Republican rivals focus on New Hampshire primary
The remaining candidates vying for the US Republican presidential nomination have begun a week of campaigning ahead of the New Hampshire primary.
Iowa loser Michele Bachmann dropped out of the race on Wednesday, but Texas Governor Rick Perry vowed to fight on.
Iowa victor Mitt Romney unveiled 2008 nominee John McCain as his latest backer, while surprise runner-up Rick Santorum said donors were backing him.
Mrs Bachmann's exit leaves six men battling to take on Barack Obama.
She bowed out of the contest with an emotional speech, pledging to continue her battle against President Obama's healthcare legislation, which she dubbed "socialised medicine".
Mitt Romney is a strong favourite to win next week's New Hampshire primary.
Caucuses and primary elections will be held in all 50 states over the next six months before the Republican presidential nominee is confirmed at the party convention in Florida this August.
In New Hampshire, Mr Romney heard words of encouragement from one of the men who beat him during the 2008 primary season, veteran Senator John McCain.
Mr McCain told a crowd "no-one will ever say that Mitt Romney will lead from behind", adding that the former Massachusetts governor's private sector experience would help turn around the economy.
"Our message to President Barack Obama is: 'You can run but you can't hide from your record.'"
Mr Romney emerged victorious in Tuesday's Iowa caucuses by the narrowest of margins, overcoming Rick Santorum by just eight votes.
He squeaked to victory in the early hours of Wednesday with 30,015 ballots (24.55%) against 30,007 for social conservative Mr Santorum (24.54%).
Texas congressman Ron Paul, a libertarian-leaning anti-war candidate, came a strong third.
Mr Santorum, whose small-scale campaign saw him spend months touring many of Iowa's small towns and villages, faces an uphill struggle to mount an effective campaign in New Hampshire and South Carolina, which will hold a primary after the Granite State.
In a campaign speech in New Hampshire on Wednesday night, Mr Santorum said fundraising had risen sharply since Tuesday's caucuses.
Mr Romney is already spending hundreds of thousands of dollars on TV ads in New Hampshire and South Carolina.
He will be joined on the airwaves by former House Speaker Newt Gingrich, who has pledged to attack Mr Romney after a savaging in Iowa.
The Gingrich campaign took out a full-page advert in the New Hampshire Union-Leader newspaper on Wednesday portraying their man as a "bold Reagan Conservative" and Mr Romney as a "timid Massachusetts moderate".
Former Utah Governor Jon Huntsman, who skipped Iowa and has bet his campaign on New Hampshire, released a TV ad on Wednesday evening portraying him as the only candidate capable of beating Barack Obama.
Mrs Bachmann, who came sixth with just 5% of the vote, told a Wednesday morning news conference in Des Moines, Iowa: "I have decided to stand aside."
The 55-year-old - who had courted the evangelical Christian vote and was briefly the front-runner back in August - said she had "no regrets".
Mrs Bachmann did not endorse another candidate, but said: "I believe that we must rally around the person that our country and our party and our people select to be that standard bearer."
She spoke at length about her reasons for entering the race, denouncing President Obama's healthcare reforms as "left-wing social engineering", which "endangered the very future" of the US.
A record 122,000 straw ballots were cast by Republicans in the largely rural state.
There had been speculation that Texas Governor Rick Perry might also drop out after he said he would return to his home state to reassess his campaign.
But instead of going back to Texas, he tweeted on Wednesday: "Here we come South Carolina!!!"
The Perry campaign also confirmed he would attend a Republican debate on Saturday in New Hampshire.
And his well-financed political machine has snapped up television advertising space in the key swing state of Florida, which holds its primary at the end of this month.
Meanwhile, former House Speaker Newt Gingrich, who came fourth in Iowa, signalled that he would campaign more aggressively against Mr Romney, whom he has linked to a series of bruising TV attack ads.
Appearing on US breakfast shows, Mr Romney acknowledged he was now a "big target" for attacks.
A Suffolk University opinion poll on Wednesday showed him with 43% of support in New Hampshire, while Mr Santorum was at just 6%.