Republican hero John McCain has backed Mitt Romney to be the party's 2012 standard bearer, after a bruising Iowa battle reshaped the unruly field, forcing one candidate to quit.
Veteran Senator McCain, who was the 2008 Republican presidential nominee against Democrat Barack Obama, called on supporters to crown Romney this time around and then catapult him into the White House in the November elections.
"Our message to President Barack Obama is: 'You can run but you can't hide from your record'," said the decorated Vietnam War veteran, who accused Obama of "destroying our national security".
Romney, a former Massachusetts governor who ran against McCain in 2008, squeaked to victory in Tuesday's first test in the 2012 White House race, defeating devout Christian conservative Rick Santorum by a mere eight votes out of the 120,000 cast.
The result cemented Romney's status as the frontrunner for the Republican nomination among the remaining six candidates, but failed to dispel lingering doubts about the depth of his popularity especially among the Republicans traditional conservative base.
McCain, who carries huge political weight, remains popular in New Hampshire, where he beat Romney 37-31 per cent four years ago.
His backing for Romney reflected an effort by the Republican establishment to avoid an intra-party bloodletting that could leave the eventual nominee in a weakened position to take on Obama.
The first casualty of Tuesday's vote was Iowa-born Representative Michele Bachmann, who announced she was quitting the race after her disappointing sixth place finish in the heartland state.
"Last night the people of Iowa spoke with a very clear voice and so I have decided to stand aside," the former darling of the conservative right said.
But Texas Governor Rick Perry, having hinted he might drop out after coming in fifth in Iowa, teased supporters, saying in a tweet on Wednesday: "And the next leg of the marathon is the Palmetto State ... Here we come South Carolina!!!"
After an agonising delay that stretched into Wednesday morning, the final Iowa count gave Romney a razor-thin victory of 30,015 votes to Santorum's 30,007 - or 25 per cent each.
Both candidates were swift to declare that they were moving on to the next battle-ground in northeastern New Hampshire, which will hold the nation's first primary of the 2012 presidential race on January 10, followed on January 21 by South Carolina.
Romney however dismissed concerns that the Iowa split - in which firebrand libertarian Representative Ron Paul took some 21 per cent to finish third - marked an inconclusive showing.
"This was a seven-person field. You can't do in a field of seven what you can in a smaller field. And I also ran a national campaign," Romney insisted on America's ABC.
He also said he had "broad shoulders" as former House speaker Newt Gingrich, who finished fourth in Iowa, signalled he was gunning for Romney, slamming the multimillionaire businessman and venture capitalist as a "liar".
"I know the attacks are going to come. They're going to come more fast and furious now," Romney said.
New Hampshire polls taken before the Iowa results showed Romney polling strongly in the state, with around 43 per cent support.
Pundits have raised doubts whether Santorum - a devout Catholic whose pro-life stand and fierce opposition to gay marriage resonated with Iowa's evangelical voters but might find less traction in New Hampshire - has the necessary resources and on-the-ground organisation to take on Romney's well-oiled, well-funded political machine.
Santorum, who came from behind to give Romney a run for his money in Iowa, insisted however there was still everything to play for.
"Game on!" he told cheering supporters in Iowa late on Tuesday.
Former Utah governor Jon Huntsman - who did not campaign in Iowa so he could focus on New Hampshire - said Tuesday's results, in which he finished last, showed Republican voters still haven't quite made up their minds.
"You've got three people sharing a tie. And a whole lot of people are looking for an alternative," the former ambassador to China told MSNBC television.
