Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.) on Wednesday said that White House budget director Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE might have broken the federal Hatch Act by meeting with Republican donors and campaign officials earlier this month.

"Your appearance at this private event with Republican donors and campaign officials raises questions about your compliance with the Hatch Act and other federal laws, your judgment, and your management of the Office of Management and Budget (OMB) and the Consumer Financial Protection Bureau (CFBP)," Warren wrote in an open letter to Mulvaney Wednesday.

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Warren quoting a May 23 letter from Ana Galindo-Marrone, the Office of the Special Counsel's Hatch Act Unit chief, to Mulvaney, wrote, "you are prohibited from...soliciting...political contributions from any person...[and from] soliciting...the political activity of persons who have business pending before their employing office."

“By custom, top executive branch officials and independent agency heads — financial regulators in particular — steer well clear of campaign- or donor-related activity," Warren added.

"Indeed, I could find no no [sic] examples of your immediate predecessors at OMB or CFPB, or of any top Securities and Exchange Commission, Commodity Futures Trading Commission, Federal Deposit Insurance Corporation, or Federal Reserve official, meeting with top political party officials and campaign donors to provide advice and insight into upcoming elections — and I do not understand how your decision to do so was consistent with your responsibilities either as OMB Director or as Acting Director of the CFPB," she wrote.

Warren asked Mulvaney to answer 10 questions — two of which have multiple parts — about the meeting prior to Sept. 29, giving him a 10-day window in which to respond.

Mulvaney spoke alongside Republican National Committee Chairwoman Ronna Romney McDaniel at an event in New York earlier this month, telling GOP donors it was a "very real possibility" that Republicans will win Florida's Senate race, but lose a Senate seat in Texas.

Warren has been one of Mulvaney's most outspoken critics, supporting a legal challenge to his appointment and calling for answers to dozens of questions about his agenda.

Mulvaney has fired back at Warren, stating that he doesn't want the CFPB "to be Elizabeth Warren's baby." The bureau was Warren's brainchild and has been defended staunchly by her party.