Fireworks flying in their first one-on-one debate, Hillary Clinton accused Bernie Sanders Thursday night of subjecting her to an "artful smear" while Sanders suggested the former secretary of state was a captive of America's political establishment.

The two Democrats embraced a markedly more contentious tone than when they last debated before the year's presidential voting began in Iowa, and it signalled how the race for the nomination has tightened five days ahead of the first-in-the-nation primary in New Hampshire next Tuesday.

The two argued over ideas, over tactics and over who has the liberal credentials to deliver on an agenda of better access to health care, more affordable college, fighting income inequality and more.

It was Clinton who was the main aggressor, saying Sanders could never achieve his ambitious and costly proposals. Then she took after the Vermont senator for his efforts to cast her as beholden to Wall Street interests because of the campaign donations and speaking fees she's accepted from the financial sector.

"It's time to end the very artful smear that you and your campaign have been carrying out," she said.

Sanders, for his part, suggested Clinton's loyalties were coloured by a reliance on big corporate donors.

"Secretary Clinton does represent the establishment," he said. "I represent — I hope — ordinary Americans."

Clinton may say the right things, he suggested, but "one of the things we should do is not only talk the talk but walk the walk."

"I am very proud to be the only candidate up here who does not have a super PAC, who's not raising huge sums from Wall Street and special interests," he said, referring to outside groups who can receive unlimited funds to support candidates.

Where Clinton aimed considerable criticism at Sanders, the Vermont senator focused much of his fire on what he says is a political system rigged against ordinary Americans.

He said that when a "kid gets caught with marijuana, that kid has a police record." But when "a Wall Street executive destroys the economy" and pays a $5 billion US settlement, he has no criminal record.

"That is what power is about, that is what corruption is about," he said.

Clinton, unwilling to cede the issue to Sanders, insisted her regulatory policies would be tougher on Wall Street than his.

"I've got their number," she said, "the Wall Street guys."

Asked if she would release transcripts of her paid speeches to Wall Street interests and others, Clinton was noncommittal, saying "I'll look into it." She had struggled a day earlier to explain why she accepted $675,000 for three speeches from Goldman Sachs.

In fresh evidence of the tightening race, Clinton reported that her campaign had raised $15 million in January — $5 million less than Sanders and the first time she's been outraised by her opponent. Her finance director called the numbers "a very loud wake-up call."

The Durham debate was the first faceoff for Clinton and Sanders since former Maryland Gov. Martin O'Malley dropped out of the race after a poor showing in Iowa.

Republicans debate Saturday

The election campaign has so far been dominated by the crowded and cacophonous Republican contest. GOP candidates, who debate again Saturday, were all over New Hampshire ahead of the primary.

Donald Trump, who finished second in the Iowa caucuses, took fresh aim Wednesday at caucus winner Ted Cruz, the firebrand Texas senator who appears to be running behind in the Granite State and is hoping for momentum to carry him into South Carolina later this month.

For other Republicans such as former Florida governor Jeb Bush and New Jersey Gov. Chris Christie, New Hampshire may be their last stand as they try to chisel establishment support away from Florida's junior Senator Marco Rubio, who finished a strong third in Iowa.