It's a move that appears to set the stage for another contempt vote to retaliate against the Trump administration for rejecting the demands of Congress, as committee Chairman Jerry Nadler threatened late Monday to do whatever was necessary to make McGahn appear before his panel, which convenes Tuesday at 10 a.m. ET.
"As with the subpoena for documents, Mr. McGahn again finds himself facing contradictory instructions from two co-equal branches of government," McGahn's attorney William A. Burck said in a letter to Nadler. He added later, "Under these circumstances, and also conscious of the duties he, as an attorney, owes to his former client, Mr. McGahn must decline to appear at the hearing tomorrow."
Hours later, in a letter sent Monday night, Nadler warned McGahn that the committee would use "all enforcement mechanisms at its disposal" to compel his testimony.
"The Justice Department has no place informing you about the potential remedies that congress may pursue in the exercise of its own Article I powers," Nadler wrote. "The Committee has made clear that you risk serious consequences if you do not appear tomorrow."
In an interview on CNN's "Cuomo Prime Time," Nadler added that "the first thing we're going to have to do is hold McGahn in contempt."
"That's the next step," he said.
Earlier Monday afternoon, the White House argued that as a former senior adviser to the President, he is exempt from having to appear before Congress. The Justice Department's Office of Legal Counsel concluded that former McGahn was not legally required to appear before the House Judiciary Committee and testify about matters related to his official duties as counsel to the President, according to a memo issued Monday and obtained by CNN.
"The Department of Justice 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," White House counsel Pat Cipollone wrote in a letter to Nadler.
White House press secretary Sarah Sanders said in a statement that McGahn "cannot be forced to give such testimony, and Mr. McGahn has been directed to act accordingly."
"This action has been taken in order to ensure that future Presidents can effectively execute the responsibilities of the Office of the Presidency," she said.
The White House's move to keep McGahn off of Capitol Hill is the latest in a slew of current and former Trump administration officials defying subpoenas from House Democrats, who are now grappling with how best to respond to the Trump administration's blanket resistance to their investigations into the President.
President Donald Trump sought to distance himself from his decision to direct McGahn not to testify Tuesday and sought to cast it as one rooted in presidential precedent.
"As I understand it they're doing that for the office of the presidency for future presidents. As I understand it it's a very important precedent," Trump said in comments on the White House lawn. "The attorneys say they're doing it not for me, they're doing it for the future."
It marks the second time McGahn has not complied with the Judiciary Committee's subpoena. He also deferred to the White House in refusing to provide documents that the committee subpoenaed related to McGahn's special counsel interview preparations, which the White House argued were covered by executive privilege.
Nadler, a Democrat from New York, did not move to hold McGahn in contempt after he would not provide those documents, but he has made clear he is likely to do so if McGahn does not appear Tuesday. The committee is expected to hold the hearing without him, as it did earlier this month for Attorney General William Barr when he did not testify over a dispute about the hearing format.
McGahn's testimony is of interest to Democrats in Congress because of the role that he played in special counsel Robert Mueller's investigation into whether the President obstructed justice. One of the key episodes the special counsel cited in the investigation, which did not exonerate Trump, was when the President told McGahn to fire Mueller and McGahn would not do so.
McGahn is now one of a number of officials who could be held in contempt by Congress.
Earlier this month, Nadler's committee voted to hold Barr in contempt for refusing to provide the unredacted Mueller report and underlying evidence to Congress. House Intelligence Chairman Adam Schiff says he will take an unspecified "enforcement action" against the Justice Department for not complying with the committee's subpoena for Mueller's counterintelligence information. And Treasury Secretary Steve Mnuchin defied the House Ways and Means Committee's subpoena for Trump's tax records last week.
The House is still considering how to handle contempt issues on the floor. One option being considered is to bundle up all of the contempt citations into a single vote to highlight the Trump administration's stonewalling. Another is to invoke the House's so-called "inherent contempt" powers to fine or jail officials who are held in contempt without using the court system, although such an action hasn't been taken in nearly a century.
In its memo Monday, the Justice Department argued that Congress cannot use its inherent contempt powers to punish McGahn for asserting immunity, in what appears to be a preemptive challenge to House Democrats as they contemplate their next steps.
"The constitutional separation of powers bars Congress from exercising its inherent contempt power in the face of presidential assertion of executive privilege," the memo says. "An attempt to exercise inherent contempt powers in such a circumstance would be without precedent and 'would immensely burden the President's ability to assert the privilege and to carry out his constitutional functions.'"
The White House's move to block McGahn has similarities to President George W. Bush's efforts to prevent his former White House counsel, Harriet Miers, from testifying. A federal judge in 2008 ruled against the Bush administration.
But that court ruling didn't stop the Obama administration from making similar arguments. In 2014, the Obama administration's Office of Legal Counsel wrote a memo that a senior White House adviser subpoenaed by the House Oversight Committee did not have to testify, arguing the President's immediate advisers had "absolute immunity from congressional compulsion to testify about matters that occur during the course of discharging their official duties."
Democrats argue the situation with McGahn is different than that case because he has already testified before the special counsel, therefore waiving executive privilege.
But the White House has argued that testifying before Mueller was different because it was a criminal investigation. And on Monday, the Justice Department said that privilege is a separate question than immunity, arguing that the precedent for claiming immunity goes back decades.
"We provide the same answer that the Department of Justice has repeatedly provided for nearly five decades: Congress may not constitutionally compel the President's senior advisers to testify about their official duties," DOJ wrote in its memo. "This testimonial immunity is rooted in the constitutional separation of powers and derives from the President's independence from Congress."
If Nadler goes to court to try to force McGahn's testimony, it's likely to be one of a number of judicial battles pitting congressional Democrats against the Trump administration.
Democrats' efforts to obtain the full, unredacted Mueller report and Trump's tax returns are also likely headed to court, and Trump and the Trump Organization have already sued to block subpoenas to Deutsche Bank, Capital One and an accounting firm that has prepared the President's financial statements.
This story has been updated to include additional developments Monday.
CNN's Jeremy Diamond, Kara Scannell and Jim Acosta contributed to this report.
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