicans can vote.

LeMieux, meanwhile, will struggle to distance himself from former Gov. Charlie Crist, who alienated the GOP base by leaving the party and running for the Senate last fall as an Independent. LeMieux is a former Crist chief of staff and was nominated to the Senate by Crist to fill the remainder of GOP Sen. Mel Martinez’s term.

Republicans in Washington, D.C., concede they haven’t decided who is the strongest candidate. Haridopolos and LeMieux have baggage, while Hasner is not as well-known but is considered to be an ally of the popular newly elected Sen. Marco Rubio (R).

Democrats are hoping for an ugly GOP primary. And they think Nelson will ultimately benefit from the resources that Democrats pour into Florida to help President Barack Obama win the key swing state. Case in point, Obama made his first campaign stop this year at a Nelson fundraiser in the Sunshine State.

Massachusetts

Serving in this deep-blue state, Sen. Scott Brown is supposed to be among the most vulnerable Republican incumbents in the nation. But Democrats have been slow to identify a top-tier challenger.

To date, only Somerville activist Bob Massie has announced a campaign. A crowd of more formidable challengers remains on the fence.

The prospects include Newton Mayor Setti Warren, City Year co-founder Alan Khazei and Reps. Mike Capuano and Stephen Lynch. Gerry Kavanaugh, former chief of staff to the late Sen. Edward Kennedy (D), is one to watch as well.

The Democrats’ extended deliberation could be because Brown is far stronger than many expected he would be at this point. He is boosting his profile and fundraising — with $1.7 million raised in the first quarter — while he’s embarked on a national book tour. Recent polling suggests the former tea party darling may be the most popular politician in the Bay State. And a campaign aide said his first-quarter fundraising report will show a whopping $8.3 million in his campaign account.

Tea party leaders are grumbling about Brown’s moderate positions on some key votes such as financial reform and the repeal of “don’t ask, don’t tell,” but don’t expect him to face a serious challenge from the right.

Still, whichever Democrat emerges from the primary will make this race competitive, based on the Massachusetts voter registration numbers, if nothing else.

Missouri

Sen. Claire McCaskill is widely considered among the most vulnerable Democrats, and recent ethics troubles combined with Missouri’s difficult terrain for Democrats all but ensures the first-term Senator is in for a tough re-election bid — no matter her Republican opponent.

The Democrat has been snared by revelations that she expensed taxpayers for chartered flights she took aboard a plane she partly owned, and her troubles continued days later when it was revealed she owed more than $300,000 in unpaid property taxes.

The GOP field remains in flux as Rep. Todd Akin takes a serious look at the race. State insiders think he would instantly leap to frontrunner status ahead of former state Treasurer Sarah Steelman and former House candidate Ed Martin, who are both already in the race, as well as former state party Chairwoman Ann Wagner. Former Sen. Jim Talent, whom McCaskill narrowly defeated in 2006 as part of the Democratic wave, is taking a pass.

The race will likely become a target for outside spending from both national party campaign committees. The Democratic Senatorial Campaign Committee is already criticizing Steelman for public records that have gone missing from her tenure as treasurer, and the National Republican Senatorial Committee is expected to keep “Air Claire” at the front of voters’ minds for the campaign’s duration.

McCaskill, though, is a skilled, experienced politician whom national Democrats think can handle any of the potential GOP foes. Making it tougher for McCaskill is the fact Missouri is a red-leaning presidential swing state where Democrats have struggled to earn more than 50 percent of the vote in recent years.

Nebraska

Republicans like to draw comparisons between Democratic Sen. Ben Nelson’s re-election bid and that of former Sen. Blanche Lincoln (D-Ark.) in 2010: Nelson’s numbers are so far in the tank that it will be nearly impossible to climb out of the hole with local voters.

In fact, not a single public independent poll released so far this year gives Nelson a lead over either of his GOP opponents: state Attorney General Jon Bruning and state Treasurer Don Stenberg. A Public Policy Polling survey taken in late January gave Bruning an 11-point lead over Nelson, while Stenberg had a 4-point advantage over the Senator. The poll of 977 Nebraska voters had a margin of error of 3.1 points.

