Lis Smith, a senior adviser to Mr. Buttigieg, responded to Ms. Warren on Twitter by calling on her to release old tax returns from her years in the private sector.

“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,” Ms. Smith wrote.

Ms. Warren has released 11 years of tax returns on her website, covering her time in public office. Mr. Buttigieg has said he has asked McKinsey to release him from the nondisclosure agreement that covered his work.

Ms. Warren has also focused criticism in recent weeks on former Mayor Michael R. Bloomberg of New York, who has spent tens of millions of dollars from his multibillion-dollar fortune since entering the Democratic primary. She took out an advertisement on Mr. Bloomberg’s own television network calling out the former mayor by name. And she sat for an interview on Bloomberg Television, saying, “I don’t believe that elections ought to be for sale.”

In Boston on Thursday, Ms. Warren reiterated her attacks on Mr. Bloomberg, saying his candidacy validated her campaign’s theme of battling corruption in politics.

“If campaigns are nothing more than billionaires and people who suck up to billionaires buying tens of millions of dollars of TV ads, then we’re just not going to have any America where everybody gets an opportunity,” she said.