President Barack Obama's Democrats face a day of reckoning as US voters headed to the polls on Tuesday in an election likely to see Republicans seize control of the House of Representatives and gain broad new powers to attack his agenda.
Democrats feared heavy losses as well in the Senate, 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 re-election bid.
After a bitter year-long campaign shaped by voter anger at the sour economy, election day kicked off with eight eastern states opening their voting booths in the pre-dawn chill at 6:00 am (1000 GMT), and would wrap up after midnight (0400 GMT Wednesday) in remote Alaska and Hawaii.
By 7:00 am voting stations in several more locales were open, including the key populous states of Florida, Ohio, Pennsylvania, and Illinois, where polls show the Republican candidate with a slight lead in the race for Obama's old Senate seat.
Vote counting in some especially contested races are expected to take days, though experts forecast that there would be little doubt late today as to the overall winners and losers.
Bad weather was not expected to play a role in keeping voters away from the polls.
Republicans, energised by the ultra-conservative Tea Party movement, hoped their enthusiasm would carry them to victory -- much as a Democratic tide swept that party into power two years earlier.
Democrats hoped to stem the 2010 election onslaught and defy opinion surveys with an aggressive get-out-the-vote drive.
In an 11th-hour plea yesterday, Obama warned that the vote "will have an impact for decades to come" and called upon demoralised Americans not to give up on his two-year-old campaign for change.
He warned that Republicans would bring back the policies he blamed for the 2008 economic meltdown that left nearly one in 10 Americans unemployed.