White House Counsel Don McGahn (R) attends a cabinet meeting with U.S. President Donald Trump in the Cabinet Room at the White House October 17, 2018 in Washington, DC.

President Donald Trump on Monday directed former White House counsel Don McGahn not to comply with a subpoena to testify about special counsel Robert Mueller's Russia report, intensifying a clash between the Trump administration and the House Democrats who subpoenaed McGahn to appear.

In a 15-page legal memo from the Department of Justice and a separate letter to House Judiciary Committee Chairman Jerrold Nadler, D-N.Y., legal officials in the Trump administration argued that McGahn is "absolutely immune" under the law from being compelled to testify.

"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," Trump's counsel, Pat Cipollone, wrote in the letter to Nadler.

McGahn, who was scheduled to testify Tuesday morning, is cited more than any other witness in Mueller's report on Russian election meddling, possible conspiracy between the Kremlin and the Trump campaign, and possible obstruction by Trump himself. He was subpoenaed in late April by the House Judiciary Committee to appear before that panel and answer questions, as well as hand over a slew of documents.

"Congress may not constitutionally compel the president's senior advisors to testify about their official duties," Assistant Attorney General Steven Engel wrote in the legal opinion.

The Judiciary Committee had received Cipollone's letter Monday, a committee source told NBC News. But McGahn's legal team had not notified the panel about whether he would or would not appear. The committee still planned to hold a hearing at 10 a.m. ET, the source told NBC.

Trump signaled in early May that he would likely stop McGahn from testifying.

"Well, I've had him testifying already for 30 hours" with Mueller's team, Trump said of McGahn in a Fox News interview. "I don't think I can let him and then tell everybody else you can."

"I would say it's done," Trump added.

Press secretary Sarah Huckabee Sanders echoed Trump's view in a statement Monday afternoon, arguing that Democrats "want a wasteful and unnecessary do-over" of the Mueller probe.

"This action has been taken in order to ensure that future Presidents can effectively execute the responsibilities of the Office of the Presidency," Sanders said.