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 has instructed former White House counsel Don McGahn to defy a subpoena to testify before Congress on Tuesday, his latest effort to stymie congressional Democrats who are investigating him and his administration.

The House Judiciary Committee had subpoenaed McGahn to appear before the panel to answer questions about the Russia probe, but the White House on Monday released a legal opinion from the Justice Department arguing the lawyer should not attend over concerns about separation of powers.

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“I think it’s a very important precedent,” Trump told reporters at the White House before leaving for a campaign rally in Pennsylvania. “They’re doing it for the office of the presidency.”

The Justice Department’s Office of Legal Counsel (OLC) said in its 15-page letter that McGahn is not “legally required” to testify about matters related to his work for Trump, citing decades of precedent stating that 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.

If McGahn follows the White House’s instructions, it will prevent Democratic lawmakers from hearing directly from a key witness in 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 investigation into whether Trump obstructed the probe into Russian interference in the 2016 election.

McGahn’s lawyer William Burck did not immediately respond when asked if he will abide by the White House’s request.

Democrats are sure to be angered by the move. Since the release last month of Mueller’s 448-page redacted report, Democrats have demanded to hear from McGahn about his account of Trump’s alleged attempt to block the investigation. His appearance was highly anticipated, with some in Washington drawing parallels to former White House counsel John Dean’s June 1973 testimony about then-President Nixon’s misdeeds during the Watergate scandal.

House Judiciary Committee 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.) in a statement accused Trump of obstructing the panel’s investigation and said the committee would convene as planned in the morning.

"[Trump] 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 said.

"This move is just the latest act of obstruction from the White House that includes its blanket refusal to cooperate with this Committee,” he said. "The Committee will convene as planned tomorrow morning, and Mr. McGahn is expected to appear as legally required."

In a letter to Nadler, current White House counsel Pat Cipollone cited the latest Justice Department opinion and past precedent under Democratic and Republican administrations to assert the committee does not have the power to subpoena McGahn even though he has left his White House post.

“This long-standing principle is firmly rooted in the Constitution's separation of powers and protects the core functions of the Presidency,” Cipollone wrote.

The committee subpoenaed McGahn last month to hand over documents and submit to public testimony on May 21 as part of a sprawling investigation into allegations of obstruction and abuses of power by Trump and his associates.

But the White House has repeatedly attempted to thwart Democratic investigators. Earlier this month, it instructed McGahn to defy the document request and asked the Judiciary panel to go directly to Cipollone’s office with such requests, arguing the records remain under control of the White House and implicate matters of executive privilege.

The White House has since rejected a similar document request from the committee, accusing the panel of attempting a “do-over” of Mueller’s investigation and asking it to narrow the scope.

Evading the appearance would set up the likelihood that the committee could take punitive measures against McGahn for defying the subpoena.

In a letter to McGahn’s attorney earlier this month, Nadler threatened to hold McGahn in contempt if he fails to comply with the subpoena.

“Mr. McGahn is required to appear and provide testimony before the Committee absent a court order authorizing non-compliance, as well as provide a privilege log for any documents withheld,” Nadler wrote on May 7.

“Otherwise, the committee will have no choice but to resort to contempt proceedings to ensure that it has access to the information it requires to fulfill its constitutionally mandated duties,” he added.

The Justice Department OLC opinion released Monday also argues that Congress cannot invoke its inherent contempt power to punish McGahn for evading testimony.

“The constitutional separation of powers bars Congress from exercising its inherent contempt power in the face of a Presidential assertion of executive privilege,” the letter states.

The little-used inherent contempt authority allows Congress to punish individuals who fail to comply with congressional subpoenas by jailing them or imposing fines; the power has rarely been used, but Democrats such as Nadler have discussed reviving it in order to force Trump administration officials to comply with their investigations.

As of Monday afternoon, the Judiciary panel had not yet received a formal response from McGahn’s attorneys, a committee source told The Hill. The committee is still expected to convene the hearing, even if McGahn does not ultimately show up.

Trump has signaled for weeks that he would likely assert executive privilege to block McGahn, who is viewed as a key witness for the committee as Democrats look to investigate possible wrongdoing by the president. McGahn’s testimony is featured prominently in the volume of Mueller’s report examining potential acts of obstruction by Trump.

McGahn spent hours speaking with investigators from Mueller’s office, supplying testimony and contemporaneous notes that detailed Trump’s efforts to remove the special counsel and then cover it up in the media. The former White House counsel described Trump’s orders to oust Mueller as “crazy shit,” according to the special counsel’s report.

Mueller did not ultimately reach a judgment on whether Trump obstructed justice, but Attorney General William Barr Bill BarrHarris faces pivotal moment with Supreme Court battle Hillicon Valley: DOJ proposes tech liability shield reform to Congress | Treasury sanctions individuals, groups tied to Russian malign influence activities | House Republican introduces bill to set standards for self-driving cars McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE reviewed the evidence laid out in the report and judged it to be insufficient to accuse Trump of obstruction.

Trump has taken repeated steps to thwart Democrat-led investigations in the House, accusing them of trying to cause him political damage ahead of the 2020 election. The White House has repeatedly denied Democrats’ requests for documents and testimony, accusing them of overreach.

Trump has also asserted executive privilege over the entire Mueller report and underlying evidence, for which Nadler’s committee has subpoenaed Barr. The committee voted strictly along party lines in early May to hold Barr in contempt for not complying with the subpoena, but the lower chamber has not yet taken up the resolution for a vote by the full House.

--Olivia Beavers contributed to this report, which was updated Monday at 6:01 p.m.