But then things went awry, beginning with the surprise decision in February by Republican Senator Olympia Snowe of Maine to surrender her relatively safe seat and retire and, most recently, a series of attention-grabbing comments about rape by Republican candidates who had good odds of defeating 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 US vice president would cast the decisive vote. Here's a look at races that have been considered close over the past few weeks as the nation votes on Tuesday: Maine 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. Indiana State Treasurer Richard Mourdock, a favorite of the conservative Tea Party movement, defeated Senator Richard Lugar in the 2012 Republican primary, complaining that the incumbent was not conservative enough. But Mourdock may be too conservative for his Republican-leaning state. He had been running neck-and-neck with Democrat Joe Donnelly, a House of Representatives member since 2007, until Mourdock defended his position on abortion by saying that pregnancy resulting from rape was "something God intended to happen." Donnelly has attacked Mourdock as an extremist while Mourdock ties Donnelly to President Barack Obama, referring to him in TV ads as "Obama Joe." A poll released on Friday showed Donnelly with a lead of 11 percentage points. Virginia Republican George Allen and Democrat Tim Kaine, both former governors, 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. Massachusetts With voters fed up with 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 Democrat Elizabeth Warren, a Harvard professor and consumer advocate. Wisconsin The race to replace retiring Democratic Senator Herb Kohl pits a former conservative Republican governor, 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. Connecticut Republican 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 three-term Democratic Representative Chris Murphy. The winner will replace Senator Joe Lieberman, an independent who is retiring. Montana Democrat Jon Tester, who narrowly won his first term in the Senate in 2006, is in another close race. Tester is being challenged by Denny Rehberg, a six-term Republican House member. Their battle has largely focused on who better reflects Montana versus who has "gone Washington." North Dakota 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. Arizona 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 descent. He is opposed by Republican Congressman Jeff Flake, a Tea Party favorite endorsed by 2008 Republican vice presidential nominee Sarah Palin. Nevada 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. Nebraska Former Senator Bob Kerrey, a Democrat, is trying to return to 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.