Buttigieg’s campaign has been calling on Warren to release her tax returns from before 2008, when she did private legal work for some large corporations similar to the ones she now rails against. The Warren campaign is not releasing her tax returns from 1986 onward, arguing that providing over three decades of tax returns after she already released 11 years' worth is an unreasonable standard that other candidates are not being held to.

Her campaign also argued that the breakdown of earnings by client she's providing is more specific than her tax returns would have included.

Orthman said that the compensation figures were gathered from “public records, Elizabeth’s personal records, and other sources” but POLITICO was not able to independently verify the compensation.

In total, Warren's campaign disclosed income from 42 cases, with the most money coming from a 2009 Travelers Insurance case concerning victims of asbestos poisoning. Warren says she earned $212,335. The next highest compensation came as counsel to department store Bergner's in the 1990s, from which she earned $186,859.59. There are five cases that the Warren campaign says it does not have compensation records for, including consulting for the former directors of Getty Oil in the late 1980s during a bankruptcy filing by Texaco.

South Bend, Ind., Mayor Pete Buttigieg. | Scott Olson/Getty Images

Friction between Warren and Buttigieg has been growing as he's surged in Iowa, a critical state for both of them. In addition to pressing him about his clients at McKinsey — which Buttigieg says he can't reveal because of a nondisclosure agreement, which he has asked the firm to nullify — Warren has criticized Buttigieg for holding closed-door fundraisers where media access is denied.

“The mayor should be releasing who’s on his finance committee, who are the bundlers who are raising big money for him, who he’s given titles to and made promises to,” Warren told reporters on Thursday. She repeated the point throughout the weekend in New Hampshire and argued that Buttigieg’s should start letting reporters into his Manhattan fundraisers this upcoming week.

Buttigieg’s campaign did not immediately respond for comment.

Warren’s private legal work has been a prime target for past opponents who have tried to paint her as hypocritical — a self-proclaimed champion for the little guy as she takes big money from corporations.

Warren’s central message on the campaign trail is the corrosive influence of money — another reason why Buttigieg and other opponents want to draw attention to her lucrative private work.

“She is firmly entrenched in the same ‘1 percent’ she rails against, and she is more than happy to make tens of thousands of dollars defending powerful insurance companies against middle-class victims,’’ Jim Barnett, a spokesman for former Republican Sen. Scott Brown, said in 2012 when facing Warren in Massachusetts. The issue has dogged her in politics ever since. After declining for years to provide a full list of her past clients, Warren finally did so this past May.

Warren’s central message on the campaign trail is the corrosive influence of money which is another reason why Buttigieg and other opponents want to draw attention to her lucrative private work. Former Vice President Joe Biden’s campaign told Bloomberg News before the September debate that he was planning to attack Warren on the issue.