Americans are casting their votes today in elections that could sweep Democrats from power in Congress and put the brakes on President Barack Obama's legislative agenda.
In a final push, Mr Obama gave election day interviews to radio stations in several battleground states.
"Things have gotten better over the last two years," he told KPWR in Los Angeles. "We can only keep it up if I've got some friends and allies in Congress and statehouses."
Anxiety over the stumbling economy and discontent with Mr 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.
All 435 House seats, 37 of the 100 Senate seats and 37 of the 50 state governorships are at stake in today's voting. Polls opened before dawn in some areas of the eastern United States.
Mr 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 loosely organised conservative movement wary of Mr Obama that backed less government, lower taxes and reduced spending.
Mr Obama's signature overhaul of the US healthcare system, a decades-long goal for Democrats, has prompted a backlash from many voters.
Republican control of even one chamber of Congress would likely spark a long bout of legislative gridlock, weakening Mr Obama's hand in fights over extending tax cuts put in place under his Republican predecessor president George W. Bush 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 health care overhaul, but Mr Obama would wield veto power over Republican initiatives.
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.
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.
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.
Reuters