President Donald Trump gives a "thumbs-up" as he walks across the tarmac during his arrival on Air Force One on Sunday at Andrews Air Force Base, Maryland. Credit: Pablo Martinez Monsivais | AP

President Donald Trump sued his own accounting firm and the Democratic chairman of the House Oversight Committee at the same time Monday — trying an unusual tactic to stop the firm from giving the committee details about Trump’s past financial dealings.

The suit, filed in federal court in the District of Columbia, seeks a court order to block a subpoena issued by the committee to Mazars USA last week.

It amounts to Trump — the leader of the executive branch of government — asking the judicial branch to stop the legislative branch from investigating his past.

To do so, Trump wants the court to negate an idea that has guided Congress for decades: that the legislature’s investigative power isn’t just meant to research possible legislation.

Instead, Congress has used subpoenas to dig into the president’s past and present doings, conducting oversight without a particular bill in mind.

Trump says that’s wrong, and must stop.

“There is no possible legislation at the end of this tunnel,” Trump’s attorneys write in their brief, talking about the oversight committee’s inquiry into whether Trump misled his lenders by inflating his net worth. “The Oversight Committee is instead assuming the powers of the Department of Justice, investigating [dubious and partisan] allegations of illegal conduct by private individuals.”

The subpoena Trump is seeking to stop — sent by Oversight Committee Chairman Elijah Cummings, D-Maryland — relates in part to “Statements of Financial Condition” that Mazars produced for Trump before he took office.

These statements are unaudited summations of Trump’s assets, debts and net worth, which Mazars compiled annually for Trump. The statements omitted some debts, overvalued of some assets, and mis-stated some key facts in ways that made Trump seem wealthier than he was.

In testimony earlier this year, Trump’s former lawyer Michael Cohen said that Trump used these statements to solicit loans from banks, including a loan for a failed effort to buy the Buffalo Bills. Cohen said Trump had also used the statements to convince insurers to lower his premiums.

At the time of that testimony, Cohen had already pleaded guilty to lying to Congress in earlier testimony, when he was still working for Trump. He is now facing a three-year jail term.

The Oversight Committee on March 20 asked the company for copies of “statements of financial condition” and audits prepared for Trump and several of his companies, including the one that owns the Trump International Hotel in downtown Washington. The panel also requested supporting documents used to produce the reports and communications between the firm and Trump.

In a statement, Cummings called Trump’s suit “baseless.”

“There is simply no valid legal basis to interfere with this duly authorized subpoena from Congress,” Cummings said. “This complaint reads more like political talking points than a reasoned legal brief.”

Mazars did not immediately respond to a request for comment on Monday. Last week, the company said it “will respect the legal process and fully comply with its legal obligations.”

Trump’s lawsuit argues that Mazars should follow the code of professional conduct issued by the American Institute of CPAs. It quotes one section, counseling CPAs against “disclos[ing] any confidential client information without the specific consent of the client.”

The next paragraph of that code, however, says that CPAs can release client information “to comply with a validly issued and enforceable subpoena.”

Trump’s private attorney, Jay Sekulow, issued a brief statement saying, “We will not allow Congressional Presidential harassment to go unanswered.”

In the suit, Trump’s attorneys wrote that the true purpose of this investigation was not governance, but political advantage: “Its goal is to expose Plaintiffs’ private financial information for the sake of exposure, with the hope that it will turn up something that Democrats can use as a political tool against the President now and in the 2020 election.”

The Trump Organization, Trump’s private business, is also listed as a plaintiff in the lawsuit. Trump Organization officials declined comment.

Trump himself still owns the business, though he says he has given up day-to-day control to his sons Eric and Donald Jr.

Monday’s lawsuit comes amid a broader effort by Trump’s attorneys and the White House to resist congressional requests for information.

Earlier this month, the Treasury Department missed a deadline to hand over Trump’s tax returns to the House Ways and Means Committee.

White House officials have also been digging in their heels on other requests related to Trump’s actions as president.

The administration has signaled it does not plan to turn over information being sought about how particular individuals received their security clearances, Trump’s meetings with foreign leaders, and other topics that they plan to argue are subject to executive privilege, according to several aides familiar with internal discussions.

Cummings’ subpoena to Mazars put a spotlight on an accounting firm that Trump has used since the early 1980s. In addition to compiling these “statements of financial condition” for Trump personally, Mazars and its predecessor firms also handled the tax filings for his charity, the Donald J. Trump Foundation.

In a 2007 deposition, an attorney asked Gerald Rosenblum — an executive at a predecessor firm to Mazars — if Trump was their top client.

“I would think he’s top 10,” Rosenblum said.

Rosenblum was asked if Trump had ever referred other clients to the firm, in the past.

“No,” Rosenblum said.

Washington Post writers Devlin Barrett and Tom Hamburger contributed to this report.