New Hampshire primary: Republicans scramble for votes
Polls are closing in New Hampshire where voters have been choosing the Republican candidate they want to see take on President Obama in November.
Front-runner Mitt Romney faced last-minute attacks from rivals attempting to brand him a heartless job cutter.
But the latest opinion polls suggest he will easily win the first primary of the Republican nomination process.
Jon Huntsman, who has staked everything on New Hampshire, is battling with Ron Paul for runner-up, polls suggest.
Primaries and caucuses will take place in every US state over the next few months to pick a Republican nominee before the eventual winner is crowned at the party convention in August.
A poor showing could force former Utah Governor Huntsman, who was the Obama administration's first ambassador to China, to drop out of the race.
Rick Santorum - who ran former Massachusetts Governor Romney a close second in last week's Iowa caucuses - and Newt Gingrich are battling it out for right-wing conservative votes.
Former Pennsylvania Senator Santorum and former House Speaker Gingrich have one eye on a primary later this month in staunchly Republican South Carolina. Their fellow conservative Texas Governor Rick Perry is already in that state campaigning.
Most polls across New Hampshire, which has a reputation for bucking the national trend, have been open since 07:00 (12:00 GMT) are expected to close at 19:00 local time, with the remainder closing at 20:00 local time.
Election officials are predicting a near record turn-out despite a slow but steady start to the day in some parts of the state, according to New Hampshire's Union-Leader newspaper. Analysts estimate about 250,000 Republican voters will turn out.
A Reuters/Ipsos opinion poll on Tuesday suggested Mr Romney was far ahead of his Republican rivals nationally, with 30% support.
It also indicated Mr Romney was trailing Democratic President Obama by five percentage points were he to challenge him for the White House now.
If Mr Romney triumphs in Tuesday's primary it would be the first time a non-incumbent candidate had won the first two nominating contests in a Republican race for the White House.
But his rivals have sought to exploit an off-the-cuff remark he made in New Hampshire on the eve of the primary.
"I like being able to fire people who provide services to me," said Mr Romney, who was discussing how he wanted consumers to pick and choose private health insurers.
Mr Romney has also come under attack over his leadership of a private equity firm, Bain Capital.
He has claimed he helped to create more than 100,000 jobs during his time at Bain Capital.
The former governor 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.