The next few months will be stressful for Senate strategists.

With less than four months to go, the fate of the Senate majority is maddeningly  unknown — and likely to stay that way through Election Day. With multiple outcomes possible on Nov. 6, three scenarios appear most realistic: Democrats hold the Senate with a small majority, Republicans flip the chamber and win a narrow majority, or the parties split the Senate with 50 seats apiece and control is decided by the winner of the White House and his vice president’s tie-breaking vote.

With their current 53-47 majority, Democrats can withstand a net loss of only two seats if they are to retain control of the chamber, regardless of whether President Barack Obama or presumptive Republican nominee Mitt Romney emerges victorious in the presidential race.

“The outcome of the presidency is part of the overall fabric of keeping the Senate,” said J.B. Poersch, a veteran Democratic Senate strategist. “But I wouldn’t go as far as saying it’s the most critical factor.”

The battle for the Senate will play out over a cross-section of states, some highly competitive at the presidential level and others that will likely see very little of Obama or Romney. While the top of the ticket undoubtedly affects them, Senate races are largely individualized contests swayed by the quality of the candidates and their campaigns and the metrics of the state.

The Democratic majority is remarkably less tenuous than it was entering this cycle, which followed a net gain of seven seats for Republicans in 2010. Neither party can confidently put more than a couple of competitive seats in their column just yet. Democrats are defending 23 seats to just 10 for Republicans, but the competitive landscape at this point is limited to a maximum of 15 states, including a few that are likely out of reach for the challenging party.

The playing field will mature by summer’s end, when contests could move down the competitive scale as they are more clearly defined and spending picks up. Republican nominees still haven’t been picked in Wisconsin or Missouri, two states critical to the majority.

Scenario 1: Democrats Hold

Democrats don’t need everything to go their way over the next few months to hold the Senate majority, although it would help.

The party faces a potentially daunting task. But it also begins with a 53-47 edge and therefore breathing room for a few losses, some of which are expected. In fact, the Democrats’ pickup opportunities have increased over the course of the cycle, with Sen. Olympia Snowe’s (R-Maine) surprising decision to retire altering a conventional wisdom that had previously given them a slim chance of holding their majority.

Although Senate contests are influenced less by the top of the ticket than House races are, under a scenario where the Democrats hold a majority of one to three seats, a victory by President Barack Obama would go a long way, especially in some of the open-seat races the party must defend. Obama’s success against presumptive Republican presidential nominee Mitt Romney could prove particularly crucial in Florida, Ohio, Virginia and Wisconsin.

Those holds, plus flipping seats in Maine and Massachusetts, would allow for the possibility of losses in Nebraska, North Dakota and Missouri. The party is still optimistic in North Dakota and hopeful in Missouri, but Nebraska is likely out of reach. Still, by losing only three seats and picking up two, Democrats would have a 52-48 majority, a net loss of only one seat.

That scenario would require Montana Sen. Jon Tester’s (D) fundraising and advertising strategy to keep pace with the outside spending assisting Rep. Denny Rehberg (R). Tester has received outside help of his own, but he’s also running in a state Obama has no chance of winning.

This outcome would also require former Virginia Gov. Tim Kaine (D) to win what will no doubt be a tight race with George Allen (R), a former governor and Senator, and necessitate Wisconsin Rep. Tammy Baldwin (D) to win a state in political upheaval after the failed recall of Republican Gov. Scott Walker. Baldwin won’t know her opponent until the Aug. 14 GOP primary.

Thus far, Democratic Sens. Sherrod Brown (Ohio) and Bill Nelson (Fla.) are favored in their respective presidential tossup states. To keep that edge, they’ll need to withstand heavy outside spending that will flood the airwaves in both states. Nelson’s $10.8 million gives him a big advantage in prohibitively expensive Florida over his likely opponent, Rep. Connie Mack IV (R), whose fundraising has so far been dwarfed in comparison.

In the Maine contest, the widely held assumption is that former Gov. Angus King (I) will ultimately caucus with Democrats. King, the favorite in the race to succeed Snowe, told Roll Call over the weekend that he has not spoken with anyone in Democratic leadership.

Assuming Maine is in their column, Democrats also need Harvard professor Elizabeth Warren (D) to defeat Sen. Scott Brown, perhaps the only Republican who could win Massachusetts with Obama on the ticket. Both candidates are raising huge sums of money and, should she win, Warren will have translated that into winning over enough of the working-class Democrats capable of voting for Brown.

Among the Democrats’ top three pickup opportunities, Nevada is probably the most difficult. It will be far more competitive for Rep. Shelley Berkley (D) if Obama’s margin doesn’t fall too far from his 12-point win in 2008. But Democrats could hold the Senate under this scenario even with a victory for Sen. Dean Heller (R).

Democrats are expected to hold Obama’s native Hawaii and come up short in Indiana and Arizona, but those contests are unlikely to affect their ability to hold the Senate majority.

Scenario 2: Republicans Gain

Republicans entered this election cycle as favorites to win the Senate, and their chances grew in the opening months of the 112th Congress as Democrats in swing states opted for retirement.

