Durham, N.H. — Former Secretary of State Hillary Clinton said she was "100 percent confident" that her use of a private email server to conduct government business would not become a problem that derailed her campaign.

"I never sent or received any classified material. They are retroactively classifying it," Clinton said in Thursday night's Democratic debate, referring to recent disclosures that some of the emails on her server would not be released because they contain information classified "top secret." "I think the American people will know that it's an absurdity. I have absolutely no concerns about it whatsoever."

Clinton did not seek to explain why she had used the private server in the first place, although she has on past occasions.

Instead, when moderator Chuck Todd said that the issue continued to worry Democrats, Clinton implied that the email scandal was a creation of politics.

"Before it was emails, it was Benghazi, and the Republicans were stirring up so much controversy about that," Clinton said, referring to the House investigation of her actions during the attacks that killed the U.S. ambassador in Benghazi, Libya, on Sept. 11, 2012.

Vermont Sen. Bernie Sanders, Clinton's opponent, said that he had declined chances to attack Clinton on the issue.

"There's a process underway. I will not politicize it," he said.

Sanders, pressed about his experience in foreign affairs earlier in the evening, repeatedly cited his 2002 vote against the Iraq War as evidence that he could be trusted to make foreign policy decisions.

"Experience is not the only point. Judgment is. And once again, back in 2002 when we both looked at the same evidence about the wisdom of the war in Iraq, one of us voted the right way, and one of us didn't," Sanders said, addressing Clinton.

Clinton voted in favor of the war, a vote that has become a centerpiece of Sanders' case against her.

But Clinton offered a counter-argument. She has apologized for the vote, but said that Sanders' vote was not enough.

"We did differ. A vote in 2002 is not a plan to defeat ISIS" in the present day, Clinton said, using another name for the Islamic State.

Sanders, asked to give more details about his foreign policy during the first one-on-one debate between the two, said it flows out of the experience of the Iraq War.

"That lesson is . . . the United States cannot do it alone. We cannot be the policeman of the world," Sanders said. He added: "The key doctrine of the Sanders administration would be, no, we can't continue to do it alone. We need to work in coalition."

The one-on-one format and the stakes going into the New Hampshire primary might also explain the much more combative tone both candidates seemed to be taking Thursday, with both Clinton and Sanders raising their voices and appearing much more demonstrative than in previous debates.

Earlier in the evening, Clinton defended her connections to Wall Street, including her paid speeches to Wall Street firms, saying they were being used unfairly to cast her as soft on financial regulation.

"Look at my record. Look at what I am proposing. We have a vigorous agreement here. We both want to rein in the excesses of Wall Street," Clinton said.

Sanders, a "democratic socialist," has made a Wall Street crackdown a key piece of his platform.

Clinton said that she had spoken to Wall Street firms during a broader set of talks to corporate groups after she left the State Department, and had not changed her policies to favor financial firms.

In fact, Clinton said, she wants to regulate a broader set of firms whose financial risks could require future bailouts.

"If all we're gonna talk about is one part of our economy, and indeed one street in our economy . . . we're missing the big picture," Clinton said.

Sanders responded that Clinton was underplaying Wall Street's influence.

"Madam Secretary, it is not one street. Wall Street is an entity of unbelievable economic and political power," Sanders said. He returned to a broader critique of the financial industry, saying that it provided little of true value to the American economy or American workers: "In my view, the business model of Wall Street is fraud. It's fraud."

The two candidates had their sharpest argument of the evening shortly before, with Sanders accusing Clinton of representing "the establishment," and Clinton deriding Sanders as an unrealistic idealist whose campaign had carried out a "very artful smear" connecting her to Wall Street interests.

The debate, occurring just a few days before the New Hampshire primary, took place in a state where the original dynamic of the race had completely reversed.

Clinton, who began the race as the prohibitive favorite, is a huge underdog here, in the state next to Sanders's home state of Vermont.

In the 2008 campaign, losing transformed the defensive, standoffish Clinton into a more aggressive, active candidate.

In this state, on this night, it seemed to be happening again.

Clinton repeatedly challenged Sanders, at one point saying that his criticisms that she had taken too much Wall Street money was an insinuation that she was "bought."

"Enough is enough. If you've got something to say, say it directly," Clinton said.

She also accused Sanders of holding out a too-pure standard for what constituted "progressivism" and called him the "self-proclaimed gatekeeper" for progressivism.

After years in which aggressive conservatives had made "liberal" a dirty word, and Democrats won by running to the center, here were two Democrats fighting over a label in the same way that Republicans fight over "conservative."

Sanders responded with an answer that's now familiar in those intra-Republican fights.

His opponent, he said, represents the "establishment." In this party, as in the other one, that word never means anything good.

