Democrat control of the U.S. Senate is in serious jeopardy this November. Of the 33 seats up for election this Fall, Democrats are defending 22. The GOP needs a net-gain of 4 seats to take control of the chamber (3 if Romney is elected, as the GOP VP would provide a tie-breaking vote.) Of the 8 seats considered "toss up" by RealClearPolitics, Democrats are defending six. Moreover, though, Democrats are defending 8 seats in Presidential battleground states, linking their fates to the Obama/Romney contest.
In recent years, contested Senate races have tended to "break" in favor of one party. In 2006, Democrats picked up 6 seats and didn't lose any. In 2008, Dems picked up 8 and again successfully defended every seat. Same too for the GOP in 2010, who picked up 6 seats without losing any they had held. If the election contest breaks discernibly for either Obama or Romney, the Senate races in the 8 battleground states will likely break towards the candidate's party, delivering the Senate to the party of the Presidential victor.
Senate races will be hotly contested in the following battleground states: Pennsylvania, New Mexico, Ohio, Wisconsin, Missouri, Virginia, Michigan, Nevada and Florida. The GOP hasn't settled on a candidate yet in 4 of these states: WI, MO, MI and FL. Recent polling has shown MO incumbent Senate Claire McCaskill trailing all of her main rivals, but the outcome of primaries there and in the other states will determine how credible the GOP challenge will be for those seats. In 2010, Senate GOP primary winners who ran flawed general election campaigns prevented the GOP from further gains in the chamber.
Nevada is the only battleground state the GOP is defending in the Senate. That match-up pits appointed Sen. Dean Heller against Dem Rep. Shelley Berkley. That race, however, has moved recently in favor of the GOP amidst questions surrounding Rep. Berkley's ethics. Because of the serious allegations against Berkley, this race will provide move independently of the Presidential contest.
Current polling gives the edge to Democrats in PA, NM, OH and MI, but I expect this will narrow considerably as the campaign heats up. The GOP has very compelling candidates in PA, NM and OH and Dem incumbents or candidates whose views may be out-of-step with the electorate this November. If PA and NM are competitive in the Fall, as certainly seems possible now, it will signal that Romney is running a strong general campaign and boost the GOP's chances there.
There will probably be some talk about voters "splitting their ticket" in an exercise in "strategic voting". In other words, they'll vote for one party for President and another for Senate or House to act as a "check" on the Presidential victor. Its a compelling theory and may be true for a small number of voters, but evidence of it actually happening is scarce. In 2004, when then-President Bush won a very close reelection, the GOP picked up an additional four seats in the Senate. It is possible that if Obama looks likely to win reelection, voters would lean towards the GOP candidates, as voters two years ago loudly delivered a legislative "check" on him. Still, I think past elections suggest a "wave" breaking for the party of the victorious Presidential candidate the most likely outcome. If Romney wins the election, I expect the GOP will reclaim control of the Senate by a comfortable margin.