The Trump administration on Monday instructed former White House counsel Don McGahn to disobey a congressional subpoena, making official what had long been expected as part of President Donald Trump’s broad stonewalling of Democrat-led congressional investigations. McGahn, who provided critical testimony to Robert Mueller on instances of potential obstruction of justice by the president, was scheduled to appear in a hearing before the House Judiciary Committee Tuesday. The Trump administration, however, argued in a 15-page legal opinion from the Justice Department that “Congress may not constitutionally compel the president’s senior advisers to testify about their official duties.”

“This testimonial immunity is rooted in the constitutional separation of powers and derives from the President’s independence from Congress,” the DOJ legal memo reads. “Because of this constitutional immunity, and in order to protect the prerogatives of the office of the presidency, the president has directed Mr. McGahn not to appear at the Committee’s scheduled hearing on Tuesday,” current White House counsel Pat Cipollone wrote in a letter to the Judiciary Committee Monday. “This long-standing principle is firmly rooted in the Constitution’s separation of powers and protects the core functions of the presidency, and we are adhering to this well-established precedent in order to ensure that future Presidents can effectively execute the responsibilities of the Office of the Presidency.”

“Even if Mr. McGahn [-] ultimately does have to appear before Congress, the separate issue would remain of whether he could rely on a claim of executive privilege by Mr.

Trump to avoid answering questions about his communications with the president—even though the Trump administration already disclosed the substance of those talks by making the Mueller report public,” the New York Times notes.