DURHAM: Democratic presidential contender Hillary Clinton went on the attack early against rival Bernie Sanders in their first one-on-one debate on Thursday, questioning whether his proposals were viable and saying it was unfair to question her liberal credentials.

Five days before New Hampshire holds the second of the state-by-state presidential nominating contests, the intensity of the tightening race was reflected in the sharp exchanges between Clinton and Sanders on healthcare, college affordability and efforts to rein in Wall Street.

Clinton said Sanders' proposal for single-payer universal healthcare coverage would jeopardize Obamacare, calling it "a great mistake," and she said his plans for free college education would be too costly to be realistic.

"I can get things done. I'm not making promises I can't keep," Clinton said.

Sanders said he would not dismantle Obamacare but would expand it, pointing to how many other countries provide universal healthcare.

"By moving forward, rallying the American people, I do believe we should have healthcare for all," he said.

Sanders said his proposal for free tuition at public universities would be paid with a tax on Wall Street speculation. "The middle class bailed out Wall Street in their time of need. Now, it is Wall Street's time to help the middle class," he said.

Polls show Sanders, a US senator from neighboring Vermont, has a double-digit lead over Clinton in New Hampshire after surprising the front-runner by finishing just barely behind her in the kickoff contest in Iowa on Monday.

Since then, the Democrats have clashed sharply over who is more capable of carrying out the party's proposals to battle income inequality, bolster healthcare coverage and regulate Wall Street.

The exchanges had intensified ahead of the debate, the first since former Maryland Governor Martin O'Malley dropped out on Monday night. It is one of four debates Clinton and Sanders agreed to add to the schedule on the long road to the Nov. 8 presidential election.

Sanders criticized Clinton as being too cozy with Wall Street, noting her Super PAC had taken contributions from Wall Street firms and that Clinton has taken speaking fees from Goldman Sachs.

Clinton called that an "artful smear" and said she had never changed a view or a vote because of her donations.

Clinton has tried to play down expectations for her performance in New Hampshire, where she came from behind for an upset victory in the 2008 campaign just days after losing badly to Barack Obama in Iowa.

The surprisingly strong performance by Sanders in Iowa is likely to prolong a race that Clinton entered as the prohibitive favorite.

In addition to previously scheduled debates in Wisconsin and Florida, the candidates added one in March in Flint, Michigan, to draw attention to the city's contaminated water crisis ahead of the Michigan primary. They also will debate in April and May.