"Secretary Clinton does represent the establishment. I represent ordinary working Americans," Sanders said. He also said he wanted to change the Democratic party, which Sanders only recently joined after years as an official "independent."

"Let me be frank: I do want to see major changes in the Democratic Party. I want to see working people, and young people, come into the Democratic Party in a way that doesn't exist now," Sanders said. "And I want a 50-state strategy, so the Democratic Party doesn't just compete in 25 states."

Clinton responded that the "establishment" label could not fit her, despite her long experience in Washington as first lady, senator and secretary of state.

"A woman, running to be the first woman president, as the establishment. It's really quite amusing to me," Clinton said.

Earlier, Clinton had started the evening with an attack on Sanders's policy ideas, saying that they were unrealistic and unworkable.

"The numbers just don't add up, from what Senator Sanders has been proposing," Clinton said, in response to a question from moderator Chuck Todd. Clinton said that her approach was more realistic. In particular, she said that Sanders's plan for "single-payer" health care would cause a divisive and time-consuming debate which could undo the structure of President Obama's health-care law.

Clinton also criticized Sanders's plan to make public college free for all, saying it would be too expensive.

"A progressive is someone who makes progress," Clinton said. "That's what I intend to do."

Sanders replied that a number of European countries had approved single-payer health-care systems. "I do not accept the belief that the United States of America cannot do that," Sanders said.

The battle over the label "progressive" dominated the early minutes of the debate, too: Moderators asked Clinton about Sanders' charges that she did not meet the modern definition of the word. Clinton responded by criticizing Sanders' positions on gun laws, which have been a rare place where Sanders is not at the left edge of Congress.

"I don't think it was particularly progressive to vote against the Brady bill five times," Clinton said, referencing a major bill that instituted background checks for gun buyers. "We can go back and forth like this."

The debate began at 9 p.m. Eastern on MSNBC. This is the first time the two have debated without former Maryland governor Martin O'Malley, who struggled to find a niche between the former secretary of state and the "democratic socialist" senator and who dropped out of the race after Monday's Iowa caucuses.

In this debate, both Clinton and Sanders were playing unfamiliar roles.

For Sanders, that means being the front-runner.

The Vermont senator has spent most of this race as an underdog, chipping away at Clinton's by highlighting her close ties to Wall Street and her policy shifts on issues like the Iraq War and same-sex marriage.

But now, in New Hampshire, which is next door to his home state, Sanders is playing Clinton's old role: trying to hold on to a huge lead. He is leading Clinton 61 percent to 30 percent among likely Democratic primary voters, according to the latest CNN/WMUR poll. The margin was 58 to 38 percent in a new NBC News/Wall Street Journal/Marist poll.

In recent days, Sanders has tried to lower expectations here, even as the polls have raised them. He's talked about the Clinton family's long experience in New Hampshire, and Clinton's own win in New Hampshire in 2008.

Clinton, by contrast, was playing up the underdog role — trying to make expectations low, then beat them. Clinton's campaign has said New Hampshire is Sanders's "backyard," but she has poured in supporters to try to close the gap in the polls.

Clinton barely beat Sanders in the Iowa caucuses Monday. Her campaign has said, though, that the road will get easier for her after these first two states, which have a lot of white voters and very liberal voters - two groups among whom Sanders does well.

Earlier this week, Sanders said that Clinton is a progressive "some days," except when she "announces she is a moderate."

Clinton called Sanders' comments a "low blow," but her pitch to voters is essentially based on the idea that Sanders's brand of progressivism is too idealistic and uncompromising to ever work in the real world.

"I'm a progressive who likes to get things done," Clinton said at a CNN forum Wednesday in which the two candidates appeared one after the other.

Clinton's campaign announced Thursday evening that she would leave New Hampshire briefly Sunday to visit the city of Flint, Mich. In Flint, a poor city with a large African American population, a series of government breakdowns meant that residents' drinking water was highly contaminated with lead.

Democrats have said that the state's Republican governor, Rick Snyder, should resign for the role his state government played in causing the problem and failing to notify residents quickly.

Also Thursday, Clinton reported that her campaign had raised $15 million in January — $5 million less than Sanders', according to the Associated Press. The AP said this was the first time an opponent had out-raised her.

The Clinton-Sanders race was once marked by polite disagreements about governing style. Sanders, of course, famously dismissed a damaging Clinton scandal by saying he didn't actually care about "your damn emails."

But as their race has tightened, both sides have turned less polite.

The Sanders campaign has criticized the speaking fees Clinton accepted from large financial firms, including Goldman Sachs. To Sanders — who wants to have government break up these banks to reduce their power — the payments are proof that Clinton owes Wall Street a favor.

Clinton allies have pointed to a blurb Sanders wrote for a book that argues that President Obama let progressives down.

The Washington Post's Abby Phillip contributed to this report.