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.
SENATE RACES
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:
"Sometimes presidents have performed best with an opposing Congress"
In pictures: US mid-terms
High turnout
Voters were also choosing governors in 36 states.
ELECTION LANDMARKS
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.
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