Democrats have won control of the US House of Representatives in mid-term polls, and are one seat away from gaining a majority in the Senate.
The Democrats won the Senate seat in Montana after a delayed neck-and-neck vote count was completed.
The race now hangs on Virginia, where a win would give the Democrats the six seats needed for a majority.
US Defense Secretary Donald Rumsfeld has already announced he will resign in the wake of Republicans' poll losses.
In Tuesday's poll, Democrats comfortably gained the 15 seats needed to wrest power from the Republicans in the lower chamber.
Correspondents say Democratic gains reflect voter discontent over Iraq, government corruption and the economy.
In Virginia, the Democrats have claimed victory with a lead of about 8,000 votes, but the Republicans are thought likely to demand a recount.
Republican Conrad Burns (l) and Democrat Jon Tester are awaiting results in Montana
McCaskill takes Missouri
Casey gains Pennsylvania
Brown gains Ohio
Whitehouse gains Rhode Island
RESULTS TO COME:
The results give the Democrats control of the House of Representatives for the first time in 12 years.
The BBC's Jamie Coomarasamy in Washington says this has changed the political landscape in the US, and the last two years of the Bush presidency will be very different from those before.
With projected results still coming through, the Democrats had gained 27 House seats from the Republicans.
Analysts say control of the House will allow Democrats to choose to launch inquiries into the handling of Iraq, and could lead to significant changes on domestic issues like taxation and health care.
Democrats also won five of six target Senate seats.
President George W Bush has telephoned several top Congressional Democrats to offer his congratulations.
In a news conference the US president described as "thumping" the Republicans' set-back in the elections.
But he said that with victory, there had to be responsibility, and that was why he would be working with Democrats on legislation.
Mr Bush said his administration's Iraq policy was "not working well enough, fast enough", and that Mr Rumsfeld agreed that a "fresh perspective" was needed on the issue.
House Democratic leader Nancy Pelosi - poised to become the chamber's first female Speaker - pledged that the Democrats would work with "civility" and "partnership, not partisanship" in their newly empowered position.
But she said the Bush administration's "stay the course" policy in Iraq was not working.
"We cannot continue down this catastrophic path," she said.
In key results:
- In Pennsylvania, Democrat Bob Casey Jr beat one of his party's biggest Republican targets this year, arch-conservative incumbent Rick Santorum
- In Ohio, where the Republican Party has been hit by scandal, Democrat Sherrod Brown won a decisive victory over incumbent Republican Mike DeWine
- In New Jersey, Democrat incumbent Senator Bob Menendez succeeded in holding off a strong challenge from the Republicans' Thomas Kean
- The Senate seat in Connecticut has gone to Joe Lieberman, who stood as an independent after losing the Democratic primary to Ned Lamont amid strong anti-war feeling. He has said he will align himself with the Democrats, as has Vermont's new socialist senator, Bernie Sanders, also an independent
- Democrat Keith Ellison was elected as the nation's first Muslim member of Congress, taking a House seat in Minnesota
- The Senate seat in Rhode Island went to Democrat Sheldon Whitehouse after a closely-fought battle with incumbent Republican Lincoln Chafee
- Democratic Senator Hillary Rodham Clinton has strolled to re-election in New York, as expected. Her win opens the way for a potential presidential run in 2008
- Republicans lost the Florida district of Mark Foley, who resigned after the disclosure that he sent sexually explicit messages to teenage male congressional assistants.
Voters were also choosing governors in 36 states.
Democrat Deval Patrick is set to become the first black governor of Massachusetts, while Arnold Schwarzenegger has won a second term as California governor.
First female House speaker: Nancy Pelosi
First Muslim congressman: Keith Ellison
First socialist senator: Bernie Sanders
First black northern governor: Deval Patrick in Massachusetts
Longest serving Senate member: Democrat Robert Byrd, 88, of West Virginia
And in one of a series of state referendums, voters in South Dakota have overturned a near-total ban on abortions passed by the state legislature earlier this year.
A high turnout was reported in Tuesday's vote.
In Virginia, election officials told ABC News the FBI was investigating claims of voter intimidation.