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.
Polls opened before dawn in some areas of the eastern United States and will start to close at 6 p.m. EDT (10 p.m. BST), but it will be hours after that before results are known in many crucial races.
All 435 House seats, 37 of the 100 Senate seats and 37 of the 50 state governorships are at stake in Tuesday’s voting.
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 Obama that backed less government, lower taxes and reduced spending.
Obama’s signature overhaul of the US healthcare system, a decades-long goal for Democrats, has prompted a backlash from many voters.
“They’re trying to ruin all the healthcare,” said Sharon Krumins, a doctor at Walt Disney World, as she left a polling station on Tuesday morning in Winter Park, Florida.
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 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 healthcare overhaul, but 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.
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.
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 other guys say they’re going to take both houses, I’m saying we’re going to retain both houses,” Democratic National Committee chairman Tim Kaine said on MSNBC.
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 political 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.
“It’s tough right now economically, but Obama and the Democrats are putting us on the right track,” said Miami voter Jose Martinez of Miami. “I don’t see the Republicans have any ideas to move us forward.”
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-favourite Christine O’Donnell in Delaware badly trails Democrat Chris Coons in the race for Vice President Joe Biden’s old Senate seat. Reuters