After a long and bitter campaign, Americans cast their votes on Tuesday in elections that could sweep Democrats from power in Congress and slam the brakes on President Barack Obama's legislative agenda.
Anxiety over the stumbling economy and discontent with Obama and government in Washington have propelled Republicans to the threshold of huge gains that could give them a majority in the House of Representatives and perhaps even the Senate.
Opinion polls and independent analysts project Republican gains of at least 50 House seats, far more than the 39 they need to take control and topple Democratic House Speaker Nancy Pelosi from power.
Republicans are also expected to make big gains in the Senate, although it appears more difficult -- but not impossible -- for them to pick up the 10 seats they need for a majority.
Obama won office two years ago on a wave of hope he could lead the United States out of a deep economic crisis, but persistent high unemployment and a gaping budget deficit have turned many voters against him.
The public mood gave rise to the political phenomenon of the Tea Party, a conservative grass-roots movement wary of Obama that backed less government, lower taxes and reduced spending.
Republican control of even one chamber of Congress would likely spark a long bout of legislative gridlock, weakening Obama's hand in fights over extending the Bush-era tax cuts and passing comprehensive climate change or immigration bills.
Republican candidates have pushed an agenda of spending cuts, deficit reduction and the repeal of at least portions of the healthcare overhaul, but Obama would wield veto power over Republican initiatives.
Polls open before dawn in some areas of the eastern United States and will start to close at 6 p.m. EDT (2200 GMT), but it will be hours before results are known in many crucial races.
All 435 House seats, 37 Senate seats and 37 state governorships are at stake in Tuesday's voting. Many states have been conducting early and mail-in voting for weeks.
Dozens of races are considered too close to call. Candidates in both parties launched a frenetic round of last-minute campaign stops and fundraising appeals on Monday.
HARRY REID IN TROUBLE
In perhaps the country's most high-profile race, Senate Democratic leader Harry Reid is embroiled in a neck-and-neck re-election fight with Republican Sharron Angle. Former President Bill Clinton campaigned in West Virginia for Democratic Senate candidate Joe Manchin.
Republicans need to string together wins in seven of eight tight races in California, Washington, Nevada, Wisconsin, Colorado, Pennsylvania, Illinois and West Virginia to win a Senate majority.
Obama, who hit four states over the weekend trying to pump up Democratic voter turnout, stayed out of public view in the White House on Monday. He conducted radio interviews and made get-out-the-vote phone calls to key battleground states.
In an interview with a radio show, Obama said he should have called his political foes "opponents" instead of "enemies" in a radio interview he gave last week.
Republican John Boehner, in line to become the next House speaker if his party takes control, condemned Obama at a campaign rally in Ohio for his use of the word "enemies."
"There's a word for people who have the audacity to speak up in defense of freedom, the Constitution and the values of limited government ... That word isn't enemies. It's patriots," Boehner said in Cincinnati.
Democrats mounted a huge get-out-the vote operation to ensure supporters made it to the polls. They were encouraged by their lead among early voters in some key states.
"The voters are going to surprise all of these Washington pollsters when they go out," Representative Chris Van Hollen, head of the Democratic House campaign committee, told CNN. "I think there's early evidence of that fact, especially in the early vote."
Democrats have battled a sour political climate all year, with voters in a foul mood over persistent high unemployment, a growing budget deficit and the perceived failures of government in Washington.
The climate put Democrats on the defensive in dozens of once-safe House and Senate seats, with the nonpartisan Cook Political Report estimating there are now more than 90 endangered Democratic-held House seats.
Tea Party-backed Republican candidates Ken Buck in Colorado, Joe Miller in Alaska and Angle in Nevada are threatening to knock off incumbents in tight Senate races, and Rand Paul in Kentucky has a big lead in opinion polls.
Republican Tea Party-favorite Christine O'Donnell in Delaware badly trails Democrat Chris Coons in the race for Vice President Joe Biden's old Senate seat.