MANCHESTER, United States (AFP) - US presidential candidates, including billionaire Donald Trump and under-pressure Democrat Hillary Clinton, criss-crossed snowy New Hampshire in a final frantic bid to win over undecided voters before Tuesday's (Feb 9) crucial primary.

The Granite State takes the spotlight as Republican supporters determine whether to back a more mainstream candidate - Senator Marco Rubio or one of the three governors in the race - instead of frontrunner Trump and his nearest rival, the arch-conservative Senator Ted Cruz.

With everything to play for in the nation's first primary after last week's Iowa caucuses, most of the nine remaining Republicans in the race blanketed the state on Monday, pressing the flesh at coffee shops, middle schools, Elks lodges and town halls in a mad dash for support.

Snow fell heavily in the state by Monday afternoon, creating a last-minute obstacle for voters and candidates alike.

With Mr Trump well ahead in polls, it appears to be a fierce battle for second place.

A victory, strong second place or even a third-place showing can propel a candidate onward toward South Carolina and Nevada, while a poor result will likely mark an end to some candidates' White House dreams.

Democrats made their final pitches too, as Mrs Clinton scrambled to narrow the gap against her insurgent challenger, Senator Bernie Sanders of neighbouring Vermont, who may well gallop to victory in the state.

The RealClearPolitics poll average shows Mr Sanders - who has called for nothing short of a "political revolution" - leading 53.3 per cent to 40.5 percent for Mrs Clinton in New Hampshire.

Mrs Clinton won Iowa by a hair, but New Hampshire is essentially Sanders territory, and the independent US senator is keen to show that his campaign, built on economic fairness for all, can give the former secretary of state a run for her money deep into primary season.

"For all those of you who are still deciding, still shopping, I hope I can close the deal between now and the time the polls close," Mrs Clinton told a rally in Manchester.

It is a more volatile affair on the Republican side, with 30 per cent of voters still undecided ahead of the primary, according to a CNN poll released on Sunday.

Mr Trump was seeking to reclaim his edge and Mr Rubio sought to regain his composure after taking a drubbing in Saturday night's debate when New Jersey Governor Chris Christie eviscerated the first-term senator for regurgitating scripted talking points.

It appeared to be do-or-die for the governors in the establishment lane - Christie, Ohio Governor John Kasich and former Florida governor Jeb Bush.

Mr Kasich's stock has steadily risen in New Hampshire, and he has signalled that a poor showing here would likely mean the end of his campaign.

Mr Bush, who tangled with Mr Trump in Saturday's debate, sharpened his attacks on the real estate tycoon, calling him unfit to be president.

"You aren't just a loser, you're a liar and a whiner," Mr Bush added on Twitter on Monday.

The pair traded insults, with Mr Trump calling Mr Bush desperate and declaring on CBS that "every time he attacks me he melts like butter."

"The last thing we need, frankly, is another Bush," he said.

Mr Trump, who for months packed thousands into his campaign rallies while keeping voters at arm's length, was engaging in more traditional retail politics in New Hampshire, holding four scheduled events on Monday.

But for Mr Jack Jordan, a sea captain who lost his job one month ago, it was too late.

Fed up with the frontrunner's bombast and arrogance, he said he has traded "nasty" Mr Trump for Mr Cruz, who won in Iowa.

"It's just too much," Mr Jordan told AFP in Keene, where he listened to Mr Cruz address his supporters. "I don't want someone representing me who's that mean."

Mr Clinton has launched a full-court press, enlisting husband Bill Clinton as the ultimate campaign surrogate.

The popular former president has hammered Mr Sanders for his supporters' "sexist" attacks on his wife and for peddling an unaffordable single-payer health care plan.

Mirthia Prince, a 17-year-old student who will be eligible to vote in November, saw Mr Sanders speak on Friday, but while waiting for Mrs Clinton's Manchester rally, she said she was leaning towards voting for her.

"I like her goals, she's very experienced, she's very realistic" and can get things done, she said.

"Of course, it doesn't hurt that she's a woman. It would be nice to have her in the White House."

With Mrs Clinton eager to avoid a repeat of 2008, when her campaign collapsed and she lost the nomination battle to Mr Barack Obama, news outlet Politico reported on Monday that a campaign team shakeup was in the works.

Mrs Clinton's team swiftly shot it down, with campaign chairman John Podesta saying there was "zero truth" to the rumour.

"Hillary stands behind her team, period."