Factbox: Close races in the battle for Senate
(Reuters) - A year ago, Republicans seemed headed toward capturing the U.S. Senate, now held by Democrats, 53-47.
But then things went awry, beginning with the surprise decision in February by Republican Senator Olympia Snowe to surrender her relatively safe seat and retire and, most recently, a series of attention-grabbing comments about rape by Republican candidates who could have unseated Democrats.
Thirty-three of the Senate's 100 seats are being contested, of which 23 are held by Democrats.
Democrats are now slightly favored to retain the Senate though a tie is also possible, in which case the vice president of the United States would cast the decisive vote.
Here's a look at races that have been considered close over the past few weeks:
Angus King, a popular and independent former governor, is in a three-way race with Republican Secretary of State Charlie Summers and Democratic state senator Cynthia Dill.
National Democrats are not backing Dill, figuring King will win and side with them in deciding Senate control. But with King declining to say what he will do, the race for Snowe's seat is seen as a tossup.
State Treasurer Richard Mourdock, a Tea Party favorite, defeated Senator Richard Lugar in the 2012 Republican primary, complaining that the incumbent was not conservative enough.
But Mourdock may be too conservative for this Republican-leaning state. He had been running neck and neck with Democrat Joe Donnelly, a House of Representatives member since 2007. Donnelly has attacked Mourdock as an extremist; Mourdock ties Donnelly to President Barack Obama, referring to him in TV ads as "Obama Joe."
A poll released on Friday, after Mourdock made some more controversial comments about rape and abortion, showed Donnelly with a lead of 11 percentage points.
Former Governors George Allen and Tim Kaine are battling to fill the Senate seat being vacated by retiring Democrat Jim Webb, who ousted Allen from the Senate job six years ago.
Kaine, a former Democratic Party chairman, is running left of center; Allen, son of Hall-of-Fame football coach George Allen, is running to the right.
With voters fed up with a partisan gridlock, Republican Senator Scott Brown has focused on his credentials as one of the most bipartisan members of Congress.
Yet two years after being elected to fill the seat vacated by the death of Democratic Senator Edward Kennedy, Brown is fighting for political survival. He is being challenged by Elizabeth Warren, a Harvard professor and consumer advocate.
The race to replace retiring Democrat Herb Kohl pits a former conservative governor, Republican Tommy Thompson, against a liberal congresswoman, Democrat Tammy Baldwin, who would be the chamber's first openly gay member.
Polls initially showed Thompson leading. But Baldwin moved up by blasting him for having worked as a high-priced lobbyist after serving in the administration of President George W. Bush.
Linda McMahon, a former executive director of World Wrestling Entertainment, is running a stronger campaign for the Senate this year than she did her first time around in 2010.
Having revamped strategy to spend more face time with voters, she is in a close race with third-term Democratic Representative Chris Murphy. The winner will replace retiring Joe Lieberman, an independent.
Democrat Jon Tester, who narrowly won his first-term in the Senate in 2006, is in another close race for a second term.
Tester is being challenged by Denny Rehberg, a six-term House member. Their battle has largely focused on who better reflects wide-open Montana versus who has "gone Washington."
When Democratic Senator Kent Conrad announced last year he would not run for re-election, his conservative state of North Dakota looked like a potential pickup for Republicans.
But former state Attorney General Heidi Heitkamp, running as a centrist Democrat who has disagreements with Obama, has waged a surprisingly strong campaign against first-term Republican Congressman Rick Berg.
Former U.S. Surgeon General Richard Carmona is trying to become the first Democrat elected to the Senate from Arizona since 1988.
Party leaders figure Carmona may benefit in this border state from his Hispanic surname. He is opposed by Republican Congressman Jeff Flake, a Tea Party favorite endorsed by 2008 Republican vice presidential nominee Sarah Palin.
The House Ethics Committee announced in July it would investigate Democratic Representative Shelley Berkley, complicating her bid to unseat Republican Senator Dean Heller.
But she has denied any wrongdoing in helping save a hospital program associated with her husband, and has stayed in a close race with Heller. He was appointed to the Senate last year to replace John Ensign, who resigned amid his own ethics problems.
Former Senator Bob Kerrey, a Democrat, is trying to get back into the Senate but has faced a strong opponent in conservative Nebraska state Senator Deb Fischer, who upset two opponents in the state's Republican primary race.
Kerrey has suffered from the fact that he has been living in New York for the past 10 years. He lagged for months in polls but appeared to be closing the gap in the final two weeks of the campaign.
The seat is now held by retiring Democratic Senator Ben Nelson.