US voters head to the polls on Tuesday in key elections widely expected to hand President Barack Obama's Republican foes control of the House of Representatives and with it, broad new powers to attack his agenda.
Democrats feared heavy Senate losses, but analysts forecast they would cling to a narrow majority, dividing power in Washington and setting the stage for a superheated political war ahead of Obama's 2012 reelection bid.
After a bitter year-long campaign shaped by voter anger at the sour economy, election day was to open along the US East Coast in the pre-dawn gloom at 6 am and wrap up after midnight in remote Alaska and Hawaii.
Both parties dispatched platoons of lawyers to battleground states in case too-close-to-call races turned into court battles, though experts forecast that there would be little doubt late Tuesday as to the overall winners and losers.
Republicans, energized by the ultra-conservative Tea Party movement, hoped their enthusiasm would carry them to victory, while Democrats aimed to stem the onslaught and defy opinion surveys with an aggressive get-out-the-vote drive.
In an 11th-hour plea Monday, Obama warned that the vote "will have an impact for decades to come" and called upon demoralized Americans not to give up on his two-year-old campaign for change.
He warned that Republicans would bring back the very policies he blamed for the 2008 economic meltdown that has left nearly one in 10 Americans still unemployed while many more have given up on finding work.
"The bottom line is this: We're making progress, we're moving in the right direction," the president said in a radio interview. "If the other side is more enthusiastic, we could end up having problems moving this country forward."
Obama was notably targeting voters in the critical battleground states of Florida, Minnesota, Ohio and Pennsylvania as well as his birth state of Hawaii, the White House said.
But his own soaring win of 2008 seemed an age away with all 435 House seats, 37 of 100 Senate slots, and 37 of 50 governorships up for grabs Tuesday.
Republicans vowed to reverse Obama's sweeping, signature health care overhaul and promised a budget crunch and tax cuts they said would slash the deficit, ignite growth and create jobs.
"We just can't afford another two years like the past two," Republican House Minority Leader John Boehner, who would all but certainly replace Democrat Nancy Pelosi as House speaker, said in an op-ed in Monday's USA Today.
Boehner has led top Republicans in vowing "no compromise" with the White House on key issues, and the party's leader in the Senate candidly declared last week that their number one goal would be to defeat Obama in 2012.
Obama has charged Republicans would hand power back to predatory health insurance companies and credit card and finance barons by repealing health care and Wall Street reforms that add up to an already full presidential legacy.
But in a sign of Democratic angst, party chairman Tim Kaine told ABC television Monday that Obama would start making "adjustments and corrections" over the next few weeks in the face of a reenergized opposition.
Polls and forecasters predicted Republicans will pick up anywhere between 45 and 70 seats in the House, more than the 39 they need to gain a majority, in a reversal of the huge losses they suffered in 2006 and 2008.
Control of the House would allow Republicans to thwart Obama's ambitious plans to tackle global warming and overhaul US immigration, and to control committees that could launch damaging probes into the administration.
Democrats worked desperately to deny Republicans the 10 new Senate seats needed to retake the chamber, battling in California, Colorado, Ohio, Pennsylvania, Wisconsin and over Obama's old Senate seat in Illinois.
"I think we don't get the majority back, but we've come awfully close, and we finish the job in 2012," Republican Senator John Cornyn, his party's Senate campaign field marshal, told NBC television.
The list of endangered Senate Democrats included Majority Leader Harry Reid, who was locked in a tight race with Republican and Tea Party favorite Sharron Angle in Nevada, home to the country's highest unemployment and home foreclosure rates.
But voters seem more inclined to punish the party in power, rather than to have suddenly fallen for Republicans, who remain broadly unpopular.
Despite a grim two years fighting economic malaise, Obama remained the most popular, active senior US political leader, though his approval ratings now in the mid-40s have declined from their stratospheric heights.
Historically, sitting US presidents have seen their party lose seats in mid-term elections.