Nelson would benefit if the primary between Bruning and Stenberg gets ugly. But even that might be wishful thinking for Democrats given the state’s early May primary, which would give either Republican ample time to fundraise and recover before the general election.

Regardless of what the polls read, Nelson is preparing for a strong race. He has already assembled a campaign team that includes his former campaign manager, Paul Johnson. He also had almost $1.5 million in the bank at the end of last year — money that will go a long way in a cheap media state.

Nevada

This race is shaping up to be a battle between two Members of Congress, Reps. Dean Heller (R) and Shelley Berkley (D), who represent vastly different populations in the state — Berkley’s Las Vegas and Heller’s Reno and rural Nevada.

Berkley has reportedly not decided yet whether to give up her safe House seat for only a crack at the Senate, but she is seriously weighing a bid as evidenced by a recent poll conducted on her behalf. Testing her strength against Heller, the poll found the race statistically tied.

The poll proved that in a tough Senate landscape for Democrats, Berkley could give the party its best chance to pick up a Senate seat.

However, the Democratic Senatorial Campaign Committee is covering its bases and has met with other potential candidates, including Secretary of State Ross Miller, Attorney General Catherine Cortez Masto and Treasurer Kate Marshall. DSCC Executive Director Guy Cecil also released a memo touting the party’s chances, citing President Barack Obama’s 12-point win in 2008 and Democratic Sen. Harry Reid’s unexpected 5-point win last year.

But getting Heller in the race was a recruiting win for the National Republican Senatorial Committee, which two years earlier approached him to run against Reid. The GOP also caught a big break when scandal-plagued Sen. John Ensign (R) announced in early March that he would not seek re-election, sparing the party an ugly primary.

New Mexico

Democratic Rep. Martin Heinrich’s recent entrance into the race represented the first recruiting victory for the Democratic Senatorial Campaign Committee, and his decision to leave his Albuquerque-based seat for a chance at statewide office gave Democrats a top-tier candidate in one of the five open-seat Senate races.

Most political observers assumed Heinrich would run after Sen. Jeff Bingaman (D) announced his retirement in February. The two-term Congressman won the open 1st district in 2008 when then-Rep. Heather Wilson (R) ran for Senate.

Wilson, a moderate, is running for Senate again, but a matchup between the two Albuquerque-based candidates is not assured.

On the Democratic side, state Auditor Hector Balderas is looking at the race, though state insiders say there is a chance he could decide to run for Heinrich’s House seat instead.

The Republican field is more crowded, and the GOP has a history of bloody primaries in the state. Lt. Gov. John Sanchez met recently with reporters in Washington, D.C., and said he is giving it serious consideration. Conservatives Greg Sowards and Bill English are already in the race, but Sanchez would likely give Wilson her toughest challenge. Like much of the West, New Mexico is a must-win state for President Barack Obama’s re-election campaign.

Virginia

The most likely scenario in the Old Dominion is a matchup between political heavyweights Tim Kaine (D) and George Allen (R). Both former governors went to Washington, D.C., after their tenures in Richmond, and they are now on a crash course for November 2012.

Republicans have been tying Kaine to President Barack Obama and other Democratic leaders, as they seek to make the race a referendum on the president. In 2008, Obama became the first Democrat to win Virginia since 1964, but Republicans swept the statewide races a year later and picked up three House seats in 2010.

Sen. Jim Webb’s retirement announcement left Democrats with a large void and in search of a candidate who could handle the beefed-up campaign that Allen had already assembled. With some help from Obama, the Democratic Senatorial Campaign Committee got its man when Kaine made his bid official.

Several Republicans are challenging Allen, including tea party leader Jamie Radtke and conservative businessman Tim Donner.

While some establishment Republicans were caught off-guard by tea party primary challenges last year, the same fate is not expected for Allen. He has been working since last year to shore up support and emphasize his conservative credentials, and he is now raising money and campaigning well more than a year before the primary, an open contest that is highly likely to make him the victor.

Connecticut

The Democratic primary field may be coming together in the race to succeed Sen. Joe Lieberman (I), but major questions remain over whether any Republican — or maybe an Independent — can make this race competitive.

Look for Republican Linda McMahon to try. The former wrestling executive, who spent $50 million on a failed 2010 Senate run, appears to be edging closer to launching a second Senate campaign, according to a source familiar with her thinking.

