No one running for the Democratic Presidential nomination seems to irk his or her opponents quite like Pete Buttigieg, the thirty-seven-year-old mayor of South Bend, Indiana. The dynamic is so obvious that it has received its own headlines. But until Thursday night, it hadn’t spilled out into full view on a debate stage. The topic that set things off, initially, was campaign finance, which made some sense. The leading Democratic candidates agree on many areas of policy, and the big points on which they don’t—health care and education spending, most notably—are projections, plans for what might be in a future Democratic Administration. Campaign finance, on the other hand, involves decisions that the candidates are making right now. Elizabeth Warren, like Bernie Sanders, has sworn off holding high-dollar fund-raisers and private meetings with big donors. Buttigieg, who has gone from long shot to contender thanks, in part, to the money he has raised from wealthy donors, supports campaign-finance reform in the long term but isn’t turning down dollars now. He has made the argument that Democrats can’t unilaterally disarm while Republicans continue to raise money any way they please.

On Thursday, after speaking about her decision to freeze out big donors, Warren described the details of a recent Buttigieg event in California’s Napa Valley. “The mayor just recently had a fund-raiser that was held in a wine cave, full of crystals, and served nine-hundred-dollars-a-bottle wine,” Warren said. “Think about who comes to that. He had promised that every fund-raiser he would do would be open-door. But this one was closed-door.”

There were cheers in the crowd. Buttigieg responded, “According to Forbes magazine, I’m literally the only person on this stage who’s not a millionaire or billionaire. This is the problem with issuing purity tests you cannot yourself pass.” He raised a hypothetical: suppose that Warren went home after the debate and donated the maximum allowable donation to his campaign, twenty-eight hundred dollars. “Will that pollute my campaign because it came from a wealthy person?” Buttigieg said. “No, I would be glad to have that support.”

Warren changed course. She brought up her pledge, if elected President, to stop the practice of awarding ambassadorships to big donors. “This ought to be an easy step, and here’s the problem,” she said. “If you can’t stand up and take the steps that are relatively easy, can’t stand up to the wealthy and well connected when it’s relatively easy, when you’re a candidate, then how can the American people believe you’re going to stand up to the wealthy and well connected when you’re President?”

Whether you think Warren or Buttigieg—who, for the record, told Warren, “If you can’t say no to a donor then you have no business running for office in the first place”—got the better of this exchange probably depends on which candidate you prefer. This has been something of a problem with the debates, generally, this year: even when they produce moments of sustained engagement between the candidates, they’ve tended only to energize one faction or another. This perhaps explains, in part, why Amy Klobuchar chose to get involved. She’s the candidate who, in vying for the Party’s moderate wing, has been the most outspoken in her skepticism of Buttigieg’s youth and inexperience. “I did not come here to listen to this argument,” she said. “I came here to make a case for progress, and I have never even been to a wine cave. I’ve been to the Wind Cave in South Dakota, which I suggest you go to.” (Klobuchar cannot resist a Midwest joke.) Then she widened the debate by calling for a constitutional amendment to overturn Citizens United and the passage of the suite of reforms that passed the House earlier this year, known as H.R. 1.

Klobuchar focussed on experience a few minutes later, when the debate turned to the subject of immigration. “When we were in the last debate, Mayor, you basically mocked the hundred years of experience on the stage,” Klobuchar said. “I think this experience works, and I have not denigrated your experience as a local official. I have been one. I just think you should respect our experience.”

“You actually did denigrate my experience, Senator,” Buttigieg said. “It was before the break, and I was going to let it go because we’ve got bigger fish to fry here.”

“I don’t think we have bigger fish to fry than picking a President of the United States,” Klobuchar replied.

Buttigieg was speaking of his experience in the Navy Reserves, and of his commitment to the U.S. Constitution that he said accompanied that service. “I certainly respect your military experience,” Klobuchar said. “That’s not what this is about. This is about choosing a President.” Klobuchar spoke of her “track record” and of the importance of winning big elections and having “coattails.”

“If you want to talk about the capacity to win,” Buttigeg said, “try putting together a coalition to bring you back to office with eighty per cent of the vote as a gay dude in Mike Pence’s Indiana.”

Buttigieg has been surging in the polls in Iowa and New Hampshire, but, nationally, he’s still averaging below ten per cent. Klobuchar is someone who people think might surprise in the Iowa caucuses. The strength of Warren’s organizing operation means that no one can rule her out. It was interesting, watching the exchanges with Buttigieg on Thursday, to notice who wasn’t getting involved. Joe Biden and Bernie Sanders, the race’s two front-runners, seemed content to let things play out without getting dirty themselves.