The battle for the Senate will be key in congressional elections. At present, Congress is divided. Democrats have a slim majority in the Senate, Republicans a much larger margin in the House of Representatives. The outcome will have a major influence on the ability of the next president to reach a crucial deficit-reduction deal.
Missouri
Democratic senator Claire McCaskill was thought to be vulnerable but Republican challenger Todd Akin's comments about rape and pregnancy (he claimed a woman's body could "shut the whole thing down" in cases of "legitimate rape") backfired.
Virginia
Two former governors, Democrat Tim Kaine and Republican George Allen, are competing for an open Senate seat. Polls consistently show this race is closely tracking the presidential one.
Massachusetts
Republican incumbent senator Scott Brown is in a close battle with Democratic challenger Elizabeth Warren. Latest polls indicate Warren has a slender lead.
Ohio
Democratic senator Sherrod Brown is seeking a second term, but Republican war veteran Josh Mandel is standing in his way. Brown is ahead in the polls and has begun to pull away in recent weeks.
Indiana
The Democrats believe their candidate, Joe Donnelly, has a chance of winning the Senate seat from challenger and Tea Party favourite Richard Mourdock, whose recent comments on rape – he said pregnancies arising from rape were an act of God – will not have helped his case.
Montana
Democrats do not expect Barack Obama to carry Montana but believe their incumbent senator Jon Tester will eke out a victory over challenger Denny Rehberg.
New York
Republican Ann Marie Buerkle defeated Democrat Dan Maffei in the House elections two years ago. This time Democrats are more confident of their candidate's victory.
Utah
In Salt Lake City, a conservative bastion of Mormonism, Democratic incumbent Jim Matheson is fighting to hold on to his House seat in a battle with Republican opponent Mia Love. An African-American and a former Catholic, Ms Love converted to Mormonism and made a big splash with a hard-line speech at the recent Republican convention.