The U.S. Justice Department said Monday that President Donald Trump could block former White House Counsel Donald McGahn from complying with a congressional subpoena demanding his testimony related to Special Counsel Robert Mueller III's investigation.

The release of the Justice Department Office of Legal Counsel's 15-page opinion came as the White House for a second time instructed McGahn, now a Jones Day partner, to defy a subpoena issued by the U.S. House Judiciary Committee, escalating a standoff between the Trump administration and Democratic lawmakers. The judiciary committee had subpoenaed McGahn to appear in a Tuesday morning hearing.

Read the opinion here:

“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,” Assistant Attorney General Steven Engel said in the Monday opinion. “This testimonial immunity is rooted in the constitutional separation of powers and derives from the President's independence from Congress.”

The OLC pointed to the office's previous guidance, including one 1999 opinion published during the Bill Clinton administration, that explained that a congressional subpoena requiring a senior presidential adviser to testify would be “akin to” requiring the president himself to appear.

OLC argued that a former White House Counsel fits the description of a senior adviser.

The opinion also rejected arguments from Nadler and other Democratic lawmakers that the White House's decision to not assert executive privilege over Mueller's report amounted to a waiver of privilege over McGahn's communications with Trump.

While the opinion said Congress cannot compel McGahn to testify before lawmakers, it does not necessarily bar McGahn from voluntarily speaking to Congress as a private citizen.

McGahn's attorney, William Burck of Quinn Emanuel Urquhart & Sullivan, informed Nadler in a letter Monday that his client would not appear before lawmakers. Burck's letter said McGahn was following the White House's instruction, consistent with the OLC opinion.

“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,” the letter, obtained by the National Law Journal, said.

Burck also noted Nadler's earlier threat to potentially hold McGahn in contempt for defying the subpoena. “While we disagree with the Committee's position and hope it will instead seek an accommodation with the White House, Mr. McGahn also must honor his ethical and legal obligations as a former senior lawyer and senior advisor to the President. In short, it is our view that the Committee's dispute is not with Mr. McGahn but with the White House,” Burck said.

Nadler slammed the White House in a statement Monday, describing its move Monday as the “latest act of obstruction from the White House that includes its blanket refusal to cooperate with this Committee.” His statement also noted that a federal court previously rejected similar arguments advanced by the George W. Bush White House in a case dealing with a congressional subpoena for former White House counsel Harriet Miers' testimony.

McGahn had earlier refused to comply with the earlier House Judiciary Committee subpoena, which also sought records related to his cooperation with the special counsel. Burck said at the time he was deferring to the instructions of the White House.

Burck did not immediately respond to a request for comment Monday.