A chastened President Barack Obama offered an olive branch to jubilant Republicans Wednesday after their crushing election victory halted his reform quest and cast doubt on his 2012 reelection hopes.

Obama was set to make a first show of conciliation after a bitter congressional election campaign in a formal White House press conference hours after Republicans shattered his Democratic Party's stranglehold on power.

Republicans, bouncing back from their own election drubbing by Obama in 2008, had picked up 60 seats in the 435-seat House of Representatives by early Wednesday, more than the 39 they needed for a majority.

They also grabbed an extra six seats in the 100-member Senate, with three outstanding races yet to be decided, setting the stage for a likely gridlock in Washington, despite voter demands for both parties to work together.

Obama spoke to Republican House leader John Boehner around midnight, and offered to seek "common ground" with foes whom he repeatedly accused in the mid-term election campaign of driving the economy into a "ditch."

Republicans also spoke the language of compromise, but left no doubt that they believed they were emboldened by the election to turn back the social reform drive that Obama launched after his euphoric election in 2008.

Boehner, in line to replace the first woman speaker, Democrat Nancy Pelosi in January, said that Obama must "change course."

"I think it's pretty clear that the Obama-Pelosi agenda is being rejected by the American people," Boehner told reporters, and called Obama's signature health reform law a "monstrosity."

Eric Cantor, Boehner's number two, reflected the view that Tuesday's election was less a vote of confidence for Republicans than an angry repudiation of incumbent US politicians.

"The American people have had it with Washington. Last night's vote was a vote to say, you know what, Washington better start listening to the people again," he said.

Obama's top Senate ally Harry Reid meanwhile savored his victory over conservative Tea Party favorite Sharron Angle, and admitted on MSNBC that Obama was in a "hole," but argued history suggested he could bounce back.

"We have to work together. We have so many problems in this country that we can't have people saying no to everything," Reid told MSNBC.

Obama's press conference in the ornate East Room of the White House, at 1:00 pm (1700 GMT), represented a first chance to set the tone of the second half of his term, and was effectively the first shot in his 2012 reelection campaign.

He faces a choice of trying to work with Republicans who have already said they want to halt his agenda and ensure he loses in two years time, or to draw a line in the stand and reject Republican policies with his veto pen.

The Republican rout is all the more stunning given the moribund state of the party after the Democrats' sweeping victory of 2008 and is evidence of a period of sharp volatility in US politics.

"The president single-handedly resurrected the conservative movement by his policies," Karl Rove, former top advisor to Obama's predecessor George W. Bush, told Fox. "It's remarkable."

But Democrats will hand over the House after amassing a historic legacy, including healthcare reform and a Wall Street overhaul over the last two years, and claim they staved off a second Great Depression.

But they paid a heavy price for a sluggish economic recovery that has yet to be felt countrywide and unemployment pegged at a stubborn 9.6 percent.

In a true embarrassment for Obama, Republicans won his former US Senate seat, as Mark Kirk beat presidential friend Alexi Giannoulias in Illinois.

Democrats did manage to reclaim claim the governor's mansion in the most populous state, California, as Jerry Brown defeated former eBay CEO Meg Whitman to replace Arnold Schwarzenegger.

Democrats kept the Senate after Joe Manchin won the firewall seat of West Virginia, and Barbara Boxer fought off a tough challenge from former Hewlett-Packard boss Carly Fiorina in California.

But Republicans were jubilant.

"We've come to take our government back!" cried Rand Paul, a hero of the Tea Party movement, after winning a Senate seat in Kentucky.

"There's a Tea Party tidal wave," he said, in a coming-of-age moment for the movement set up to challenge what critics see as Obama's "big-government" takeover of American life.

Charismatic Florida Tea Party favorite and Cuban-American Marco Rubio also surged to victory in the Senate under the conservative group's banner.

Under Boehner, Republicans are promising to reverse Obama's health reforms and promise a budget crunch and tax cuts which they claim will cut the deficit and ignite growth.

Obama, who leaves Friday on a nine-day tour of Asia, must now launch a re-examination of his presidency, as he seeks to renew his bond with voters in the run-up to 2012.