Senator Ted Stevens is locked in a close race with his Democratic rival
Three races in the US Senate are still hanging in the balance, after the Democrats extended majorities in both houses of Congress.
A recount will take place in Minnesota, where the contest ended with the candidates only a few hundred votes apart.
Winners have also not yet been declared in Georgia and Alaska.
In the House of Representatives, the Democrats secured their biggest majority in 15 years.
They made gains of at least 19 seats, with eight still to be called.
All 435 seats in the House and 35 in the 100-seat Senate were up for grabs in tandem with Tuesday's historic presidential election, which saw Democratic candidate Barack Obama swept to power.
In the Senate, the Democrats gained five seats, leaving them with a total of 57 (including two independents who vote with the majority). Republican incumbents were unseated in New Hampshire, New Mexico, North Carolina, Virginia and Colorado.
Winning 60 seats would give the Democrats a so-called "super-majority" - meaning that they could prevent Republican senators from using procedural blocks such as filibustering to hold up legislation.
In Minnesota, a manual recount of 2.9 million votes will take place after incumbent Republican Senator Norm Coleman and his Democrat rival, Al Franken, ended the race with fewer than 1,000 votes separating them.
Mr Coleman declared victory on Wednesday, but Mr Franken said his party was investigating reports of irregularities at some precincts.
"There is reason to believe that the recount could change the vote tallies significantly," he told a news conference.
In Oregon, Democratic challenger Jeff Merkley ousted Republican Senator Gordon Smith. The result, which was not called until Thursday, brought the Democrats' total to 55 plus the two independent senators.
In Georgia, a run-off will take place on 2 December following a three-way race in which none of the candidates secured the required majority.
And in Alaska, the incumbent Republican Senator Ted Stevens was locked in a tight race with the Democrat Mayor of Anchorage, Mark Begich.
Mr Stevens has held his seat since 1968, but was convicted in October of lying about gifts he had received from a business executive.
By late on Wednesday only 3,500 votes separated the two men. Absentee ballots and early votes will be counted in the coming days.
In the House of Representatives, the Democrats built on the 235 seats they had secured in the 2006 mid-term elections. In the new Congress they will have at least 254 seats, projections suggest.
House Speaker Nancy Pelosi, a California Democrat, said lawmakers were ready to focus on "the growth of our economy, the education of our children, the health of our people, the end of dependence on foreign oil and the end of the war in Iraq".
But she called on Mr Obama to govern from the middle, to "reach consensus on solutions that are sustainable and acceptable to the American people".