The Department of Justice on Monday moved to block former White House counsel Don McGahn from testifying to Congress, citing the principle of the separation of powers in a legal opinion.

McGahn had been subpoenaed by the House Judiciary Committee to answer questions on Tuesday about Mueller's investigation of President Donald Trump and Russian interference in the 2016 election, but it is unclear whether he will appear. The Justice Department opinion backs the White House, which has refused to cooperate with Congress as House Democrats ramp up their oversight investigations into the administration.


Sources close to McGahn say they have received the Justice Department's legal opinion and a letter from the White House instructing McGahn not to testify, which are being reviewed.

Current White House counsel Pat Cipollone wrote in a letter to House Judiciary Chairman Jerrold Nadler, D-N.Y., Monday, that the Justice Department "has advised me that Mr. McGahn is absolutely immune from compelled congressional testimony with respect to matters occurring during his service as a senior adviser to the President."

Cipollone added that, "the President has directed Mr. McGahn not to appear at the Committee's scheduled hearing" on Tuesday.

Last month, Trump said that his administration is "fighting all the subpoenas" and that he has been the "most transparent president and administration in the history of our country by far."

McGahn was a key witness in Mueller's probe into whether the president attempted to derail the investigation. Mueller's 448-page report said, for example, that Trump ordered McGahn to tell Deputy Attorney General Rod Rosenstein that "Mueller has to go." McGahn resisted those efforts despite Trump’s insistence, the report said. But, when news of those events were first reported in The New York Times months later, Trump sought to have McGahn deny it and write a letter "for our records" changing his story.