Analysts were eager to see how well Mitt Romney, the former Massachusetts governor and millionaire venture capitalist, does compared to polls that have given him a commanding lead all year -- and to see who comes in second.
Romney hopes a romp -- after a squeaker win in Iowa -- will propel him into South Carolina's January 21 primary -- the first in the South, where his more conservative rivals could get their best shot at a victory.
By tradition, the first ballots were cast shortly after midnight in two tiny New Hampshire towns with a combined turnout of 32 voters.
In Dixville Notch -- population 75 -- Romney tied former US China envoy Jon Huntsman with two votes each out of nine cast. In Hart's Location -- population 42 -- Romney was the Republican frontrunner with five votes.
Overall, New Hampshire officials expect some 325,000 voters to head to the polls, most of them Republicans because Democratic President Barack Obama is running unopposed for his party's nomination ahead of the November 6 elections.
The Republican primary is closed to Democrats but open to Republicans and to the state's undeclared voters -- its term for registered independents who are notoriously unpredictable and can be late to make up their minds.
The candidates themselves were expected to make made-for-television public appearances -- telephoning undecided voters, exhorting volunteers to get their supporters to the polls -- before settling in to await the verdict.
A daily tracking poll released Monday by Suffolk University in nearby Boston found Romney leading the pack with 33 percent of likely voters, followed by Representative Ron Paul at 20 percent.
Huntsman was at 13, former House speaker Newt Gingrich at 11, and Christian conservative and former senator Rick Santorum at 10 percent, following his extremely close second-place finish in last week's Iowa vote.
Texas Governor Rick Perry and former Louisiana governor Buddy Roemer trailed the pack in low single digits.
A vast campaign war chest and high-profile endorsements have fed Romney's image as the man to beat, but he faces stubborn doubts about his conservative credentials and has been unable to push his nationwide Republican support above 30 percent.
South Carolina is the first contest for the Republican nomination in the US South, where Romney faces pushback from conservatives who think he is a moderate flip-flopper and from evangelical Christians wary of his Mormon faith.
But Romney faced a blistering attack of a different sort in the days before the New Hampshire primary, as Gingrich, Perry and Huntsman assailed him over jobs lost at companies bought by his prosperous venture capital firm, Bain.
Gingrich, trailing in the polls after a blitz on his own record by Romney allies, led the charge against his rival over his time at Bain, which dismantled some of the firms it bought while reaping vast profits.
"They apparently looted the companies, left people totally unemployed, and walked off with millions of dollars," Gingrich told NBC television.
"Look, I'm for capitalism," he said. "But if somebody comes in, takes all the money out of your company, and then leaves you bankrupt while they go off with millions, that's not traditional capitalism."
Romney struck back, telling the Nashua Chamber of Commerce "I have been in business. I have learned some things fail and some things succeed, that's how it works ... those that succeed make our overall nation stronger."
"Our system works. Free enterprise works," said Romney, who touts his leadership of the private equity firm as his signal qualification to take on Obama in the November 6 elections.
Romney's foes have also seized on a comment he made as he argued that Americans ought to be able to switch health insurance companies -- "I like being able to fire people" -- to argue that Romney would target US workers.
"The insurance company will have an incentive to keep you healthy. It also means that if you don't like what they do, you can fire them. I like being able to fire people that provide services to me," he said in Nashua.
Huntsman charged that Romney "enjoys firing people, I enjoy creating jobs," and added: "It may be that he's slightly out of touch with the economic reality playing out in America right now."
But the Bain-related attacks effectively opened a new front against Romney, who has had to walk a difficult line between boasting of his private sector success and avoiding being tied to Wall Street, which is broadly unpopular.
The attacks could help Obama, who faces an uphill fight to reelection because of the sour US economy and high joblessness.