She has been increasingly active in local politics in recent weeks, a likely sign that her political career isn’t over. Democrats say that if McMahon couldn’t win an open seat in 2010, under what were likely far better political circumstances, she doesn’t have a chance with President Barack Obama at the top of the ticket in 2012.

Her presence and pocketbook, however, could dramatically change the shape of the contest. And it could affect whether Independent David Walker, a former comptroller under Presidents Bill Clinton and George W. Bush, enters the race.

The smart money, however, is that whoever emerges from the Democratic primary will ultimately earn a Senate seat.

Three-term Rep. Christopher Murphy (D) is perhaps the best-known in Washington, D.C., circles. A 37-year-old progressive, he has already made his opposition to the war in Afghanistan a central theme of his campaign.

Former Secretary of State Susan Bysiewicz has also announced a bid. And while she may not enjoy the same popularity in Washington, she may have better name recognition in Connecticut than Murphy, having won statewide election three times. Her $500,000 first-quarter haul proved that she could be a force in the primary, but Murphy raised more than double that.

Former state Treasurer Frank Borges, a centrist Democrat, is also considering a run.

Leans Democratic

Hawaii

This Democratic-leaning state is expected to vote heavily for its native son, President Barack Obama, in 2012, and almost any Democratic Senate candidate will be favored in this open-seat race. The competitiveness will depend on whether Republicans can successfully recruit former two-term Gov. Linda Lingle. Former Rep. Ed Case (D) has announced a bid after doing polling on the Democratic primary field.

Prospective GOP candidates, including former Rep. Charles Djou, are giving Lingle the right of first refusal in the race, as she represents the party’s best chance at picking up the seat.

There have been only five Senators in the state’s history, including Sen. Daniel Akaka (D), who announced his retirement in March. And no Republican has been elected to the Senate since Hiram Fong in 1970.

But Case is unlikely to have a clear shot at the nomination. Democratic Reps. Mazie Hirono and Colleen Hanabusa, who was just elected in November, could be candidates. Other potential candidates include former Honolulu Mayor Mufi Hannemann and Lt. Gov. Brian Schatz.

Michigan

Democratic Sen. Debbie Stabenow’s re-election bid has all of the ingredients of a competitive race — except, so far, a strong Republican opponent.

The Senator has average approval ratings, Republicans swept Michigan in the 2010 cycle, and the state remains one of the hardest hit by the recession, but no top-tier Republicans have yet to float their name.

The state Republican Party chairman openly admitted the field was lackluster earlier this month, telling a local newspaper that he expects a candidate “head and shoulders” above the current potential contenders to emerge. State GOP Chairman Robert Schostak said he was not sure who that candidate would be, but he expects another contender.

Some of the Republicans who are openly considering a bid did not take too kindly to Schostak’s call for a better candidate, especially former Rep. Pete Hoekstra (R), whom national Republicans see as a potentially strong candidate. Other possible candidates include former Secretary of State Terri Lynn Land and former state GOP Chairman Saul Anuzis, both of whom are openly mulling bids.

New Jersey

The Garden State is an example of a Democratic stronghold where the GOP says the incumbent is vulnerable but has been slow to identify a viable challenger.

Sen. Bob Menendez, the Democratic Senatorial Campaign Committee chairman in the 2010 cycle when his party lost seven seats, looks to be in strong shape for now. But recent polling suggests he could have challenges should a formidable opponent emerge.

Republicans in Washington, D.C., and New Jersey are excited about the potential candidacy of biotech CEO and Navy reservist John Crowley, whose deep pockets and literally made-for-Hollywood life story (Harrison Ford starred in a 2010 film) could give Menendez cause for concern.

Crowley flirted with a run in 2008 but appears to be more serious this time, according to a source familiar with his thinking.

The list of other potential GOP candidates, however, is long and largely uncertain. It is topped by Lt. Gov. Kim Guadagno, followed by state Sen. Tom Kean Jr., who lost to Menendez in 2006. Others to watch include state Sens. Michael Doherty and Joseph Kyrillos, New York Jets owner Woody Johnson and unsuccessful House candidate Anna Little, the former mayor of Highlands.

