A look at this year's top Senate races as Democrats to try take back control from Republicans, who now hold a majority in the chamber:
Democratic former Sen. Evan Bayh, who retired six years ago, is running to get his old seat back. He is favored over GOP Rep. Todd Young, a former Marine. Incumbent GOP Sen. Dan Coats is retiring.
Incumbent GOP Sen. Mark Kirk is the most endangered Republican this year given Illinois' Democratic leanings in a presidential year. His Democratic opponent is Rep. Tammy Duckworth, a double-amputee war veteran.
Wisconsin features a rematch between former Sen. Russ Feingold and GOP incumbent Sen. Ron Johnson, who defeated Feingold six years ago. Democrats are optimistic about the state.
GOP Sen. Kelly Ayotte is facing a tough challenge from Gov. Maggie Hassan. With two strong candidates in the race, Ayotte's challenge is overcoming Donald Trump's unpopularity in the state.
GOP Sen. Pat Toomey has broken with his party in support of limited gun control measures and refused to endorse Trump as he struggles for support from Democratic-leaning voters. He could have trouble surviving a big Trump loss. His opponent is Democrat Katie McGinty, a former gubernatorial chief of staff.
GOP Sen. Marco Rubio announced his retirement from the Senate in an unsuccessful bid for the White House, but like Bayh on the Democratic side, he got back into the race at the urging of party leaders. He is slightly favored over Democratic Rep. Patrick Murphy.
The retirement of Senate Minority Leader Harry Reid gives Republicans their one pickup opportunity. They are bullish about their candidate, GOP Rep. Joe Heck, but the race is shaping as a hard-fought toss-up with former state Attorney General Catherine Cortez Masto.
Democrats had high hopes for defeating GOP Sen. Rob Portman but he's run a strong and disciplined campaign, and former Democratic Gov. Ted Strickland has failed to gain traction.
Both parties are keeping an eye on GOP Sen. Richard Burr's re-election race as a potential surprise. His Democratic opponent is former state legislator Deborah Ross, a former official with the American Civil Liberties Union. In a sign of GOP nervousness, a Republican-leaning group plans to spend $8.1 million on television ads.
Republicans are confident that incumbent GOP Sen. Roy Blunt will keep his seat, but Democrats are enthusiastic about their young candidate, Missouri Secretary of State Jason Kander.
Republican Sen. John McCain, 80 and seeking his sixth term, faces a spirited challenge from Democratic Rep. Ann Kirkpatrick in a state with a significant Hispanic population.