Republicans have lost two seats in the Senate, boosting Democratic prospects for keeping control of the upper chamber.
In Indiana, Democrat Joe Donnelly beat Tea Party-backed Republican Richard Mourdock, whose clumsy comment about rape and abortion in the closing days of the race damaged his chances. The seat had been held by moderate Republican Richard Lugar, who lost a primary to Mourdock.
Republicans also lost a seat in Maine, where the surprise retirement of Senator Olympia Snowe opened the way for independent Angus King’s win. King is expected to vote with the Democrats.
Only about a dozen Senate races were seen as competitive and the first of them to be called – in Indiana, Maine, Connecticut, Ohio, Pennsylvania and Florida - all went the Democrats’ way.
Democrat Sherrod Brown won re-election in Ohio, denying the Republicans a pick-up. Another Democrat Bob Casey won a new term in Pennsylvania.
Republicans have yet to pick up a Democratic seat, as the two parties fight for control of the Senate.
Mitt Romney’s party had seemed poised to snatch control of the Senate from the Democrats, who were defending 23 seats and losing several retiring veterans in Republican-leaning states.
But although Barack Obama’s party looks to be on course to keep control of the upper chamber, it has little hope of capturing the House of Representatives.
All 435 House seats were on the ballot, but Republicans were expected to keep control of the chamber, despite the possibility of some Democratic gains.
That means the winner of the presidential vote will face a divided Congress that would make passing any major pieces of legislation difficult.
And although Democrats were expected to narrowly retain control of the Senate, they are likely to remain nowhere near the 60-vote supermajority needed to easily pass legislation under Senate rules.