Democrats held their narrow majority in the Senate on Tuesday, grabbing GOP seats in Massachusetts and Indiana and turning aside Republican challenges in Virginia and Ohio.
Republicans were well on the way to retaining control of the House, ensuring that Congress will be divided at the start of President Barack Obama's second term in office.
In Massachusetts, Democrat Elizabeth Warren has beaten Republican Senator Scott Brown.
In Indiana, Democratic Joe Donnelly defeated Republican Richard Mourdock, whose clumsy comment about rape and abortion in the closing days of the race damaged his chances.
That seat had been held by moderate Republican Senator Richard Lugar. Republicans also lost a seat in Maine, where the surprise retirement of Senator Olympia Snowe opened the way for a win by independent Angus King, who 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.
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 - President Barack Obama or Republican Mitt Romney - will likely face a divided Congress that would make passing any major pieces of legislation difficult.
Republicans 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 Republican prospects were undermined by a firestorm that erupted after their candidates in Indiana and Missouri made explosive comments about abortion, rape and pregnancy, and the surprise retirement of Snowe in Maine.
Snowe voiced her frustration with the gridlocked Congress when she announced her retirement earlier this year. King, a former governor, won a three-way race to replace her.
King has vowed to be a bridge between the parties and has not said whether he would caucus with Democrats or Republicans and the outcome of the presidential election is certain to have a bearing on any decision.
However, he was expected to side with the Democrats after Republican groups spent hundreds of thousands of dollars attacking him.
In Connecticut, Democratic congressman Chris Murphy won the seat being vacated by retiring independent Joe Lieberman.
Republicans had once hoped that the race would be won by Linda McMahon, the former head of World Wrestling Entertainment who spent more than US$42 million of her own fortune in the race.
Three Democrats senators also won re-election in competitive races.
Sherrod Brown in Ohio and Bob Casey in Pennsylvania, who rode a Democratic wave to the Senate in 2006, were elected to second terms, while Bill Nelson won in Florida.
Democrats were expected to narrowly retain control of the Senate but would remain nowhere near the 60-vote supermajority needed to easily pass legislation under Senate rules.
Republicans were expected to retain their solid advantage in the House. Congress consistently rates low in public opinion surveys, but incumbents still tend to get re-elected.
They benefit from a system that gives them huge financial advantages in their re-election bids, and enjoy support from voters who tend to favor their own lawmakers even if they dislike Congress overall.
Many incumbents in the House were also helped by the once-a-decade redrawing of district boundaries, which has just been completed.