Democrats swept Republicans from power in the US House of Representatives and moved to the brink of capturing the Senate, where their final victory could be delayed by a possible recount in Virginia.
Democrats rolled up gains of about 30 seats in the House in Tuesday's elections, riding to a huge victory on a wave of public discontent with the Iraq war, corruption and Republican President George W Bush's leadership.
In a harsh setback to Bush and Republicans, Democrats picked up four of the six Senate seats they needed for a majority and led in races for the other two, in Montana and Virginia, threatening to take control of both chambers of Congress for the first time in 12 years.
A potential recount and possible legal challenges in Virginia could delay the final result, dredging up memories of the 2000 presidential election recount that lasted five weeks.
Virginia Democrat James Webb had an 8,000-vote advantage over Republican Sen. George Allen out of more than 2 million cast. A potential recount could stretch into December, leaving Senate control uncertain.
In Montana, Democrat Jon Tester also held a narrow lead on Republican Sen. Conrad Burns, but a final result was not expected until later on Wednesday.
The narrow governing majorities in Congress, especially the Senate, were almost certain to spawn more partisan gridlock and political warfare during Bush's final two years in the White House.
Bush scheduled a news conference for 1 pm EST (1800 GMT) to discuss the results.