New Hampshire voters are picking a Republican presidential candidate as the US state hosts the first primary of this year's election season.
Eve-of-vote opinion polls showed Mitt Romney's big lead shrinking slightly as rivals savaged his business record.
Jon Huntsman, who has all but staked his entire candidacy on Tuesday's vote, was competing for second place with Ron Paul, according to pollsters.
The eventual nominee will challenge President Barack Obama in November.
Primaries and caucuses will be held in every US state over the next few months to choose a Republican candidate before the eventual winner is crowned at the party convention in August.Romney under fire
New Hampshire is expected to give former Massachusetts Governor Romney his second victory, after he was declared the winner over Rick Santorum by the slimmest of margins in last week's Iowa caucuses.
Most polls open at 07:00 (12:00 GMT) and close at 20:00 local time.
But the tiny community of Dixville Notch enjoyed its quadrennial moment in the political spotlight by voting at midnight, with all of nine registered voters casting ballots.
Among the Republicans, Mr Romney and Mr Huntsman were tied on two votes each, with Barack Obama winning three Democratic votes.
Two opinion polls on Monday from the small New England state showed Mr Romney's lead clipped to between 13 and 17 percentage points ahead of his nearest rival, Texas Congressman Paul.
Former Pennsylvania Senator Santorum and former House Speaker Newt Gingrich are vying for conservative votes, with one eye on a primary later this month in South Carolina, where the other contenders hope to halt Mr Romney's momentum.
Mr Romney unwittingly gave ammunition to his rivals on Monday in an off-the-cuff mark 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, the former governor of Utah and onetime 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 his remark at the weekend that he knew was it was like to fear a "pink slip", or being fired.
"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.
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.