Treasury Secretary Steve Mnuchin officially denied House Democrats’ request for President Donald Trump’s tax returns in a letter Monday, saying the request “lacks a legitimate legislative purpose.”

If Democrats decide to continue to pursue the issue, they can either subpoena the Internal Revenue Service or file a lawsuit. Either way, a lengthy legal battle is expected to follow that could wind up before the Supreme Court.

As Mother Jones previously reported, experts maintain that Congress does, in fact, have the authority to request a president’s tax returns, and denying Congress’ request would be unprecedented:

Trump was the first presidential candidate since Gerald Ford to refuse to release his tax returns But according to a tax historian who spoke to Mother Jones last year, Congress has always had the option to request them. “They can’t do it because they want to show it off at a cocktail party, but as long as there is a legitimate need for them—something consistent with Congress’ prerogatives and responsibilities, yes, they can request them, and Treasury is supposed to supply them,” said Joseph Thorndike.

Monday also marks the deadline set by House Judiciary Chair Jerrold Nadler for Attorney General William Barr to share the unredacted Mueller report and underlying evidence with Congress. Barr did not comply, and the committee plans to vote whether to hold him in contempt on Wednesday.

Read the letter here: