Currently, the U.S. Senate has 51 sitting Democrats, two independents and 47 Republicans. Of the 33 seats up for grabs this year, 21 are held by Democrats, two are held by independents that caucus with the Democrats, and 10 are held by Republicans.

To take control of the Senate, Republicans would need a net gain of four seats if President Barack Obama is re-elected, but only three if Romney won. In that case, Paul Ryan, as Romney's vice president, would become the Senate's tie-breaking vote. Stuart Rothenberg thinks the most likely range is no net change to a Republican gain of three seats. At this time, Democrats still have an edge in their effort to retain control of the Senate.

The Republican road to the majority remains a high-wire act requiring GOP candidates to catch almost every break in the last days of the campaign. But many of the most important races have become more competitive in recent days, and their outcomes harder to predict.

Possible Outcomes:
Democrats Outperform Expectations

  • It’s unlikely to happen, but if Democrats have a better than expected night, they could gain one or two seats -- this would be an impressive feat. A

    Democratic gain of four seats is their absolute limit, but this would require them to hold all ten endangered seats – after their expected loss in Nebraska – and then win all of the endangered Republican seats; don’t expect that to happen.

A Status Quo Election

  • There is a real possibility that Democrats will manage to lose zero net seats. It would be a good night for Democrats, but not a completely surprising one. A no-net-change result would leave Democrats with a 53-47 edge over Republicans.

Democrats Manage to Limit Their Losses

  • There is a good chance that Democrats will lose only one or two seats – many expect this outcome. A loss of two seats would leave them with a 51-49 edge over Republicans; a loss of one seat would leave them at 52-48.

Republicans Manage to Split the Senate

  • Republicans still have a fair shot at winning three seats. That would get Republicans to a 50-50 tie in the Senate, in which case the vice president would cast tie-breaking votes. The big question would then be: will the next vice president be Biden or Ryan?

Republicans Win the Senate

  • Republicans have a realistic – but somewhat slim – chance of gaining four seats and seizing control of the chamber, leaving them with a 51-49 edge over the Democrats.

Republicans Outperform Expectations

  • There are a lot of Democratic seats in play – don’t expect them all to go to the Republicans – but Republicans could gain five or six seats on a very good night. A gain of seven seats would be a real shocker given the current landscape.

A Note on Maine: Angus King, who is running as an independent to succeed retiring GOP Sen. Olympia Snowe, is the frontrunner. He has said that he will not decide which party he will caucus with in the Senate until after the election. Most expect him to caucus with the Democrats. Without knowing which party King will choose, Charlie Cook says it’s impossible to give this race a rating that reflects King's frontrunner status. For that reason, Cook and others keep the race in the Toss Up column, not because they expect the Republican, Charlie Summers, to win.