DURHAM: Hillary Clinton mounted a sharp attack on White House rival Bernie Sanders Thursday, accusing her Democratic challenger of waging a smear campaign and warning his promises of revolution "don't add up".

The nearly two-hour debate at the University of New Hampshire in Durham comes five days before the state votes in the first primary of the 2016 election race, after Clinton clinched the narrowest victory in Iowa caucus history against Sanders this week.

While most expect Clinton to ultimately secure her party's nomination, she faces a much steeper uphill climb than once imagined as the 74-year-old senator from Vermont whips up passionate support among young voters ─ earning him a double-digit poll lead in neighbouring New Hampshire.

In their first one-on-one duel, the former secretary of state made a play to woo Sanders's fan base by presenting herself as a battle-hardened progressive who can improve the lives of struggling Americans ─ and whose experience makes her far better suited to the job.

"Senator Sanders and I share some very big progressive goals," the former first lady said. "But the numbers just don't add up from what Senator Sanders has been proposing."

Sanders, who identifies as a democratic socialist ─ unique for a United States (US) presidential candidate ─ hammered home his opposition to what he believes is the corrosive power of Wall Street on American democracy.

He insisted Clinton cannot claim to be both a moderate and a progressive, criticising her for raising $15 million from Wall Street ─ and prompting some of the night's sharpest exchanges.

"Enough is enough," Clinton said. "I don't think these attacks are worthy of you."

'Artful smear'

"I think it's time to end the very artful smear that you and your campaign have been carrying out in recent weeks," the 68-year-old added to a smattering of boos among the in-house audience of 600. "You will not find that I ever changed a view or a vote because of any donation that I ever received."

Sanders hit back by insisting that billionaires undermine democracy in the US by spending unlimited funds on election campaigns.

"We need a political revolution where millions of people stand up and say loudly and clearly that our government belongs to all of us and not just a handful of wealthy campaign contributors," he said.

Sanders presented himself throughout as a fresh voice championing a political revolution to raise the voice of working Americans and young voters, offering universal health care and free public colleges.

The more moderate Clinton advocates incremental change: slow but steady reforms on Wall Street, tweaks to Obama's Affordable Care Act and expanding university scholarships.

She listed her record on racial injustice, children's rights and the fight to widen American access to health care.

Clinton appealed on listeners to vote next Tuesday with "their heart" but also "their head" ─ and left the door open to considering Sanders as her potential running mate for vice president.

"If I'm so fortunate to be the nominee, the first person I will call and talk to about where we go from here will be Senator Sanders," she said.

'100 times better'

The Vermont senator ─ who has steadfastly refused to attack Clinton over her use of a personal email account as US top diplomat ─ was also unfailingly polite.

"I happen to respect the secretary very much. I hope it's mutual and on our worst days I think it is fair to say we are 100 times better than any Republican candidate," he said.

Sanders took 49.6 per cent to Clinton's 49.8pc in the Iowa caucuses, in the first vote of the US election cycle having trailed as a distant second only months earlier.

An NBC, Wall Street Journal, Marist poll released hours before the debate gave Sanders 58pc support among likely Democratic primary voters in New Hampshire and Clinton 38pc.

Sanders leads Clinton among voters aged 18-29 by 76 to 24pc, the poll found.

Regardless of the outcome of Tuesday's primary, Clinton moves into friendlier territory later this month in Nevada and South Carolina, where she commands greater support among black and Latino voters.

Republicans are also swarming to New Hampshire for the crucial February 9 vote, with Donald Trump eager to reclaim the lead after he was beaten in Iowa by arch-conservative Senator Ted Cruz.

On the Republican side, Trump is firmly ahead in New Hampshire, with the telegenic Rubio moving into second place ahead of Iowa winner Ted Cruz, according to the latest University of Massachusetts/Lowell poll.