Ohio

Senate Republicans made it clear early this cycle that Sen. Sherrod Brown (D) was one of the names on the top of their target list. But so far, some of the more prominent Republicans in the Buckeye State — including Attorney General Mike DeWine, the former Senator whom Brown unseated in 2006, and Reps. Jim Jordan and Patrick Tiberi — have indicated they will likely take a pass at challenging Brown.

State Treasurer Josh Mandel filed candidacy paperwork with the Federal Election Commission on Monday. Republicans are high on Mandel’s profile as a veteran and cite the former state Representative’s predominantly Democratic district as proof of his statewide appeal, but Democrats say his relative inexperience — he was just elected statewide last November — makes him no match against Brown.

The other Republican openly considering a bid is former Secretary of State Ken Blackwell, who lost a gubernatorial bid in 2006.

Blackwell’s base is in southwest Ohio, which is the opposite part of the state as Mandel’s home turf, and he is widely considered to be a more conservative candidate — a scenario that could lead to a divisive GOP primary if both Republicans decide to run.

Meanwhile, public polling shows that Brown is increasingly less vulnerable as the election cycle continues. A Public Policy Polling survey of 559 Ohio voters in early March showed Brown leading all of his potential opponents by at least 15 points.  The poll had a 4.1-point margin of error.

Brown is also preparing for what Democrats anticipate to be a challenging campaign and recently tapped Sarah Benzing, who managed the successful re-election bid of Sen. Kirsten Gillibrand (D-N.Y.) last cycle, to run his race.

Pennsylvania

Given GOP Sen. Pat Toomey’s success in 2010, Republicans should have an opportunity to knock off first-term Sen. Bob Casey (D) in 2012.

But Republican leaders concede Pennsylvania is one of the states where they have some recruitment holes. That could be because Democrats enjoy a substantial voter-registration edge, which will likely be a far greater factor with President Barack Obama atop the ballot in 2012 than it was in the 2010 midterms.

The outlook for Republicans has become a joke in the state, prompting a local political blog, PoliticsPa.com, to run an April Fools’ Day story titled “GOP Recruits Potted Plant to Challenge Casey in 2012.”

That said, there are at least two who have announced bids: Marc Scaringi, a former aide to Sen. Rick Santorum (R), and Scranton Tea Party leader Laureen Cummings.

Neither has generated substantial excitement among local Republicans, however. Look for the field to develop in the coming months.

Longtime Dick’s Sporting Goods CEO Edward Stack is mentioned as a possible candidate, as are state Sens. Kim Ward and Jake Corman. Reps. Charlie Dent and Jim Gerlach have been part of the conversation but aren’t expected to run.

West Virginia

In his special Senate election bid last year, then-Gov. Joe Manchin (D) ran an ad that featured him loading a rifle and shooting a hole in the cap-and-trade bill, as well as pronouncing that he has sued the Environmental Protection Agency, would “take on” the Obama administration, would cut federal spending and would “repeal the bad parts of Obamacare.”

Since being elected in November and instantly starting a new election cycle as he runs for a full term, Manchin has continued to speak out against President Barack Obama and Democratic Congressional leaders. That will likely serve him well, as Obama lost the state by 13 points in 2008.

No top-tier Republican has stepped forward to run, in part because the focus this year is on the special gubernatorial election to replace Manchin. The National Republican Senatorial Committee has been targeting Manchin since last year and is not expected to give him a free pass. This would become a far more competitive race if Rep. Shelley Moore Capito (R) decides to run. She declined to run against Manchin last year in the special election but is keeping her options open this time around.

Wisconsin

The competitiveness of this seat depends on whether 76-year-old Sen. Herb Kohl (D) decides to retire. The wealthy NBA team owner would have been down to about $4,000 in cash on hand by the end of 2010 had he not loaned his campaign $1 million, a signal that the four- term Senator wants to run.

An open-seat race would put the seat in jeopardy, even with no GOP opponent yet apparent. A Kohl re-election run, however, does not mean the race could not be competitive for Republicans. Former Sen. Russ Feingold (D) saw now-Sen. Ron Johnson (R), a wealthy businessman with no political experience, come out of nowhere and pull off a 5-point win.

