Democratic leaders predicted they would recapture the House of Representatives for the first time since 1994, but needed to pull off a less likely clean sweep of battleground seats to seize the Senate.
Both sides fired up huge voter turnout drives but there were some early signs of problems with electronic voting machines, notably in Ohio, in a nail-biting election could define Bush's final two years in the White House.
"I think we are going to win the House tonight," Democratic National Committee Chairman Howard Dean told CNN, but declined to predict where the chips would fall in the Senate.
Bush voted in his tiny adoptive hometown of Crawford, Texas, and urged Americans to do their duty by going out to cast a ballot.
"Do your duty, cast your ballot and let your voice be heard," said Bush a day after wrapping up a 10-state blitz for his Republicans. He was to return to Washington with wife Laura to watch the results roll in Tuesday evening.
But problems with electronic voting machines which haunted US elections in 2000 and 2004 again emerged, as soon as polls opened.
In one elementary school in the predominantly black district of East Cleveland, Ohio, all 12 machines went down when voting opened at 6:30 am (1130 GMT), according to an AFP correspondent at the scene.
The machines were not started up until two hours later and poll officials refused to hand out paper ballots until a lawyer for watchdog group Election Protection showed up.
"The machines weren't working and they were just turning people away," said the attorney, Fred Livingstone. "They are not allowed to do that."
Other incidents were reported in Ohio, and other states including New York, Pennsylvania and Florida. Both parties have sent battalions of lawyers to polling booths to challenge irregularities on the spot.
A bitterly partisan campaign marred by nasty rows over racism, sex, war and terrorism saw Democrats claim Bush was in denial over mounting carnage in Iraq, where more than 2,800 US troops have died.
"This is our last chance to rebuke George Bush. Despite six years of disastrous leadership, George Bush has not been held accountable for a single thing he has done," Democrat Senator Chuck Schumer told supporters.
Bush accused his rivals of having no plan to win the war, and told Americans to vote Republican if they wanted America's full protection from terrorists.
"Harsh criticism is not a plan for victory. Second-guessing is not a strategy," Bush told thousands of supporters in Texas Monday.
Voters turning up to polling booths on a chilly, misty morning in Friendship Heights, Maryland seemed exhausted by the mudslinging in the campaign.
"I hope the Democrats win because I have absolutely no use for the people running the government now," Donald Rollert.
"They are incompetent," but he added that he didn't think the Democrats were much better.
Most national polls and experts gave the Democrats a good shot at picking up a net gain of 15 seats out 435 on offer that they need to win the House.
A net gain of at least two seats, Ohio and Pennsylvania, looks likely in the Senate. So control of the upper chamber will depend on whether Democrats can sweep another four toss-up races in Virginia, Rhode Island, Missouri and Montana.
With voters turnout likely to be key in marginal races, Dean called on voters to not just cast their own ballot, but to pressure friends and relatives to get to the polls as well.
"The power is in your hands -- by getting as many people to vote as you can, you'll bring a change to our country and end Republican rule," Dean said in an overnight email to supporters.
Polling booths opened at 6:00 am (11OO GMT) in eastern states including New York, Connecticut and New Jersey and in Virginia. Last votes will be cast in Alaska where polls close at 8:00 p.m. (0500 GMT Wednesday).
As well as Iraq, the outcome of the race will also hinge on how much issues like skyrocketing health care costs, "values" issues like stem-cell research, gay marriage and abortion, the economy and illegal immigration, weigh on voters' minds.
Some 200 million Americans are eligible to vote in the elections for all 435 seats in the House of Representatives, 33 in the Senate and for 36 state governors.
Republicans were cheered in the final hours of the campaign Monday by surveys suggesting the gap between parties was closing.
A Washington Post and ABC television poll showed likely voters favouring Democrats to Republicans by 51 percent to 45 percent. In an October 22 survey, the Democrats had led 55-41.
The non-partisan Pew Research Center found a 47-43 lead for Democrats among likely voters, down from 50-39 two weeks ago.

Copyright AFP (Agence France-Presse), 2006