BUTTIGIEG: You know, according to Forbes magazine, I’m literally the only person on this stage who is not a millionaire or a billionaire. So this is important. This is the problem with issuing purity tests you cannot yourself pass. If I pledge — if I pledge never to be in the company of a progressive Democratic donor, I couldn’t be up here. Senator, your net worth is 100 times mine. Now, supposing you went home and felt the holiday spirit — I know this isn’t likely, but stay with me — and decided to go on pete.com and gave the maximum allowable by law, $2,800, would that pollute my campaign because it came from a wealthy person? No. I would be glad to have that support. We need the support to everybody who is committed to helping us to defeat Donald Trump.

WARREN: I do not sell access to my time. I don’t do call time with millionaires and billionaires.

BUTTIGIEG: As of when, Senator?

WARREN: I don’t meet behind closed doors with big-dollar donors. Look, I have taken one that ought to be an easy step for anyone here. I said to anyone who wants to donate to me, if you want to donate to me, that’s fine. But don’t come around later expecting to be named ambassador, because that’s what goes on in these high-dollar fund-raisers. I said no, and I asked everybody on this stage to join me. This ought to be an easy step. And here’s the problem. 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 is relatively easy, when you are a candidate, then how can the American people believe you will stand up to the wealthy and well connected when you are president and it is really hard?

BUTTIGIEG: Senator, I’ve got to respond. First of all, if you can’t say no to a donor, then you have no business running in the first place. But also, Senator, your presidential campaign right now as we speak is funded in part by money you transferred, having raised it at those exact same big-ticket fund-raisers you now denounce. Did it corrupt you, Senator? Of course not. So to denounce the same kind of fund-raising guidelines that President Obama went by, that Speaker Pelosi goes by, that you yourself went by until not long ago, to build the Democratic Party and build a campaign ready for the fight of our lives, these purity tests shrink the stakes of the most important election. We’d like to bring everyone in.

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Later, Senator Bernie Sanders of Vermont joined in the criticism, using the debate to further his argument that the campaign finance system is broken. Before the debate, Mr. Sanders’s campaign had bought the web address peteswinecave.com and redirected it to the Sanders campaign website.

A few minutes later, Senator Amy Klobuchar of Minnesota went directly at Mr. Buttigieg with a different critique, objecting to his attacks on Washington experience, ticking off achievements of others onstage in D.C., including Mr. Sanders and Mr. Biden.

“So while you can dismiss committee hearings, I think this experience works,” she said. “And I have not denigrated your experience as a local official. I have been one. I think you should respect our experience.”