DURHAM, N.H. — Hillary Clinton and Bernie Sanders, in by far their testiest encounter of the campaign, tangled Thursday night over the role of money in politics and the philosophical underpinnings of the Democratic Party in their first one-on-one debate.

The candidates scowled, frowned, and cut each other off as they traded attacks that had before been launched from the less personal remove of press releases and tweets.

Sanders said Clinton’s speaking fees and campaign finance donations from Wall Street would hinder her from bringing sweeping changes needed to protect the middle class.

Clinton accused Sanders’ operation of engaging in a “very artful smear campaign’’ to make her appear too cozy with Wall Street. Instead, she said, her ties to financial executives give her valuable insights into how they operate.

“I don’t think these kinds of attacks by insinuation are worthy of you,’’ Clinton told the Vermont senator. “Enough is enough.’’

Sanders shot back that millions of dollars in campaign donations and lobbying fees have been spent to push Congress to deregulate Wall Street and protect drug companies.

The debate, hosted by MSNBC at the University of New Hampshire, offered a clearer choice, now that former Maryland governor Martin O’Malley has dropped out, between one candidate carrying the banner of an idealist and the other selling deeper experience.

It was by far Sanders’ strongest and smoothest performance.

For a senator with limited experience on the national stage, he was finally able to go toe-to-toe with one of the most skillful debaters in the party —toggling between his signature anger and the occasional joke.

This was the last time New Hampshire voters were scheduled to see them on stage together before voting Tuesday’s primary election, and came as the contest has taken a more personal turn.

The candidates debated what it means to be a progressive in the Democratic Party. Sanders said Clinton represents the party establishment and that he represents “ordinary Americans.’’

Clinton, who seemed well-prepared, had a ready answer.

“Senator Sanders is the only person to characterize me — the first woman running to be the first woman president, as part of the establishment,’’ Clinton said. “It’s really quite amusing to me.’’

Clinton eked out a slender victory in the Iowa caucuses earlier this week, and polls show her down in New Hampshire by double digits.

That’s been enough for Sanders to get a second look from voters across the country —and enough of a scare for Clinton, a skilled debater, to drop her objections and agree to the hastily schedule debate Thursday.

At times it was the moderators pushing the most sensitive topics.

NBC’s Chuck Todd pointedly asked Clinton whether she would release transcripts of her paid speeches, including the ones to banks.

“I’ll look into it,’’ Clinton said. “I don’t know the status, but I’ll look into it.’’

Clinton downplayed the reasons why moneyed interests, including Goldman Sachs,paid her six-figure speaking fees, saying they largely wanted to know about her experiences around the world.

“I probably described more times than I can remember how stressful it was advising the president in going after bin Laden,’’ she said.

Later, Todd pressed Clinton on whether she could offer assurances that an FBI review of how classified information was handled during her tenure as secretary of state will leave her unscathed.

“Absolutely I can!’’ Clinton said. “I have absolutely no concerns about it whatsoever.’’

About halfway through the debate, the pair returned to a fight that broke out via social media this week, when Sanders unleashed a Twitter offensive over Clinton’s progressive credentials.

“You can be a moderate,’’ Sanders wrote. “You can be a progressive. But you cannot be a moderate and a progressive.’’

During the debate, Clinton confronted Sanders, calling him “the self-proclaimed gatekeeper of progressivism.’’

“I am a progressive who gets things done,’’ Clinton said. “The root word of the word progressive is progress.’’

She said that by Sanders’ definition, President Obama wouldn’t be considered a liberal because he took campaign donations from Wall Street.

“Do I think President Obama is a progressive?’’ Sanders said. “Yeah. I do.’’

The moderators brought up Clinton’s endorsements, including those from a number of top officials in Sanders’ home state. Clinton used it to demonstrate that people who have worked with her liked her, but Sanders suggested it was an indication of her career in politics.

“She has the entire establishment, or almost the entire establishment, behind her,’’ he said. “That’s a fact.’’

The two laid out different ideas for the future of the Democratic Party, with Sanders saying the wants a “50 state strategy’’ to welcome more young people and working class voters.

Clinton pointed out that the originator of that idea, former Vermont governor Howard Dean, endorsed her.

When the debate turned to international affairs, an area that Clinton says he’s ignored, Sanders reminded the moderators and the audience that he recently gave a speech about foreign policy and Democratic socialism at Georgetown. In a rare moment of levity, he quipped that maybe he should have just limited the talk to one issue.

Sanders said it was “not arguable’’ that Clinton has more experience on foreign affairs, but argued that judgment matters more.

“Once again, back in 2002, when we both looked at the same evidence of the wisdom of the war in Iraq,’’ he said, “one of us voted the right way and one of us didn’t.’’

Clinton countered that Obama put his faith in her in appointing her as secretary of state.

“I know from my own experience that you’ve got to be ready on Day One,’’ she said. “There’s just too much unpredictable threat and danger in the world today.’’

Both candidates took pains to compliment each other from time to time. At one point, Sanders and Clinton shook hands. Later Sanders clapped after Clinton gave her closing remarks.

Both face risks if they appear too negative. Sanders, unlike a traditional candidate, is seen by millions of Democratic primary voters as the leader of a movement. Attacking Sanders without also angering the movement is a complicated maneuver that Clinton has yet to master.

As Clinton has ramped up her negative rhetoric this month, the senator reported an eye-popping $20 million in campaign donations for January. That meant he outraised the storied Clinton money machine by $5 million in those 31 days.

Sanders, too, faces dangers. Clinton performs at her best when she’s taking incoming fire.

In 2008, when Clinton’s back was against the wall, an overconfident Barack Obama made an off-handed comment that she was “likeable enough’’ during a New Hampshire debate. It earned sympathy for her, helping her overcome a deficit in the polls and win the state.

Responding to a question from a moderator,Sanders refused to concede that he lost in Iowa, a contest that the local Democratic Party called the closest caucus in history. Several of the precincts were decided by the flip of a coin. Sanders wants the raw vote totals released.

The senator said he agreed with the Des Moines Register, the state’s largest paper, which has called for a complete audit of the results.

“What happened Monday night at the Democratic caucuses was a debacle, period,’’ the paper opined.

But he downplayed the extent of his concern, noting that the state awards only a handful of delegates. “Lets not blow this out of proportion,’’ he said. “This is not the biggest deal in the world.’’

Annie Linskey can be reached at annie.linskey@globe.com. Follow her on Twitter @AnnieLinskey.