The conventional wisdom is that odds favor Republicans winning control of the Senate next year. But an examination of the 2012 landscape at the end of the third quarter shows the chamber’s majority could go either way.

The overall numbers favor Republicans — Democrats are defending 23 seats to the GOP’s 10 — but controlling at least 50 seats may once again hinge on how some primaries shake out.

Several races still have maturing to do, and the third-quarter reports due Oct. 15 will offer the latest clues about the Republican primaries in states such as Indiana, Missouri and Florida, where the nomination is up for grabs. It will be months before GOP nominees are selected in those states, and in some cases, the primary winner could change the race’s outlook.

Despite that uncertainty, there are more than enough vulnerable Democratic seats to give National Republican Senatorial Committee strategists several paths to the majority. Winning Missouri, Montana, Nebraska and North Dakota would be enough, and there are plenty of opportunities with varying degrees of likelihood beyond those.

Other certain Senate battlegrounds next year include Wisconsin, Montana, Ohio and Virginia, where fundraising isn’t expected to be an issue for any of the top candidates. All are Democratic-held seats — as are most of the top races.

It’s highly likely that the GOP will pick up North Dakota’s open seat. But Republicans have a couple of tossup races of their own to defend in Massachusetts and Nevada. Losses there would complicate the math and help the Democratic Senatorial Campaign Committee hold the Senate, barely, for a second straight cycle. Nomination surprises last cycle helped keep Republicans from winning seats in Nevada, Colorado and Delaware, giving Democrats a 53-47 edge in the 112th Congress.

As for the Senate campaign committees, there may still be some recruiting to do for the NRSC, which lacks top-tier candidates in competitive states such as Pennsylvania and West Virginia, as well as against Sen. Bob Menendez in New Jersey. But for both committees, the top priority at this point is stocking up on cash for independent expenditures next fall, former NRSC Political Director Chris LaCivita said.

LaCivita, who works for Rep. Todd Akin’s Senate campaign in Missouri and former WWE CEO Linda McMahon’s campaign in Connecticut, said President Barack Obama will be a drag on the ticket in states across the country.

“What we encountered in 2006 and 2008, the Democrats got a taste of in 2010,” he said. “Now they’re going to get the full sample in 2012.”

J.B. Poersch, managing director at SKDKnickerbocker and a former DSCC executive director, said the 2012 map looks far better for Democrats when it’s sized down.

“While Republicans may have an opportunity to take back the Senate, this thing is a lot closer than anyone wants to admit,” Poersch said.

Massachusetts is the Democratic Party’s best opportunity to pick up a seat, but consumer advocate Elizabeth Warren must prove her chops as a candidate against Sen. Scott Brown, who has a lengthy head start on fundraising and strong approval ratings. Warren’s third-quarter report will be one to watch, even though she’s had just a few weeks to raise money.

Beyond Massachusetts and Nevada, which is a pure tossup, there are only a couple of second-tier pickup opportunities for Democrats. They include Arizona, where a few candidates are emerging but will face a tough opponent in Rep. Jeff Flake, and Indiana. For now, Republicans are favored to hold Indiana, but that could change if Sen. Dick Lugar loses the GOP nomination to state Treasurer Richard Mourdock.

Mourdock has received endorsements from the Tea Party Express and tea party favorite Sharron Angle, but his fundraising was dwarfed through June by the six-term incumbent. His next report will indicate whether he’s catching up or falling further behind Lugar. Democrats would feel better about the race if Mourdock is Rep. Joe Donnelly’s (D) opponent, but it would remain a tough race given Indiana’s Republican lean.

In Florida, observers will look to see who has the fundraising edge in the GOP primary now that state Senate President Mike Haridopolos is no longer in the race. He dropped out shortly after posting
$2.8 million in cash on hand at the end of June, a figure well ahead of former Sen. George LeMieux and former state House Majority Leader Adam Hasner.

They’re running to take on Sen. Bill Nelson (D), who could be in far better shape than it appeared at the start of the cycle if none of the remaining Republicans receive a surge of excitement and money. Florida is among the most expensive and culturally diverse states of the 2012 battleground.

Missouri Sen. Claire McCaskill (D), running in a state with more affordable media markets, will be a top GOP target regardless of her opponent, who won’t be known for 10 months. St. Louis businessman John Brunner is expected to enter the race today, joining Akin and former state Treasurer Sarah Steelman.

Along with McCaskill, Nebraska Sen. Ben Nelson is one of the most vulnerable Democrats. Nebraska Attorney General Jon Bruning (R) has long been the favorite to take him on, but the outcome of the GOP primary could be in doubt if state Treasurer Don Stenberg, who’s earned recent tea-party-affiliated endorsements, including one from Sen. Mike Lee (R-Utah), can pick up the fundraising pace.

Another top target for the NRSC is Sen. Debbie Stabenow in Michigan, where many strategists will be waiting to see former Rep. Pete Hoekstra’s third-quarter fundraising totals. Hoekstra is the favored candidate of the GOP establishment in Washington, D.C., and in Michigan, although he still must win a primary next year.

Fundraising reports to watch in other Senate primaries include both parties’ top candidates in New Mexico, where Rep. Martin Heinrich and state Auditor Hector Balderas are battling for the Democratic nomination and to keep the seat in Democratic hands. Republicans Heather Wilson, a former Congresswoman, and Lt. Gov. John Sanchez are squaring off for the GOP nod.

The most high-profile potential candidate still sitting on the sidelines is former Hawaii Gov. Linda Lingle (R), who has said she will announce her plans this fall. While the two-term governor would have a tough road to win Obama’s home state in a presidential cycle, her entrance would help the NRSC expand the map.

“The goal is always to expand the playing field as much as possible, especially if you’re on offense,” LaCivita said.