The head of the House Judiciary Committee blasted the White House on Monday for ordering former White House counsel Don McGahn not to testify before Congress, calling the request the "latest act of obstruction" by the Trump administration.

Judiciary Chairman Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.) said special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's report revealed "a shocking pattern of obstruction of justice," a pattern he claims is continuing because the administration is obstructing congressional oversight.

"This move is just the latest act of obstruction from the White House that includes its blanket refusal to cooperate with this Committee. It is also the latest example of this Administration’s disdain for law," Nadler said in a statement.

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McGahn is seen as a key witness for Democrats, particularly because of the extensive interview he gave to the special counsel, including testimony about the president's attempts to fire Mueller during his 22-month long investigation into Russian interference in the 2016 election.

"Don McGahn personally witnessed the most egregious of these acts," Nadler said. "President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE knows this. He clearly does not want the American people to hear firsthand about his alleged misconduct, and so he has attempted to block Mr. McGahn from speaking in public tomorrow."

Nadler's comments come hours after President Trump instructed McGahn to defy Nadler's subpoena to testify publicly before Congress on Tuesday.

“The former counsel to the president cannot be forced to give such testimony, and Mr. McGahn has been directed to act accordingly,” White House press secretary Sarah Huckabee Sanders said in a statement. “This action has been taken in order to ensure that future presidents can effectively execute the responsibilities of the office of the presidency.”

The White House released a 15-page legal opinion letter from the Justice Department's Office of Legal Counsel (OLC) that argued McGahn—who is under subpoena to appear—should not attend due to concerns about separation of powers. The OLC also argued that McGahn is not “legally required” to testify about matters related to his work for Trump.

The DOJ opinion pointed to decades of precedent in claiming White House aides cannot be compelled to speak to Congress about their official duties.

“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,” the letter states.

Nadler argued otherwise.

"This identical approach was rejected by a federal court in the Miers case, which held that even senior advisors to the President cannot simply refuse to appear in response to a congressional subpoena. It is absurd for President Trump to claim privilege as to this witness’s testimony when that testimony was already described publicly in the Mueller Report. Even more ridiculous is the extension of the privilege to cover events before and after Mr. McGahn’s service in the White House," he said.

Nadler noted that the committee will gavel in as planned on Tuesday morning.

Nadler's statement is the latest in an ongoing tug-of-war as House Democrats attempt to compel both current and former Trump administration officials to cooperate with their multiple, sprawling investigations into the president, while the White House seeks to fight back against their investigative inquiries.