Washington — Treasury Secretary Steve Mnuchin called the request by Congressional Democrats for the disclosure of President Trump's tax returns an "unprecedented" move that raises questions about the scope of Congress' "investigative authority" and lawmakers' "legislative purpose."

In a letter that is likely to intensify the latest showdown between the White House and Democrats in the House, Mnuchin said his department, which oversees the Internal Revenue Services (IRS), will not be able to review the request for the president's tax returns by Wednesday, the deadline set by the chairman of the House Committee on Ways and Means, Rep. Richard Neal, a Democrat from Massachusetts.

"The Committee's request raises serious issues concerning the constitutional scope of Congressional investigative authority, the legitimacy of the asserted legislative purpose, and the constitutional rights of American citizens," Mnuchin wrote in the letter addressed to Neal on Wednesday.

Last week, Neal sent a formal request to the IRS for six years of Mr. Trump's tax returns, arguing the committee "has a responsibility to conduct oversight of our voluntary Federal tax system and determine how Americans — including those elected to our highest office — are complying with those laws."

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The president responded by saying he's under audit — a pretext he has often cited — and "would not be inclined" to release his returns until the audit is completed. No federal statute bars individuals under audit from releasing their tax returns. Mr. Trump's lawyer in the matter, Will Consovoy, also asked the IRS not to hand over the president's tax returns until it receives a legal opinion from the Justice Department.

Over the weekend, Mick Mulvaney, who is the acting White House chief of staff, told Fox News that Democrats would "never" see Mr. Trump's tax returns.

Referring to Neal's request as a "long-planned inquiry," Mnuchin cited concerns raised by the House Ways and Means Committee when it was controlled by Republicans during the last session of Congress, as well as comments made by GOP lawmakers when Democrats made the request last week. He said a report issued by House Republicans in 2017 suggested the request by Democrats was politically-motivated and not for the purposes of congressional oversight.

"The Committee determined that such a request would be an 'abuse of authority and set a dangerous precedent by targeting a single individual's confidential tax returns and associated financial documents for disclosure' for political reasons," Mnuchin wrote.

He said his agency is consulting with the Justice Department to determine whether the request for Mr. Trump's tax returns is legal and constitutional. "The legal implications of this request could affect protections for all Americans against politically-motivated disclosures of personal tax information, regardless of which party is in power," he added.

Mnuchin pledged to review the request "carefully," but did not provide a timeline for doing so.

Responding to Mnuchin's letter, Neal said the Treasury Department "decided not to allow the IRS to comply with my request by the April 10 deadline." In his statement late Wednesday, the Democratic chairman indicated he would seek legal advice to make an "appropriate response" to the IRS "in the coming days."

Rebecca Kaplan contributed to this report.