Warren criticized the South Bend, Indiana, mayor for raising money in private and accused the former New York mayor of buying his way into the campaign.
Sen. Elizabeth Warren of Massachusetts, who rarely criticizes her opponents by name, went on the offensive Thursday night, directly challenging Pete Buttigieg and Michael Bloomberg, two of her rivals for the Democratic presidential nomination.
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."
"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."
Noting that neither Harris nor Sen. Cory Booker of New Jersey, who failed to qualify, would be part of the next Democratic debate, Warren said: "There will not be a single person of color on that stage, 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.
"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.
Warren signaled the new tactic on Wednesday night, when she she 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."
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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.