So far, the focus in Wisconsin has been squarely on Gov. Scott Walker (R) and the state Legislature, with a movement toward recall elections for state Senators and possibly even Walker. What happens there could also play a big role in what the Senate race looks like a year from now.

Leans Republican

Arizona

The Obama campaign and Senate Democrats really want to make the Grand Canyon State competitive, but their nominee will be facing a top-tier Republican recruit.

Rep. Jeff Flake (R) jumped into the race in February, immediately after Senate Minority Whip Jon Kyl (R) announced his retirement. Flake has since picked up an endorsement and fundraising assistance from the Club for Growth and successfully avoided a competitive primary against fellow Rep. Trent Franks.

Democrats in the state are hanging back in deference to Rep. Gabrielle Giffords as she recovers from the gunshot wounds that she suffered in January. Giffords was considered a possible Senate contender before that and a miraculous return to the campaign trail would instantly turn this race upside down. Unless she or another top-tier Democrat enters the race, it will be difficult for the party to make this a competitive seat.

Rep. Ed Pastor (D) has said he is looking at the race, and other names mentioned by Democrats in the state include Democratic National Committeeman Fred DuVal, who sits on the state Board of Regents and is a former Clinton administration official; attorney and former Arizona Democratic Party Chairman Don Bivens; and Felecia Rotellini, a former assistant state attorney general and 2010 attorney general nominee.

Maine

Sen. Olympia Snowe is among a dying breed of Republicans.

The moderate Senator may face the most challenging re-election fight of her career in 2012. Identified as a top target by some tea party groups, she is expected to face attacks from the right and left.

But both sides have been slow to yield a viable challenger.

Local tea party groups are warring among themselves and have yet to rally around either Republican candidate, former town selectman Scott D’Amboise or Andrew Ian Dodge, a freelance writer and the state coordinator for the Tea Party Patriots.

It remains to be seen whether another Republican will step up.

And at least two prominent Democratic women will likely decide whether to run based on the nature and outcome of the GOP primary.

The first is Rosa Scarcelli, a businesswoman who lost the Democratic gubernatorial primary in 2010 but has a continued interest in higher office and the ability to self-fund. Term-limited state House Minority Leader Emily Cain, just 30 years old, is also watching the primary closely.

Likely Republican

Indiana

Out of any Senator up for re-election this cycle, Dick Lugar has earned the dubious distinction of being the first incumbent to have a serious primary challenge — and so far, prospects do not look promising for the most senior Republican in the Senate.

State Treasurer Richard Mourdock announced early that he would run against Lugar, pitching himself as a Republican with support from both local tea party activists and local party leaders and attacking the Senator for his moderate voting record. Mourdock came out of the gates with the support of more than half of the state’s county GOP chairmen — although Lugar’s aides dispute that list, arguing that it overstates the challenger’s support.

If there is a path for Lugar’s re-election, it will rely heavily on his cash reserves, including the more than $3 million that he will report on hand as of March 31. That’s thanks in part to his $974,000 fundraising haul in the first quarter.

Lugar’s campaign also boasted that it raised $400,000 at one in-state fundraiser in January — three times that of Mourdock’s haul for the entire first quarter, which the GOP challenger has said is around $125,000.

Lugar would also potentially benefit if Republican state Sen. Mike Delph, who is still openly considering running, jumps into the race and splits the opposition vote in the primary. Delph told Roll Call that he will make a final decision about the race after the state legislative session is over at the end of April.

Senate Democrats think they have a better shot at the seat if Lugar loses the primary, especially if there is a bloody GOP battle, but it is still an uphill climb for the party. Nonetheless, Rep. Joe Donnelly is openly considering a bid for Senate — a relatively good option for the three-term Democrat as his district is expected to be redrawn to include more Republicans this next cycle. He’d considered a bid for governor, but Senate is looking more likely.

President Barack Obama won the state in 2008, but it’s looking less likely that he can repeat that feat again in 2012. That’s one reason the seat is still likely to remain in GOP hands should Lugar lose the primary.

Correction: April 12, 2011

The article misidentified potential New Jersey Republican Senate candidate Anna Little’s current job. She is the former mayor of Highlands, N.J.