If Republicans win the majority Nov. 6, that victory is likely to be narrow — in the realm of one to three seats. There are multiple scenarios for such an outcome, but most would probably include defeating Democratic Sens. Claire McCaskill (Mo.) and Jon Tester (Mont.) while winning the Republican open seats in Nebraska and North Dakota. Those are the party’s top four opportunities to play offense.

Under this particular scenario — which assumes that Sen. Scott Brown (R-Mass.) loses and former Gov. Angus King (I) wins the open Maine seat and caucuses with the Democrats — the Republicans would need to capitalize on a couple of other existing pickup opportunities.

One way to do this would be to win two out of four Democratic-held seats in Virginia, New Mexico, Wisconsin and Ohio. If Republicans did this, they would achieve a
51-49 majority.

All four are winnable, but under the scenario envisioned here, Republicans pick up the open seats in Virginia and Wisconsin, with New Mexico Rep. Martin Heinrich (D) able to hold on in a tough race with former Rep. Heather Wilson (R) and Sen. Sherrod Brown (D) winning in a tossup presidential state against Ohio Treasurer Josh Mandel (R).

Virginia might be the state most closely tied to the presidential election, especially with former Gov. Tim Kaine’s most recent post as chairman of the Democratic National Committee. As well-known as both former governors are, it’s likely a George Allen (R) win means presumptive Republican presidential nominee Mitt Romney also prevailed in the Old Dominion.

A Republican win in Wisconsin, meanwhile, would mean that the GOP nominee chosen in the upcoming
Aug. 14 primary was successful in branding Rep. Tammy Baldwin (D) as too liberal. Wisconsin voted out three-term Democrat Russ Feingold in 2010 and refused to recall Gov. Scott Walker (R) in June.

This GOP takeover scenario also assumes that the Republicans are able to hold Nevada, currently a tossup race between appointed Sen. Dean Heller (R) and Rep. Shelley Berkley (D), as well as the less competitive seats in Indiana and Arizona.

If the Republicans do win Montana, it probably means that Rep. Denny Rehberg (R) was able to tie Tester to President Barack Obama and the Democratic agenda Tester supported over the past six years.

It would also mean that Tester’s multitude of ads featuring him working on his farm and the ads tying Rehberg to national Republicans were not enough to withstand the barrage of outside spending in this relatively inexpensive state.

In North Dakota, a GOP victory would probably mean that former state Attorney General Heidi Heitkamp (D) was competitive but unable to overcome the state’s Republican lean. Gravy for the Republicans would be a win by former Gov. Linda Lingle (R) in Obama’s native Hawaii, but her loss doesn’t affect Republicans’ ability to win the Senate.

Scenario 3: Tied Senate

It is completely within the realm of possibility that each party will end election night with 50 Senate seats.

This scenario would add even more intrigue and consequence to the presidential contest, with Vice President Joseph Biden or presumptive Republican nominee Mitt Romney’s vice president providing the tie-breaking vote in the Senate possibly for the next two years.

Here is one of a number of ways split Senate control could happen.

With Democrats currently holding a 53-47 majority, the Republicans would cut into that by picking up the Democratic-held seats in Missouri, North Dakota, Nebraska and Montana, while Democrats would win the seat of retiring Sen. Olympia Snowe (R-Maine). This pickup assumes that former Maine Gov. Angus King (I) caucuses with the Democrats, which he is widely expected to do despite being publicly noncommittal.

Republican Sens. Dean Heller (Nev.) and Scott Brown (Mass.) would hold on in close contests, while Democrats would hold competitive open seats in Virginia, New Mexico and Wisconsin. Democratic Sens. Sherrod Brown (Ohio) and Bill Nelson (Fla.) would also keep their seats under this scenario.

Arizona and Indiana would stay in Republican hands, while Democrats would hold on in Hawaii.

This scenario would most likely be coupled by an excruciatingly tight presidential contest between Romney and President Barack Obama. With the economy slow to recover but with Obama still seen as a likable figure, that’s not hard to imagine. National polling has the two candidates within the margin of error, and the list of competitive states remains lengthy.

Among the other plausible ways the Senate could end Nov. 6 split: Democrats pick up Massachusetts but lose Virginia, where the Senate results could end up following the top of the ticket.

Nevada Rep. Shelley Berkley (D) could defeat Sen. Dean Heller (R), while her colleague Rep. Tammy Baldwin (D) falls short against a still-undetermined Republican nominee in Wisconsin.

However, it’s more difficult to see how the parties split the Senate if Democrats are able to hold on in Montana, a difficult state for Democrats where Sen. Jon Tester (D) is running a strong campaign but facing a tough challenger in Rep. Denny Rehberg (R).

While the presidential race has more of an effect on House races in general, a Tester win in Montana — where Obama is not competing — could portend Democratic success with Senate seats in more competitive presidential states such as Wisconsin and Virginia. Of course, being open seats, the dynamics of those contests could differ from a state such as Montana.

Nothing is certain, anything can happen, and Senate strategists will likely be entertaining many of these scenarios until Election Day.