Despite some last-minute gaffes, Mitt Romney remained the overwhelming favourite to win the New Hampshire presidential primary today, but the margin of victory could determine whether he quickly clinches the Republican nomination or faces a gruelling battle.
His five opponents will try to shrug off a Romney victory as the expected outcome for a former governor of next-door Massachusetts who owns a holiday home on a New Hampshire lake.
But a narrow win in the nation's first presidential primary - or a surprisingly strong finish from one of his rivals - will be played up as more evidence that Republicans still have their doubts about him.
Those doubts were on display in Dixville Notch, the tiny New Hampshire village which traditionally votes at midnight. Mr Romney received two of the six votes cast in the Republican primary, as did Jon Huntsman. Newt Gingrich and Ron Paul received a vote apiece. President Barack Obama got three votes in the Democratic primary.
The rest of New Hampshire voters go to the polls today after receiving months of attention from the Republican candidates and witnessing an increasingly sharp tone in the intraparty struggle for the nomination.
“If I am president of the United States, I will not forget New Hampshire,” Mr Romney said during a rally in Bedford last night, hinting at the impact of today's contest while surrounded by his wife, children and grandchildren.
Last week he narrowly won Iowa's caucuses, the first contest in the race to select a challenger to Mr Obama in the November election.
With polls showing Mr Romney holding a shrinking but still double-digit lead, the focus is now largely on the wide-open contest for second place, with Mr Paul, Mr Huntsman, Mr Gingrich and Rick Santorum in contention.
A strong showing in New Hampshire could give a candidate momentum and galvanise the anti-Romney vote ahead of the crucial January 21 primary in South Carolina. The conservative state which holds the first primary in the South has a strong track record of picking the eventual Republican nominee.
Mr Romney has been considered vulnerable in South Carolina, where he finished fourth in the 2008 primary. Some conservatives see him as too moderate and some evangelicals, a key constituency, are wary of his Mormon faith. But recent polls show him leading there against a split field. A win in South Carolina, following victories in Iowa and, probably, New Hampshire, could make his nomination seem inevitable.
So far, he has benefited from a fractured opposition that has allowed him to remain the front-runner even though national polls have shown his support among Republicans at only about 25% to 30%.
None of his rivals have proved to be a consistent and credible threat to the former Massachusetts governor.
The latest challenger to emerge from the pack is Mr Santorum, the former Pennsylvania senator who used a passion for social conservatism and a populist economic message to come within eight votes of Mr Romney in Iowa's caucuses.
In New Hampshire, “second place would be a dream come true”, Mr Santorum said yesterday as he raced through a campaign schedule that spanned more than 14 hours.
New Hampshire, which allows independents to vote in its primary, will help decide whether a candidate with Mr Santorum's focus can appeal to a broader electorate as would be required in a successful general election. On the other side, Mr Huntsman is relying upon independents and moderate Republicans to fuel a late surge to relevancy.
A former ambassador to China in the Obama administration, Mr Huntsman has spent the last 48 hours trying to capitalise on a notable debate exchange with Mr Romney.
A relentless critic of Mr Obama, Mr Romney had criticised Mr Huntsman for serving in the Obama administration.
Mr Huntsman, a former Utah governor, countered that he had put his country ahead of partisan politics, and accused Mr Romney of putting politics first and “dividing this country”, a message he hopes will resonate in New Hampshire.
Polls suggested Mr Huntsman may be on the rise, but it may be too little, too late. He skipped the Iowa caucuses to focus on New Hampshire, but could be pushed out of the race if he finishes below third place in the six-man field.
Mr Paul, the 76-year-old Texas congressman, is also counting on a solid performance in New Hampshire and has run a strong second to Mr Romney for much of the year in polls of Republicans in New Hampshire, where his small-government, libertarian message has some appeal.
Mr Paul has a loyal core of supporters, but goes against the Republican mainstream with his calls to pull troops out of Afghanistan, cut defence spending, and end the war against drugs, making him unlikely to win the nomination.
Mr Gingrich, the former Speaker of the House of Representatives, has promised a tougher tone in the race after finishing a disappointing fourth in Iowa following a barrage of negative adverts from Romney allies.
Mr Gingrich and his allies are criticising Mr Romney's work as the head of a private equity firm, Bain Capital. Yesterday, he said Bain “apparently looted” many of the companies it took over.
“Now we'll see if he has the broad shoulders and can stand the heat,” said Mr Gingrich.
In the final day before the primary, Mr Romney gave his opponents more ammunition to target his business background, which he has made the centrepiece of his campaign to unseat Mr Obama by touting his record as a job creator.
Yesterday, Mr Romney came under fire for saying: “I like being able to fire people who provide services to me.”
Obviously weary, he was talking about Americans being able to “fire” their private health insurers and choose another. But Mr Huntsman seized on the opening in an election expected to be dominated by lingering high unemployment and other economic issues.
“It may be that he's slightly out of touch with the economic reality playing out in America, and that's a dangerous place for someone to be,” Mr Huntsman said.
The Republican establishment has largely rallied behind Mr Romney, hoping to avoid a long, divisive primary fight which could undermine prospects of defeating Mr Obama, whose popularity has fallen because of the slow US economic recovery.
A Romney victory in New Hampshire would make him the first Republican in a contested presidential nomination battle to capture the first two races of the campaign since Iowa began leading off for the Republicans in 1976.