DURHAM, N.H. — In a caustic debate on Thursday night, Hillary Clinton accused Senator Bernie Sanders of leveling "attacks by insinuation and innuendo" against her integrity and her credentials as a progressive by portraying her as beholden to wealthy interests and corporations.
Mrs. Clinton, lobbing her harshest assault yet in their race for the Democratic presidential nomination, said months of criticism by Mr. Sanders over her taking speaking fees from Wall Street banks amounted to a suggestion that she was corrupt — or, as she put it, a "very artful smear." It was the sort of cutting remark she usually reserves for Republicans, and it drew boos from many in the audience at the University of New Hampshire.
"There is this attack that he is putting out, which really comes down to, anybody who took donations or speaking fees from any interest group has to be bought," she said. "And I just absolutely reject that, senator. And I really don't think these attacks by insinuation and innuendo are worthy of you. Enough is enough. If you've got something to say, say it directly."
Mr. Sanders, who largely kept his cool in the debate, ignored the broadside and instead reiterated his familiar critique that a "super PAC" supporting Mrs. Clinton is funded in part by banks.
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"There is a reason why these people are putting huge amounts of money into our political system," Mr. Sanders said. "It is undermining American democracy and it is allowing Congress to represent wealthy campaign contributors and not the working families of this country."
The ferocity of Mrs. Clinton's remarks in the debate was risky, given that many voters, including some Democrats, already have an unfavorable opinion of her. She is also running far behind Mr. Sanders in the polls leading up to Tuesday's primary in New Hampshire, and her attacks — while geared toward undercutting his popularity here — might backfire with some undecided voters who have positive feelings about Mr. Sanders.
Mrs. Clinton, appearing tense and even angry at times, was particularly sensitive about receiving millions of dollars in speaking fees, including $675,000 for three speeches from Goldman Sachs. She downplayed her turn on the lecture circuit, noting that she had also addressed other groups such as the American Camping Association and that the banks merely wanted to hear her views on world affairs.
But she did acknowledge that she had not "done the job I should in explaining my record" about financial regulation, suggesting that is why she had been unable to beat back questions about her speaking fees. Yet when asked if she would release all the transcripts of her speeches to banks, she hedged.
"I will look into it," she said. "I don't know the status, but I will certainly look into it."
From its opening moments, the debate devolved into a series of searing exchanges over one overarching theme: which of the two Democrats was the most progressive, an issue that they have been fighting over in recent days as they compete in New Hampshire. On one level, the debate was over semantics: The candidates share a similar worldview about aggressive government support for universal health care, public education and aid to the poor, and both of them used the word "rigged" to describe the American economy.
But they kept putting their own personal spin on the word "progressive," with Mrs. Clinton deriding Mr. Sanders as the "self-proclaimed gatekeeper for progressivism" and accusing him of judging her unfairly. Rebutting Mr. Sanders's claim that she is a political moderate, Mrs. Clinton sought to align herself with some of the most popular names in Democratic politics to suggest that Mr. Sanders was impugning them, not just her, with what she described as a purity test.
"The root of that word, progressive, is progress, but I've heard Senator Sanders's comments and it's really caused me to wonder who's left in the progressive wing of the Democratic Party," Mrs. Clinton said. "Under his definition, President Obama is not progressive because he took donations from Wall Street."
Mr. Sanders, asked if President Obama was a progressive, at first did not answer the question, instead bringing up a comment by Mrs. Clinton when she called herself a moderate. But he ultimately took on the question. "Do I think President Obama is a progressive? Yeah, I do," Mr. Sanders said. "I disagree with him on a number of issues, including the trade agreement, but, yes, I think he has done an excellent job."
Mrs. Clinton, who is leading in national polls and narrowly won the Iowa caucuses on Monday, departed from the approach she used at Democratic forums late last year when she largely ignored Mr. Sanders and focused her fire on Republicans. On Thursday, though, there was only a fleeting mention of Donald J. Trump as she went after Mr. Sanders with an intensity that reflected a nomination fight worthy of the name.
She portrayed herself as a veteran of the country's political wars who is better equipped to enact a progressive agenda than her more uncompromising rival. It was not a message likely to resonate among Mr. Sanders's idealistic young legions in New Hampshire, but it presaged the one she is likely to carry when the primary moves to Nevada and South Carolina, more moderate states, later this month.
The debate was the first to feature only Mrs. Clinton and Mr. Sanders, and the narrower aperture revealed significant differences. Mrs. Clinton seemed more tense at times in their rapid-fire exchanges. Mr. Sanders seemed surprised at times by the level of Mrs. Clinton's vitriol, but it never threw him off his central message.
"Millions of Americans are giving up on the political process, and they're giving up on the political process because they understand that the economy is rigged," he said. He zeroed in on wealthy Americans and Wall Street firms giving huge sums of money to candidates — an implicit criticism of Mrs. Clinton, though he did not say her by name.
Mr. Sanders twice let opportunities go by when he could have pressed Mrs. Clinton on her taking speaking fees from banks. Instead, he ignored her and presented his own well-honed lecture about what he described as a corrupt system in which the financial institutions have outsize political influence and are treated with kid gloves when they break the law.
"One of the things we should do is not only talk the talk, but walk the walk," he said. "We have raised 3.5 million individual contributions, averaging $27 dollars apiece. That is what the political revolution means."
Mrs. Clinton repeatedly tried to frame Mr. Sanders as a politician who has had ambitious ideas for decades but little to show for it. "The numbers just don't add up from what Senator Sanders is proposing," Mrs. Clinton said. "A progressive is someone who makes progress. That's what I intend to do."
Mr. Sanders dismissed the idea that his record in Congress showed he was unable to enact major changes. "Well I haven't quite run for president before," he said.
While their disagreements were more muted, the two also clashed over foreign policy. And, once again, it was Mrs. Clinton going on the offensive. She suggested the senator was naïve for wanting to insert Iranian troops into Syria and suggested he would be too hasty to normalize relations with the government of Iran.
"You are voting for both a president and a commander in chief," she said, speaking directly to voters.
Mr. Sanders responded by invoking her 2002 vote to authorize the Iraq war to question her judgment and recalled that, in the 2008 Democratic primary, she had called Mr. Obama naïve "because he thought it was a good idea to talk to our enemies."
Mrs. Clinton, asked if Mr. Sanders could win the general election, said she had "great respect" for the Sanders campaign but believed he would "face the most withering onslaught from Republicans." She then tacitly suggested that Mr. Sanders's left-wing views would not be popular in some states in November.
"It will put the nominee into the spotlight," she said. "I've been vetted. There's hardly anything you don't know about me."
Mrs. Clinton said she was "100 percent confident" that she did not face any legal threat from the federal investigations into her use of a private email server as secretary of state. Mr. Sanders, in turn, said he would stand by his past pledges not to politicize the issue.
The biting exchanges at the start of the debate, though, seemed long gone as the forum concluded.
"If I'm so fortunate as to be the nominee, the first person I will call to talk to about where we go and how we get it done will be Senator Sanders," Mrs. Clinton said.
Mr. Sanders returned the peace offering, noting that campaigns can "get out of hand." But he said he respected Mrs. Clinton and that their shared views were far superior to those held by Republicans.
This prompted broad smiles from them both, an impromptu handshake and a loud ovation from appreciative Democrats in the audience.