Sen. Elizabeth Warren of Massachusetts, who rarely criticizes her rivals for the Democratic presidential nomination by name, went on the offensive Thursday night to directly challenge Pete Buttigieg.

After her speech at the Democratic National Committee's I Will Vote event in Boston, Warren called on Buttigieg, the mayor of South Bend, Indiana, to let the news media into his private campaign events "so that anyone can come in and report on what's being said" to big financial donors.

Warren had been asked about a New York Times editorial examining Buttigieg's career at the management consulting firm McKinsey & Company, but she chose to talk, instead, about his private fundraising events, referring to him as "Mayor Pete."

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"I think that voters want to know about possible conflicts of interests," she said. "It is even more important that the candidates expose possible conflicts of interests right now, and that means, for example, that the mayor should be releasing who is on his finance committee, who are the bundlers who are raising big money for him."

She added: "No one should be left to wonder what kind of promises are being made to the people who can pony up big bucks to be in the room."

In response, Buttigieg's top spokeswoman, Lis Smith, tweeted: "If @ewarren wants to have a debate about transparency, she can start by opening up the doors to the decades of tax returns she's hiding from her work as a corporate lawyer — often defending the types of corporate bad actors she now denounces."

During a forum in Iowa on Friday with Buttigieg, Chicago Mayor Lori Lightfoot pressed him on the matter and asked if it would be better for him to break his non-disclosure agreement with McKinsey to "have a moral authority and high ground against somebody like" the president "who hides behind the lack of transparency to justify" his actions.

"I'm asking my former employer to do the right thing to not make me choose between claiming the moral high ground and going back" on the written agreement, Buttigieg said about asking McKinsey. "I'm going to give them the chance to do the right thing and then we will take it from there."

Warren also used the withdrawal from the race of Sen. Kamala Harris of California to go after Bloomberg, the former mayor of New York, who entered the race last month.

"There will not be a single person of color on that stage," said Warren, "and as I said, women have been forced out of this race at a time when billionaires can buy their way in.

"Michael Bloomberg just did a $37 million ad buy in one week in order to make himself a candidate while he skips the usual parts of democracy, like shaking hands with people and hearing directly about their concerns," she said, continuing her criticism of the former New York City mayor's late entry into the race.

"If democracy is just going to be about billionaires buying their way in, then buckle up, because we're going to have an America that just works better and better for billionaires and leaves everyone else behind," she said.

Wednesday night, Warren appeared on Bloomberg TV, which is owned by Bloomberg, and said: "What is broken in America is we've got a country that is working great for those at the top ... and that is why I'm so concerned about Michael Bloomberg jumping into this race, dropping $37 million in one week on ad buys. I don't think elections ought to be for sale."

Warren was at or near the top of Democratic polls earlier in the autumn, but in recent weeks she has started to fall behind former Vice President Joe Biden.

The most recent national poll by the Quinnipiac University Polling Institute, released last week, put Warren at 14 percent, trailing Biden, at 27.8 percent. The same poll in the first week of October had Warren leading Biden by 30 percent to 27 percent.

Warren and Sen. Bernie Sanders of Vermont, with whom she is roughly even in recent polls, have declined to raise money at private events. Biden, unlike Buttigieg, has allowed reporters at all of his fundraising events.

CORRECTION (Dec. 6, 2019, 8:50 a.m. ET): An earlier version of this article misstated Warren's comments. She did not mention Cory Booker or Kamala Harris by name as having failed to qualify for the next debate.