Voters in New Hampshire give their views on the race for the White House
The race to take on President Barack Obama in November's election has hit New Hampshire, the first primary in the Republican nomination process.
Voting is under way in the Granite State, where polls show Mitt Romney's big lead shrinking slightly.
Jon Huntsman, who has all but staked his entire candidacy on Tuesday's vote, is competing for second place with Ron Paul, according to pollsters.
Mr Romney won last week's Iowa contest narrowly over ex-Senator Rick Santorum.
Mr Santorum and former House Speaker Newt Gingrich are courting the conservative vote, with one eye on a primary later this month in staunchly Republican South Carolina.
Primaries and caucuses - different methods of choosing a party's candidate for office - will take place in every US state over the next few months before the eventual Republican winner is crowned at the party convention in August.
'Staying power'A primary is a state-level vote held to nominate a party's candidate for office, while caucuses choose a candidate via meetings of party members and activists held across a state.
In New Hampshire, analysts estimate about 250,000 Republican voters will turn out, while some 75,000 Democratic voters are expected to cast a ballot backing President Obama for re-election.
Most polls in New Hampshire opened at 07:00 (12:00 GMT) and close at 20:00 local time.
But the tiny community of Dixville Notch enjoyed its four-yearly moment in the political spotlight by casting ballots at midnight, with all of nine registered voters.
Among the Republicans, Mr Romney and former Utah Governor Huntsman were tied on two votes each, with Barack Obama winning three Democratic votes.
Two opinion polls released on Monday in the small north-eastern state showed Mr Romney's lead clipped slightly, putting him between 13 and 17 percentage points ahead of his nearest rival, Texas Congressman Paul.
Correspondents say many voters remain lukewarm in their support of Mr Romney, but see him as the most viable candidate to challenge President Obama.
"He wouldn't have been my first choice, but sometimes you have to make sacrifices," one voter, David Searles, 56, told Reuters news agency.
Mr Huntsman's New Hampshire campaign appears to have picked up some momentum, and he has closed the gap on Mr Paul in the state.
Speaking on cable network NBC on Tuesday, he said: "Tonight I think we're going to prove the point that we've got the staying power and we've got the ability to bring this thing home."
Romney v expectation?Appearing on cable news network MSNBC, former Pennsylvania Senator Santorum described himself as "the candidate who is taking on the big issues" as he pitched himself to blue-collar workers and pledged to defend the manufacturing sector.
Mr Santorum and former House Speaker Newt Gingrich are vying for conservative votes, with one eye on a primary later this month in staunchly Republican South Carolina.
Mr Romney unwittingly gave ammunition to his rivals on Monday in an off-the-cuff remark while telling business leaders how he wanted consumers to pick and choose private health insurers.
"If you don't like what they do, you can fire them. I like being able to fire people who provide services to me," said Mr Romney, who is under attack over his leadership of a private equity firm, Bain Capital.
Mr Huntsman, who is the former US ambassador to China, pounced.
"Governor Romney enjoys firing people; I enjoy creating jobs," he said.
Texas Governor Rick Perry - who has skipped campaigning in New Hampshire to try to revive his flagging candidacy in South Carolina - attacked Mr Romney over a remark at the weekend that he knew was it was like to fear a "pink slip", or being fired.
Nine Dixville Notch registered voters made their choice in the New Hampshire primary
"I have no doubt that Mitt Romney was worried about pink slips, whether he was going to have enough of them to hand out," Mr Perry said on Monday.
Mr Gingrich's supporters, meanwhile, unveiled a short film, When Mitt Came To Town, portraying Mr Romney as a greedy "corporate raider" who destroyed the lives of American families.
The pro-Gingrich group, Winning Our Future, is poised to spend $3.4m (£2.2m) on TV advertising in South Carolina after the New Hampshire vote.
The candidate himself attempted to frame Mr Romney's expected win as a loss. Mr Gingrich told Fox News he believes "the biggest story" on Tuesday is that Mr Romney will fall short of "any reasonable expectation" in New Hampshire.
Mr Romney has repelled the attacks, claiming he helped to create more than 100,000 jobs during his time at Bain Capital.
He casts himself as a job creator who can turn around the US economy at a time when the unemployment rate is 8.5%.
President Obama is seeking re-election amid voter concern at the pace of economic recovery from the recession that started in 2007 during the White House tenure of George W Bush and ended in 2009.
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