WASHINGTON — The Justice Department on Friday backed a decision by Treasury Secretary Steven Mnuchin to defy a request from Congress for President Trump’s tax returns on the basis that it lacked “legitimate legislative purpose.”
The department issued a 33-page memo, dated Thursday, that came as House Democrats were preparing a lawsuit as the next step in their effort to gain access to Mr. Trump’s returns. Mr. Mnuchin defied requests made in April and May by the House Ways and Means Committee and refused to comply with a subpoena, setting up a legal battle between the two branches of government.
House Democrats are trying to obtain six years of Mr. Trump’s personal and business tax returns using an obscure provision of the tax code that allows congressional tax-writing committees to get the returns of any taxpayer.
The memo, written by Steven A. Engel, the head of the Office of Legal Counsel, argued that House Democrats, led by Representative Richard E. Neal of Massachusetts, made the request for political purposes with the intention of releasing Mr. Trump’s tax returns. It cast doubt about the formal rationale that the committee used for the request, which Democrats said was for the purpose of examining how the Internal Revenue Service audits presidential tax returns.
“The true aim was to make the president’s tax returns public,” Mr. Engel wrote.
He detailed public comments by Mr. Neal and Speaker Nancy Pelosi about their desire to release Mr. Trump’s tax information. He also argued that seeking the returns would not appear to fit the committee’s stated goal of scrutinizing the presidential audit process.
Claiming that Congress was exceeding its constitutional authority with the request, the Justice Department concluded that Mr. Mnuchin was following the law by refusing to turn over the returns.
“Congress may not use its subpoena power to enforce an unconstitutional demand for information,” Mr. Engel wrote.
Mr. Neal’s office did not immediately comment on the memo.
Mr. Engel’s argument that Congress lacked a legitimate legislative purpose in seeking Mr. Trump’s tax returns dovetailed closely with tracked arguments made by Mr. Trump’s legal team in lawsuits over congressional subpoenas to financial firms like Mazars USA for his financial records.
A Federal District Court judge rejected that argument last month, ruling that Mazars USA must turn over the records because Congress had put forth a valid argument for why it wanted them for legislative purposes and “it is not for the court to question whether the committee’s actions are truly motivated by political considerations.” Mr. Trump is appealing that ruling.
House Democrats have been grappling with how to respond to the Trump administration’s strategy of rebuffing their oversight efforts. Mr. Neal has signaled that he is preparing to file a lawsuit soon.
Mr. Mnuchin had conferred with the Justice Department when he denied Mr. Neal’s initial request, and he indicated that a formal memo would be published to memorialize its guidance.
The Treasury secretary is one of Mr. Trump’s most loyal aides, and the fight over the president’s tax returns has put him in the difficult position of serving as a guardian of his boss’s financial information. Mr. Mnuchin has accused Democrats of trying to “weaponize” the Internal Revenue Service, and he has warned that providing them with Mr. Trump’s returns could come back to haunt Democrats if Republicans try to unlock their tax filings.
Mr. Trump broke with decades of tradition as a presidential candidate when he decided not to release his tax returns, claiming that they were under audit.
There is no rule preventing taxpayers from making their returns public during an audit.