In a letter to Nadler, current White House counsel Pat Cipollone said that McGhan had "constitutional immunity" and could not be compelled to testify before the committee, which subpoenaed him last month.
The Trump administration on Monday, via a legal opinion issued by the U.S. Department of Justice, has directed former White House counsel Don McGahn to disregard a subpoena issued by the House Judiciary Committee meant to compel testimony about the president's alleged efforts to obstruct justice by thwarting the investigation of Special Counsel Robert Mueller.
The decision is certain to deepen tensions between congressional Democrats and the Trump administration.
Lawmakers have been eager to hear from McGahn, who was a key figure in Mueller's Russian Federation investigation.
Mr McGahn left the White House in October to return to a Washington law firm, Jones Day, which represents the Trump campaign.
Team Trump thumbed its nose at House Judiciary Committee chair Jerry Nadler again Monday - telling him that ex-White House counsel Don McGhan will ignore a subpoena to testify before the panel on Tuesday. They've accused Trump and Attorney General William Barr of trying to stonewall and block Congress from carrying out its oversight duties. McGahn never publicly committed to testifying.
Shortly before the White House announcement, the Department of Justice issued a 15-page memo explaining the legal precedent for why the Trump administration is capable of keeping McGahn from testifying.
"The immunity of the president's immediate advisers from compelled congressional testimony on matters related to their official responsibilities has always been recognized and arises from the fundamental workings of the separation of powers", the Office of Legal Counsel opinion reads.
Trump has fumed about McGahn for months, after it became clear that much of Mueller's report was based on his testimony.
In a statement, White House Press Secretary Sarah Sanders said: 'At the President's direction, the White House has been completely transparent with the Special Counsel's investigation.
US Assistant Attorney General Steven Engel said: "Congress may not constitutionally compel the President's senior advisers to testify about their official duties".
A federal judge rejected a similar argument in 2008 in a dispute over a subpoena for Harriet Miers, who was White House counsel to George W. Bush.
The House Judiciary Committee voted to hold Barr in contempt of Congress for not turning over Mueller's full report and its underlying evidence. "The immunity of the President's immediate advisors from compelled congressional testimony on matters related to their official responsibilities has always been recognized and arises from the fundamental workings of the separation of powers".
