“The numbers just don’t add up…”

So said Hillary Clinton on Thursday, as she used a freshly scheduled MSNBC debate to tell New Hampshire Democrats—and, by extension, Democrats across the country—that the “political revolution” Bernie Sanders proposes is a fantasy rather than a serious vision for change.

Citing “independent experts” and “newspaper editorial boards,” the former secretary of state suggested that what Sanders plans “is just not achievable.”

In the first one-on-one debate of a campaign that will see its first primary on Tuesday, Clinton renewed her attack on Sanders’s proposal for a single-payer “Medicare for All” healthcare system. “The numbers just don’t add up, from what Senator Sanders has been proposing,” said the former secretary of state.

Sanders countered by arguing what Clinton claims is undoable has been done elsewhere—and can be done in America.

“Every major country on earth, whether it’s the UK, whether it’s France, whether it’s Canada, has managed to provide healthcare to all people as a right and they are spending significantly less per capita on healthcare than we are,” he said. “So I do not accept the belief that the United States of America can’t do that,” said the senator from Vermont. “I do not accept the belief that the United States of America and our government can’t stand up to the rip-offs of the pharmaceutical industry which charge us by far the highest prices in the world for prescription drugs.”

That was how it went through the most combative Democratic debate yet in a campaign season that—because of the unexpected strength of an insurgent challenge to a front-runner whose candidacy was once considered “inevitable”—is now expected to see a lot more Democratic debates.

Sanders said he was running against the establishment and Clinton countered that “Senator Sanders is the only person who I think would characterize me, a woman running to be the first woman president, as exemplifying the establishment.” Clinton suggested that she had a better track record of challenging financial abuses, and Sanders countered by recalling that he was the one who wrangled with former Federal Reserve chair Alan Greenspan over deregulation of the financial-services industry. “Go to YouTube today. Look up Greenspan-Sanders. Listen to what I told them then. I helped lead the effort against deregulation. Unfortunately, we lost that. The result is—was the worst financial disaster since the Great Depression.”

At one point, Clinton said, “we have a vigorous disagreement here,” and she was right. But that disagreement was about more than specific issues and agendas; that difference was about the boldness of the visions advanced by two contenders and about what can practically and realistically be achieved.

On Thursday night, the candidates drew clearer lines of distinction between themselves. Clinton presented herself as “a progressive who gets things done.” Sanders presented himself as a progressive who wants to get more done.

Both Clinton and Sanders are successful political leaders. Both have been elected twice to the Senate. Both have compromised when necessary. Both have made mistakes. Both in the course of their long careers have inspired Americans to imagine a better politics and better governance.

But Clinton argued that they are limits on what can be achieved.

The former secretary of state, who is trailing Sanders in New Hampshire polls, asserted that “Senator Sanders and I share some very big progressive goals.” She presented herself as “a fighter.” Yet she seemed to suggest that some of the fights that grassroots Democrats want to wage are unwinnable.

“I am not going to make promises I can’t keep. I am not going to talk about big ideas like single-payer and then not level with people about how much it will cost,” she said. “A respected health economist said that these plans would cost a trillion dollars more a year. I’m not going to tell people that I will raise your incomes and not your taxes, and not mean it, because I don’t want to see the kind of struggle that the middle class is going through exemplified by these promises that would raise taxes and make it much more difficult for many, many Americans to get ahead and stay ahead. That is not my agenda.”

That led moderator Rachel Maddow to ask: “Senator Sanders, have you established a list of what it means to be a progressive that is unrealistic?”

“No, not at all,” responded Sanders, who argued that there is nothing radical about guaranteeing healthcare for all, about making public colleges and universities tuition-free, about making massive investments in infrastructure improvement and about taxing corporations and eliminating tax havens.

The primary barriers to necessary change, he explained, are political.

“The reality is that we have one of lowest voter turnouts of any major country on earth because so many people have given up on the political process,” he said. “The reality is we that have a corrupt campaign finance system which separates the American people’s needs and desires from what Congress is doing. So to my mind, what we have got to do is wage a political revolution where millions of people have given up on the political process, stand up and fight back, demand the government that represents us and not just a handful of [campaign contributors].”

What followed was an intense exchange about campaign financing in which Sanders said that “one of the things we should do is not only talk the talk, but walk the walk. I am very proud to be the only candidate up here who does not have a Super PAC, who’s not raising huge sums of money from Wall Street.”

Clinton responded by suggesting that “time and time again, by innuendo, by insinuation, there is this attack that he is putting forth, which really comes down to—you know, anybody who ever took donations or speaking fees from any interest group has to be bought.… I think it’s time to end the very artful smear that you and your campaign have been carrying out in recent weeks, and let’s talk… about the issues. Let’s talk about the issues that divide us.”

Sanders accepted the invitation, delivering the most powerful statement of the night—and one of the most powerful statements of the campaign.

“Let’s talk—let’s talk about issues, all right?” he began. “Let’s talk about why, in the 1990s, Wall Street got deregulated. Did it have anything to do with the fact that Wall Street provided—spent billions of dollars on lobbying and campaign contributions? Well, some people might think, yeah, that had some influence.

The crowd was laughing.

“Let’s ask why it is that we pay, by far, the highest prices in the world for prescription drugs, and your medicine can be doubled tomorrow, and there’s nothing that the government can do to stop it. You think it has anything to do with the huge amounts of campaign contributions and lobbying from the fossil-fuel industry?”

The crowd was clapping.

“Let’s talk about climate change. Do you think there’s a reason why not one Republican has the guts to recognize that climate change is real, and that we need to transform our energy system? Do you think it has anything to do with the Koch brothers and ExxonMobil pouring huge amounts of money into the political system?”

The crowd was cheering.

“That is what goes on in America,” Sanders continued. “You know, there is a reason why these people are putting huge amounts of money into our political system. And in my view, it is undermining American democracy and it is allowing Congress to represent wealthy campaign contributors and not the working families of this country.”

That imbalance, Sanders concluded, demands not talk of what is unachievable but recognition of the need for a “political revolution.”