The Justice Department's Office of Legal Counsel released a memo Friday supporting the Treasury Department's position that Congress is not entitled to see President Trump's tax returns, alleging Congress exceeded its authority in its request. Treasury Secretary Steven Mnuchin refused to comply with a House subpoena for Mr. Trump's federal tax returns in May, saying members of Congress did not have a "legitimate purpose" to review the returns.

The House Ways and Means Committee issued the subpoena for the president's tax returns to Mnuchin and IRS Commissioner Charles Rettig last Friday after Mnuchin denied the committee's initial request. Ways and Means Chairman Richard Neal says the committee is entitled to the returns under section 6103(f) of the IRS code, in order to conduct oversight and determine whether the IRS is enforcing laws.

Mnuchin argued that Congress doesn't have a "legitimate legislative purpose" in requesting the president's private tax information, and in its memo Friday the Office of Legal Counsel agreed.

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"Under the facts and circumstances, the Secretary of the Treasury reasonably and correctly concluded that the Committee's asserted interest in reviewing the Internal Revenue Service's audits of presidential returns was pretextual and that its true aim was to make the President's tax returns public, which is not a legitimate legislative purpose," the Office of Legal Counsel said in the memo.

Democrats in Congress have argued that viewing Mr. Trump's tax returns has legitimate legislative purpose in that it would reveal the financial dealings of the president and potential ethical issues. Neal threatened to sue the Treasury Department for refusing to respond to the subpoena.

However, while members of Congress may face an uphill battle in obtaining Mr. Trump's federal tax returns, they could have more luck with his New York state tax returns. The New York state legislature passed a measure in May to allow the leaders of the U.S. House Ways and Means Committee, the Senate Finance Committee or the Joint Committee on Taxation to access any New York state tax returns filed by elected officials and top appointed officials.

Paula Reid contributed to this report.