Voters were asked to choose senators, representatives and governors
The Democrats have registered a clear victory over their Republican opponents in voting for both houses of Congress.
The party captured at least six Senate seats from their rivals, increasing their majority in the 100-seat chamber.
And they made further gains in the House of Representatives, easily exceeding the 218 seats needed for a majority.
All 435 seats in the House were up for election, while 35 of the Senate seats were available.
Analysts say winning control of both chambers should make it easier for President-elect Barack Obama to push through the legislation he wants.
In the House, the Democrats built on the 235 seats they had secured in the 2006 mid-term elections. The party said it had made a net gain of at least 20 seats.
"Tonight, the American people have called for a new direction," said House Speaker Nancy Pelosi, a California Democrat, as the results came in. "They have called for a change for America."
In the Senate, the Democrats have 55 seats compared with 40 for the Republicans. Two other seats are taken by independents who vote with the Democrats. Races in three states remain too close to call.
Senate majority leader Harry Reid, a Nevada Democrat, credited the party's gains to Mr Obama.
"Obama ran a terrific campaign, he inspired millions of people," Mr Reid told the Associated Press.
Despite their success, the Democrats appeared unlikely to win 60 Senate seats - a so-called super-majority that would prevent Republican senators from using procedural blocks such as filibustering to hold up legislation.
Republican candidates endured a miserable election day in most of the closely-fought states.
Virginia voters replaced retiring Republican veteran Senator John Warner with a Democrat, former Virginia Governor Mark Warner.
DEMOCRAT SENATE TARGETS
New Hampshire, New Mexico, North Carolina, Virginia, Colorado, Oregon
Results to come
Alaska, Georgia, Minnesota
New Mexico followed a similar pattern, with Democrat Tom Udall voted in to replace long-term Republican senator Pete Domenici. Mr Udall's cousin, Mark Udall, also unseated a Republican in Colorado.
In North Carolina - one of the states where pollsters predicted a close fight - Republican incumbent Elizabeth Dole was beaten convincingly by Democrat rival Kay Hagan.
And in another key battleground state, New Hampshire - a traditionally strong Democrat state - the Republican's John Sununu lost his seat to the popular former governor, Jeanne Shaheen.
The Democrats added Oregon to their tally on Thursday, after a close-fought race which saw Democratic challenger Jeff Merkley narrowly seize the seat from Republican incumbent Gordon Smith.
There are still some close contests to come, with the Democrats intent on moving into staunch Republican territory.
Votes are still being counted in Alaska - one of the Democrat's most surprising target states, whose Senate delegation has been solidly Republican since 1981.
Incumbent Republican Senator Ted Stevens has been a dominant figure in Alaskan politics since 1968, when he first won his seat.
But he was convicted in October of lying about gifts he had received, and was already facing a tough re-election battle against the Democratic Mayor of Anchorage, Mark Begich.
A recount is expected in Minnesota, and Georgia is preparing for a run-off after neither candidate received the necessary number of votes to be declared overall winner.
There was some positive news for the Republicans, with voters in Kentucky and Mississippi returning their candidates despite determined efforts by Democrat rivals.
Meanwhile, voters in 11 states also elected governors, and in 36 states there were 153 proposals to be decided upon.
Voters in Colorado and South Dakota rejected initiatives that could have led to abortion bans. Washington became the second US state to allow people with terminal illnesses the option of doctor-assisted suicide, while Nebraska outlawed affirmative action.
Most controversially, voters in California approved a constitutional amendment that would limit marriage to heterosexual couples. Thousands of gay couples have wed in the state since a court ruled in May that gay marriages could go ahead.