Warren attacked Buttigieg over the weekend for "conflicts being created every single day when candidates for president sell access to their time to the highest bidder," she told reporters.

Sen. Elizabeth Warren. | Elijah Nouvelage/Getty Images

The Buttigieg campaign also took more veiled shots at Warren for refusing to release her tax returns before 2008.

“No other candidate for president has released the entirety of their tax returns since their education concluded. No other current candidate for president has released the names of people raising money for their campaign," Schmuhl's statement continued. "There are important differences in this race among Democratic candidates, from creating a choice of affordable health care choices for all to removing cost as a barrier to college for those who need it, but transparency shouldn't be one of them."

Warren's campaign has argued that the 11 years of tax returns the senator already released should be sufficient, and that the breakdown of earnings by client the campaign provided was more specific. Earlier this year, Warren also released the names of legal clients she worked for during that time.

Buttigieg has also faced escalating criticism for not disclosing the names of clients he worked for during his time at McKinsey, a corporate consulting firm. Buttigieg, who signed a non-disclosure agreement, is publicly urging McKinsey to release him from it. And on Friday, Buttigieg's campaign released a timeline of the mayor's work, from 2008 to 2010, but it did not share the names.