Two Republican candidates who made controversial comments about rape during the campaign have been defeated, almost certainly helping the Democrats to retain their majority in the Senate.
The Democrats are almost assured of retaining the majority after Republicans lost seats in Indiana, Massachusetts and Maine.
In Missouri the incumbent Democrat Claire McCaskill defeated Republican Todd Akin to win a second Senate term.
The race was turned upside down on August 19 when Mr Akin said in a television interview that "legitimate rape" rarely results in pregnancy. The remark prompted party officials, including the Republican presidential candidate Mitt Romney, to urge the six-term congressman to leave the race. It was a political gift to Ms McCaskill, 59, who had been trailing Mr Akin in polls.
The Senate contest in Missouri, a Republican-leaning state where Barack Obama has low approval ratings, had been viewed as one of the Republicans' best opportunities to capture one of four seats they need to win a Senate majority.
In Indiana, the Democrat Joe Donnelly defeated Richard Mourdock, who in May defeated the six-term Republican incumbent Richard Lugar in a primary vote.
Less than two weeks before election day, Mr Mourdock, the Indiana state treasurer, imperiled his attempt to replace Mr Lugar in the Senate by referring to pregnancies resulting from rape as "something God intended to happen".
The Massachusetts Senator Scott Brown was beaten by the Democratic challenger Elizabeth Warren, television networks reported.
In Maine, the independent Angus King beat a Republican and a Democrat for the seat being vacated by Republican Senator Olympia Snowe, whose decision in February to retire was an early blow to Republicans' hopes of gaining Senate control. Mr King is expected to caucus with Democrats.
Democrats control the Senate 53-47. Republicans needed to pick up four net seats to gain a majority.
Democratic candidates in Connecticut, Ohio, Florida and Pennsylvania held on to seats the party now controls that were considered competitive.
The Democrat Chris Murphy defeated Linda McMahon, the former head of World Wrestling Entertainment, for the Senate seat in Connecticut held by retiring Senator Joseph Lieberman, an independent who caucuses with Democrats. Senator Sherrod Brown defeated Republican challenger Josh Mandel in Ohio, a presidential battleground state.
In Florida, Democratic Senator Bill Nelson of Florida won a third term, defeating Republican Representative Connie Mack IV.
Democratic Senator Bob Casey defeated Republican Tom Smith in Pennsylvania.
The Senate electoral landscape was supposed to favor Republicans, who were defending 10 seats compared with 23 Democratic seats on the ballot this year in the 100-seat chamber. The odds of a Republican majority dropped from 70 per cent in February to just below 40 per cent, according to the nonpartisan Cook Political Report in Washington.
To gain the majority, Republicans needed to hold all five of their competitive seats, including in Indiana and Massachusetts, and pick up four currently held by Democrats.
Continued gridlock would be probable next year in a Congress with an unchanged balance of power, said Jennifer Duffy, Senate analyst for the nonpartisan Cook Political Report in Washington. Both parties will "find things in this election to encourage them to continue to behave as they've behaved the last two to four years", she said.
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