New Hampshire is known as state that springs surprises.

With a snowstorm bearing down, presidential candidates scurried across New Hampshire, levelling inflammatory attacks against one another while pleading with voters for their support in today's primary election, which appeared likely to settle little in the wild nominating contests.

New York businessman Donald Trump held a sizeable lead in the Republican race and appeared poised to win his first contest of the 2016 campaign after finishing second in Iowa a week ago. But behind Trump, five other candidates waged a fierce battle for coveted top-tier finishes and the bragging rights and resources that could come with them when the campaign heads next to South Carolina.

In the Democratic race, Senator Bernie Sanders maintained his double-digit lead over former Secretary of State Hillary Clinton. After winning only narrowly in Iowa, Clinton was bracing for defeat here and hoping to keep the damage from spilling over into upcoming states where she long has been dominant.

Sanders is capitalising in part on the fact that he represents a neighbouring state, Vermont, though his campaign has been fuelled more by widespread discontent with the political system among many liberals and the enthusiasm of young voters.

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In diners, on factory floors and at big rallies, the candidates encountered voters, one after another, who had yet to make up their minds - a reminder that New Hampshire is a state where voters have sprung surprises in the closing days of past presidential primaries, despite the fact that neither contest this year appears to have a genuine race for first place. A commanding upset by Sanders that further exposes weaknesses in Clinton's coalition, with a photo finish for second and third place in the Republican race, could upend both contests.

For Republicans, the campaign trail yesterday was like a game of political billiards, with attacks flying fast and in all directions, reflecting the jumbled field. Jeb Bush fired at Trump, Ted Cruz, John Kasich and Marco Rubio. Chris Christie savaged Rubio, and Rubio smacked back. And Trump slammed Bush and Cruz.

At a meeting in Salem, Trump accused Cruz, the senator from Texas and winner of the Iowa caucuses, of being "politically correct" and "very queasy" on the issue of interrogation of terrorism suspects. Then he cut into Bush, calling him a "stiff" and accusing him of acting like a "spoiled child".

"You have to go out and vote," Trump said. "You have to do it because, you know, we have to get rid of the Bushes of the world. You know, guys like that will never straighten out this country."

Reaching for the jugular, Trump said on CNN that Bush "has to bring his mother out and lug his mother around at 90 years old. I think it's a very sad situation".

Bush offered a Trump-style retort, tweeting at him, "you aren't just a loser, you are a liar and a whiner".

Democratic presidential candidate, Hillary Clinton listens as Democratic presidential candidate, Sen. Bernie Sanders answers a question during a Democratic presidential primary debate. Photo / AP
Democratic presidential candidate, Hillary Clinton listens as Democratic presidential candidate, Sen. Bernie Sanders answers a question during a Democratic presidential primary debate. Photo / AP

The Democratic side was hardly harmonious as Clinton sharpened her knife to bloody Sanders. He has attacked Clinton repeatedly for her long ties to Wall Street and her acceptance of campaign contributions and personal speaking fees from major financial firms. Clinton portrayed Sanders as hypocritical because he had accepted "about US$200,000" from Wall Street firms through the Democratic Senatorial Campaign Committee. Sanders campaign manager Jeff Weaver called that suggestion "false" and "beyond preposterous".

The Clinton family made an all-out push to secure votes in New Hampshire, a state that has fuelled comebacks for both Bill and Hillary in past campaigns.

But the race seemed to be Sanders' to lose. At his stops yesterday, he restated his broad assault on the corrupting influence of money in politics and the power and wealth of Wall Street and corporate America.

"We are running a very radical campaign because we are telling the American people the truth," Sanders said.

For the Republicans, the character of the race appeared to change after a debate on Sunday in which Rubio faltered in the face of blistering attacks from Christie.

The senator from Florida appeared to have gained some momentum from his strong third-place finish in Iowa, but the question was whether his debate foible - repeating a line four times - set him back and how much of an opening it gave Christie, as well as Ohio Governor Kasich and former Florida Governor Bush.

Republican presidential candidates participate a Republican presidential primary debate. Photo / AP
Republican presidential candidates participate a Republican presidential primary debate. Photo / AP