Democrats looked to sweep Republicans out of power in the US Congress on Tuesday after a bruising campaign fed by public worries about the country's future, discontent with the Iraq war and doubts about President George W Bush's leadership.
Democrats appear positioned to recapture control of the House of Representatives for the first time since 1994, opinion polls showed, with Senate control hinging on several key races that are too close to call.
All 435 House seats, 33 Senate seats and 36 governorships are at stake and Democrats need to pick up 15 House seats and six Senate seats to seize control of both chambers of Congress.
Polls open in some areas of the eastern United States at 6 am EST (1100 GMT) and will start to close at 6 pm EST (2300 GMT), but it could be hours before results are known in many crucial races.
Both parties fired up intensive get-out-the-vote operations to bring core supporters to the polls and sent big-name stars on to the campaign trail in a late effort to win over independents and tip the balance in close races around the country.
About 50 contested House races and 10 Senate races are the chief battlegrounds. Independent analysts predict Democrats could gain 20 to 40 House seats, while polls show races for key Republican-held seats in Missouri, Virginia, Tennessee, Montana and Rhode Island are too close to call.
Democrats probably need to win four of those five Senate races to take control of the chamber.
Two national opinion polls on Monday showed Democrats still held a double-digit advantage when likely voters were asked which party's candidate they would support.
The new polls contradicted two surveys released on Sunday that showed Republicans closing the gap on Democrats.
History was with Democrats -- the party holding the White House traditionally loses seats in a president's sixth year.
The battle for the House will be fought largely in the East and Midwest, where scores of Republican incumbents are fighting for their political lives amid what polls show is a strong desire for change.
At least three Republican incumbents face strong challenges in Indiana and Connecticut, while four Republican-held seats in Pennsylvania and five New York seats could fall to Democrats.
A Democratic majority in even one chamber of Congress would slam the brakes on what is left of Bush's second-term legislative agenda, hasten his lame-duck status and give Democrats a chance to investigate his administration's most controversial policy decisions like the war in Iraq.
In a campaign dominated by Iraq, Bush defended his handling of the war to the end and questioned what Democrats would do differently.
"We have a plan for victory. We've got a strategy to win. And part of that is to elect Republicans to the Congress and to the Senate," Bush told a rally in Bentonville, Arkansas on the eve of the election.
Hampered by low approval ratings, Bush was limited to appearances in Republican strongholds to keep from turning off independents.
He was snubbed in Florida by the Republican candidate for governor, Charlie Crist, who did not appear with him.
But Bush said in Arkansas that Republicans were coming back and would retain control of Congress on Tuesday.
"I knew we were going to finish strong. I knew that we were going to come roaring into Election Day, because we've got the right position on taxes and we've got the right position on what it takes to protect you from attack," he said.
Democrats put leaders like former President Bill Clinton, former Vice President Al Gore and Illinois Sen Barack Obama on the campaign trail to drum up votes in the final hours.
At a rally in the northern Virginia suburbs of Washington, Clinton said the campaign against Democratic Senate challenger James Webb had been "grotesque" and accused Republican leaders of putting tax cuts ahead of spending on security and social programs.
"Nothing can stand in the way of them throwing money at people who don't need it," Clinton said. "They wage class warfare."