The 2012 election had been seen as perilous for the Democrats as they were defending more seats than the Republicans and faced several retirements, all during a nail-biting presidential race and an uncertain economy.
The Democrats, who in the previous Senate held a 53-47 majority when including two independents who usually vote with the party, looked set to hold a 55-45 advantage barring recounts or an independent siding with Republicans.
In North Dakota, Heidi Heitkamp, who distanced herself from Obama on issues such as oil exploration, held a lead of several thousand votes in a state where many pundits considered a Democratic win improbable. Her Republican rival, Rick Berg, has not conceded.
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In Montana, Democrat Jon Tester, who won a surprise victory in 2006, was projected to have narrowly held on to his seat after a campaign in which rockers Pearl Jam -- whose bassist Jeff Ament knew the senator as a child -- played a rare concert on his behalf.
Democrats also won victories in Indiana and Missouri, which voted against Obama in the presidential election but where Republican candidates came under fire for comments on rape.
Indiana Tea Party member Richard Mourdock, who ousted veteran senator Richard Lugar in the Republican primary, lost to Democrat Joe Donnelly after the conservative argued that pregnancy was a gift from God even when caused by rape.
In Missouri, Todd Akin lost to Senator Claire McCaskill, who had been seen as one of the most vulnerable Democrats, after the conservative argued that pregnancy cannot be caused by "legitimate rape."
Two independents were elected on Tuesday. In Maine, former governor Angus King won the seat vacated by Olympia Snowe, a Republican who was considered one of the chamber's most centrist members.
King told reporters Wednesday that he would decide soon on which party to align. He is widely expected to side with the Democrats, whose Senate leader, Harry Reid, said he spoke to King by telephone.
In liberal Vermont, Senator Bernie Sanders, a self-described socialist who joins the Democrats for organizational purposes, easily won re-election.
