US voters demoralised by a faltering economy, look certain to hand Republicans a mid-term elections triumph on Tuesday after souring on President Barack Obama's Democrats, just two years into his crusade for change.

Polls show Republicans set to win the house of representatives and slice into the democratic majority in the Senate, dividing power in Washington and forging a polarised prelude to Obama's 2012 re-election bid.

Republicans, electrified by the ultra-conservative Tea Party movement, vow to reverse Obama's sweeping health reforms and promise a budget crunch and tax cuts. They claim it will reduce the deficit and ignite growth.

The election for all 435 House seats and 37 in the Senate comes as the euphoric hope Obama stirred in 2008. Seems an age away and with America's trademark brash optimism drained by a narrative of national decline.

"We've tried it President Obama's way. We've tried it Washington's way. It hasn't worked. It's time to put the people back in charge," said John Boehner, the likely Republican house speaker on Saturday.

Though Republican George W. Bush was in the White House when the economy melted down in 2008, Boehner blames Obama for unemployment stubbornly pegged at 9.6 percent due to a sluggish recovery.

But Obama reminded voters of Bush's culpability for driving the US economy into a "ditch", claimed credit for staving off a second Great Depression and said his policies have put the United States back on the road to prosperity.

He also 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.

"I feel great," Obama told reporters in his adopted hometown of Chicago. "It's going to be tight. These are close races... And obviously, the other side is enthusiastic. We've got to make sure our side is too."

Polls and forecasters predict 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 reverse 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 immigration reform, and to control committees that could launch damaging probes into White House conduct.

The picture in the Senate remains uncertain although Republicans look set to fall short of the 10 seats they need to take full control.

A heavy loss on Tuesday would force Obama into a period of soul searching. He could seek common ground with emboldened Republicans who have already said their top priority is to deprive him of a second term.

Alternatively, with his veto pen and the megaphone of the presidency, he could opt to stand his ground, hoping Republicans overreach.

In close Senate races, Democrats were fighting desperately against the Republican tide in California, Colorado, Ohio, Pennsylvania, Wisconsin and Illinois.

The top casualty could be Democratic senate majority leader Harry
Reid, a Nevada senator who trails Sharron Angle, a favourite of the Tea Party movement opposing what conservatives see as Obama's "big government" liberal agenda.

Most polls show voters favouring Republicans. The final NBC News/Wall Street Journal survey before the elections showed that 49 percent of likely voters prefer a Republican-controlled Congress, versus 43 percent who want Democrats to remain in charge.

But voters seem more inclined to punish the party in power, rather than to have suddenly fallen for Republicans.

In a recent ABC News/Washington Post survey, 67 percent of voters said that they disapproved of Republican lawmakers' job performance, compared
to 61 percent who disapproved of Democrats.

And despite a grim two years fighting economic malaise, Obama remains the most popular, active senior political leader in America, though his approval ratings in the mid-40s have declined from their stratospheric heights.

Tuesday's outcome will likely hinge on turnout, and Republicans has claimed the corner on enthusiasm this year.

Historically, sitting US Presidents have seen their party lose seats in elections halfway through their first term, though George W. Bush defied that trend in 2002, and mid-term polls are a poor indicator of a first-term President's chances for re-election.