—Indiana: Tea party-backed state treasurer Richard Mourdock stunned the GOP in May when he easily knocked out six-term Sen. Richard Lugar in the primary. He is giving Republicans fits again because he could lose on Tuesday even as Romney wins the state and Rep. Mike Pence likely emerges as the next governor. Mourdock had limited goodwill after suing in 2009 to stop the federal government's bailout of Chrysler. He further damaged his hopes when he said in a debate that pregnancy resulting from rape is "something God intended." Public and internal polls show conservative Democratic Rep. Joe Donnelly with a lead in the high single digits. The question is whether he can overcome the state's Republican trend. Libertarian Andrew Horning could affect the outcome.
—Virginia: Former governors, Democrat Tim Kaine and Republican George Allen, are locked in a close race. Allen is trying to reclaim the seat he lost to Democratic Sen. Jim Webb six years ago. Webb decided to retire. Kaine has run slightly ahead of Obama in the battleground state, where the presidential outcome could weigh on the Senate results. The race that has attracted some $50 million in outside spending.
—Montana: Democratic Sen. Jon Tester is fighting for political survival against Republican Rep. Denny Rehberg in a state that narrowly went for John McCain in 2008 but is expected to solidly back Romney. The former music teacher, butcher and current farmer was a surprise winner in 2006. Democrats hope his solid campaign this year will tip the balance.
—North Dakota: Both parties give high marks to the campaign Democrat Heidi Heitkamp has run this year, but it may not be enough against Republican Rep. Rick Berg in the strong Republican state and the race to fill the seat held by Democratic Sen. Kent Conrad.
—Wisconsin: Former four-term Gov. Tommy Thompson is a familiar name to many in Wisconsin. The question is whether the 70-year-old Republican who last ran statewide in 1998 is seen as history. Liberal Democratic Rep. Tammy Baldwin is counting on a strong turnout in her hometown of Madison. If elected, Baldwin would be the first openly gay senator.
—Missouri: Democratic Sen. Claire McCaskill, once considered the most vulnerable incumbent, could be back in Washington if she can hold off Rep. Todd Akin. The Republican severely damaged his candidacy in August when he said women's bodies have ways of avoiding pregnancy in "legitimate rape," and GOP leaders, including Romney, called on him to abandon the race. Akin stayed in and is counting on support from evangelicals to lift his prospects in a state that will vote Republican.
—Nevada: Republican Sen. Dean Heller secured the seat after a sex scandal sunk GOP Sen. John Ensign. Ethics questions have dogged his Democratic challenger, Rep. Shelley Berkley, in a race flooded by outside cash and ads. The race is a test of whether Senate Majority Leader Harry Reid's vaunted political operation can push Berkley to victory. In final push, she loaned her campaign $250,000 last month.
—Ohio: Republican groups swamped Democratic Sen. Sherrod Brown with some $30 million in negative ads, helping to lift the candidacy of Josh Mandel. The state treasurer who just took office in January 2011 equivocated for months over the 2009 auto bailout, critical in the state, before finally saying he opposed it. The outcome of the closely fought presidential race will affect the Senate race.