On the same night their leader was elected president, the Democrats made gains to their majority in the U.S. Senate, but failed to garner a filibuster-proof 60 seats.
Barack Obama handily defeated John McCain to be elected the first black president of the United States.
With 31 of 35 Senate races called Tuesday night, Democrats were guaranteed at least a 54-40 advantage, including two holdover independents who vote with Democrats. The majority was declared following several key Democratic wins in Republican strongholds such as North Carolina, New Hampshire, Virginia and New Mexico.
But four states were still too close to call: Alaska, Minnesota, Georgia and Oregon.
With 99 per cent of the precincts reporting, controversial Senator Ted Stevens of Alaska was slightly leading his Democratic opponent Mark Begich, the mayor of Anchorage. Stevens, 84, the U.S. Senate's longest-serving Republican, was convicted last month on several felony charges.
The charges stemmed from claims that he received free home renovations and other gifts worth more than $250,000 US from his friend and millionaire oil contractor, VECO Corp. chief executive Bill Allen.
In another high-profile race, the Associated Press initially called Republican incumbent Norm Coleman the winner over Democrat Al Franken, the former Saturday Night Live writer and actor, in Minnesota.
But the news agency later rescinded its call because Coleman's 571-vote victory margin falls within the state's mandatory recount law.
"There is reason to believe that the recount could change the vote tallies significantly," Franken said in a statement.
Minnesota Secretary of State Mark Ritchie said the recount won't begin until mid-November at the earliest and will probably stretch into December. It will involve local election officials from around the state.
Earlier, State Senator Kay Hagan unseated Republican incumbent Elizabeth Dole in the race in North Carolina, where the seat had been held by a Republican for 35 years.
Hagan's victory followed a similar Democratic grab in Virginia, where former state governor Mark Warner was in competition to replace a retiring five-term Republican senator. Warner was up against another former governor, Republican Jim Gilmore.
New Hampshire also changed from red to blue after the first woman elected governor there, Jeanne Shaheen, beat Republican incumbent John Sununu. In New Mexico, Democrat Tom Udall won the seat that had been held by Republican Pete Domenici.
Going into Tuesday's election, the Democrats held a 51-49 majority in the Senate, but their edge was maintained only by the support of two Independents who have voted with Democrats the past two years.
One of them, Joe Lieberman of Connecticut, campaigned on behalf of McCain in the 2008 election.
Obama's running mate Joe Biden won another six-year term representing Delaware in the Senate; however, both his and Obama's Senate seats will now be filled by appointees chosen by the governors of Delaware and Illinois.
Economic woes, compounded by widespread distaste for the administration of current U.S. President George W. Bush, could help the Democrats in their drive to dominate Congress and add to the six Senate seats they gained in 2006.
Voters reportedly packed the polls to vote for 35 Senate seats being elected Tuesday night. Of the 35 races on Tuesday's ballot, 23 were held by Republicans, 12 by Democrats.
Leaders in both parties, however, have admitted that a 60-40 Democratic majority that would prevent Republican senators from blocking bills and judicial nominees is unlikely.
Democrats build on House majority
In the House of Representatives, Democrats captured Republican-held seats in every region, adding at least 19 seats to give them a stronger majority than the one they achieved in midterm elections two years ago. Democrats saw potential gains in as many as five more undetermined seats as of midday Wednesday.
"It's the night we have been waiting for," said House Speaker Nancy Pelosi.
Democrats controlled the House of Representatives by a 235-199 margin, with one vacancy, before the election. In 2006, they won 30 seats and control of Congress in a surge powered by voter anger over the Iraq war.