The US Department of Justice (DOJ) released a memo Monday arguing that Congress may not constitutionally compel senior presidential advisors to testify about their official duties, referring to an April 22 subpoena of former Counsel to the President Donald McGahn to testify on matters regarding the report of Special Counsel Robert Mueller.

The House Judiciary Committee subpoenaed McGahn over obstruction of justice claims in the Mueller report, following up on its demand for the fully unredacted report a week earlier.

The memo cites a 1971 memo from then-assistant Attorney General William Rehnquist first describing the legal basis for immunity of senior advisors to testify in matters of the Executive Office of the President, but notes that this practice of executive privilege dates back to the establishment of the office in 1939.

“The immunity of the President’s immediate advisers from compelled congressional testimony on matters related to their official responsibilities has long been recognized and arises from the fundamental works of the separation of powers.”

The memo makes reliability and efficiency arguments, pointing out that the President must be able to disclose to and rely on his senior advisors, and constant watch by the Congress undermines checks and balances between branches.

The Trump administration has vigorously fought ongoing conflict between congressional Democrats, including the filing of a lawsuit contesting the subpoena of Trump’s tax records.