Don McGahn does not have testify before Congress, according to a memo released Monday from the Department of Justice. And according to his lawyer, he won’t be.

“[T]he President has unambiguously directed my client not to comply with the Committee’s subpoena for testimony,” McGahn’s lawyer, William Burck, wrote in a letter to House Judiciary Committee Chairman Jerry Nadler (D-N.Y.) telling him McGahn would not comply with the chairman’s order that he testify on Tuesday. “In the event an accommodation is agreed between the Committee and the White House, Mr. McGahn will of course comply with that accommodation.”

McGahn’s appearance before lawmakers had already seemed improbable, given that the White House had directed him to refuse to turn over documents related to the Mueller probe. But his formal refusal to testify adds even more fuel to an already bitter political and legal fight between the parties over the boundaries of executive privilege and presidential power.

Those battles took shape in various legal memorandum on Monday—memorandum that could functionally reshape the relationship between two branches of government and will undoubtedly be tested in the courts.

The Justice Department memo, released by the department's Office of Legal Counsel and addressed to White House Counsel Pat Cipollone, argued that Congress inherently does not have the power to make the former White House Counsel talk to them about his work for President Donald Trump. The memo also said the president had the power to order McGahn not to testify and that Congress did not have the power to punish him––criminally or civilly––for following such an order.

“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 workings of the separation of powers,” reads the memo, signed by OLC chief Steven Engel, a Trump appointee. “Those principles apply to the former White House Counsel. Accordingly, Mr. McGahn is not legally required to appear and testify about matters related to his official duties as Counsel to the President.”

Citing the DOJ memo, Cipollone sent a separate directive to McGahn from Trump, telling him that he was not to testify to Congress about his time in the White House.

“Because of this constitutional immunity, and in order to protect the prerogatives of the Office of the Presidency, the President directs Mr. McGahn not to appear at the Committee's scheduled hearing on Tuesday, May 21, 2019,” Cipollone wrote in the letter, which The Daily Beast obtained. “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.”

McGahn’s decision to skip Tuesday’s testimony and the OLC memo providing the legal foundation for him to do so is the latest chapter in the administration’s strategy of fighting virtually all elements of Democratic oversight following the issuance of the redacted version of the Special Counsel report on Russia’s electoral interference. But it is, perhaps, the most far-reaching part of that strategy to date.

The OLC memo argues that presidential advisors like McGahn would have to refuse to answer many questions from members of Congress because of executive privilege––the president’s right to keep his communications with his advisors about his duties from becoming public. It also lays out an argument Trump allies have been making ever since the inception of the Mueller probe: that letting White House aides testify to Mueller was not an automatic waiver of executive privilege, since that testimony stayed under the executive branch’s control. The argument is controversial, to say the least.

Hours after the memo was released, President Trump himself defended telling McGahn to stay mum.

“Well as I understand it, they’re doing that for the office of the presidency, for future presidents,” he said of the OLC memo’s objective. “I think it’s a very important precedent. And the attorneys say that they’re not doing that for me. They’re doing it for the office of the president. So we’re talking about the future.”

McGahn was the White House’s top lawyer for the first two years of the Trump administration, advising the president on how to navigate Special Counsel Robert Mueller’s investigation. He sat for more than 30 hours of interviews with the Mueller team—with the blessing of other White House lawyers—laying out in detail conversations where Trump asked him to interfere in the Mueller probe. Mueller ultimately chose not to decide whether or not to charge the president with obstruction of justice, but outside observers who argue he should have brought the charge point to McGahn’s statements as evidence that Trump may have broken the law.

According to the Mueller report, McGahn said that the president twice pushed him to have the special counsel fired, and also urged him to deny contemporaneous reports that he had asked him to do so. McGahn told Mueller he refused all those requests. The report also says that shortly after then-Attorney General Jeff Sessions recused himself from overseeing the Russia probe, Trump asked McGahn to “talk to Sessions” about revisiting the decision. When McGahn said he wouldn’t do that, the president “screamed” at him, per ex-White House advisor Steve Bannon’s testimony to Mueller.

McGahn is the report’s most-cited witness, by The New York Times’ count––157 times.

After the report came out, the White House still turned to McGahn for reputational clean-up. According to The Wall Street Journal, Trump asked McGahn to publicly say he didn’t think the president obstructed justice.

And Trump has publicly criticized his former confidant. “ I was NOT going to fire Bob Mueller, and did not fire Bob Mueller,” he tweeted the day after the Journal story broke. “In fact, he was allowed to finish his Report with unprecedented help from the Trump Administration. Actually, lawyer Don McGahn had a much better chance of being fired than Mueller. Never a big fan!”

This story has been updated with additional reporting.