Democrat Hillary Clinton went on the attack against rival Bernie Sanders on Thursday in their most contentious presidential debate yet, questioning whether his ambitious proposals were viable and accusing him of an "artful smear" in suggesting she could be bought by political donations.

Sanders fought back repeatedly, questioning Clinton's progressive credentials and portraying her as a creature of the political establishment in a debate that featured heated exchanges on health care, college tuition funding and efforts to rein in Wall Street.

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The intensity reflected a race that has seen Clinton's once prohibitive lead in polls shrivel against Sanders as the two vie for the Democratic nomination for the Nov. 8 election.

One of Clinton's most forceful remarks came in response to a suggestion by Sanders that she could be influenced by political donations by Wall Street.

"Enough is enough. If you've got something to say, say it directly," she said. "But you will not find that I ever changed a view or a vote because of any donation that I ever received. So I think it's time to end the very artful smear that you and your campaign have been carrying out."

Clinton charged that Sanders' proposal for single-payer universal healthcare coverage would jeopardize Obamacare, calling it "a great mistake," and 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 the many other countries that provide universal healthcare.

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"I do not accept the belief that the United States of America can’t do that," Sanders said. "By moving forward, rallying the American people, I do believe we should have healthcare for all."

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.

Five days before New Hampshire holds the second of the state-by-state presidential nominating contests, opinion polls show Sanders, a U.S. senator from neighboring Vermont, with a double-digit lead over Clinton after surprising the front-runner by finishing just barely behind her in Iowa on Monday.

Clinton seemed energized by her underdog status in New Hampshire, delivering her most aggressive debate performance of the campaign. It was reminiscent of her 2008 run against Barack Obama, when she also began as the front-runner but became a sharper and less cautious candidate as she began to lose.

The debate was the first since former Maryland Governor Martin O'Malley dropped out on Monday after a poor finish in Iowa, giving the two candidates more freedom to directly engage each other.

Sanders accused Clinton of representing "the establishment," while saying he represented "ordinary working Americans." He also noted her Super PAC had taken contributions from Wall Street firms and that Clinton received big speaking fees from Goldman Sachs.

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Clinton disputed the establishment label, saying it was "quite amusing" to accuse "a woman, running to be the first woman president, as the establishment."

The two battled over who best represented progressive ideals. Sanders said he would lead a "political revolution," but Clinton questioned his ability to get his proposals through a Republican-led Congress.

"A progressive is someone who makes progress," she said.

Sanders repeated his earlier pledge not to attack Clinton on the controversy over her use of a private email account and a private server for government business when she was secretary of state.

But Sanders renewed his attacks on Clinton for her Senate vote to authorize the U.S. invasion of Iraq, which she has since called a mistake. He pointed out he voted against the war, which he said had led to the rise of the Islamic State militant group.