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:

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“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.”