The House Judiciary Committee has issued a subpoena to compel McGahn to testify by Tuesday, and the committee's chairman, Rep. Jerrold Nadler, D-N.Y., has threatened to hold McGahn in contempt of Congress if he doesn't testify.
"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.
He said the president had only suggested that Mr Mueller be "replaced" because of a perceived conflict of interest - and then instructed Mr McGahn to correct inaccurate media reports.
The move to bar McGahn from answering lawmakers' questions is certain to anger House Democrats already eager to hit back at what they view as White House stonewalling. I urge Mr. McGahn to comply with the subpoena. "This testimonial immunity is rooted in constituional separation of powers and derives from the President's independence from Congress". He has bashed the former White House counsel on Twitter and has insisted to advisers that the lawyer not be allowed to humiliate him in front of Congress, much like his former fixer Michael Cohen did, according to Associated Press, citing an official and Republican.
McGahn was also dispatched by Trump to convince the former attorney general Jeff Sessions not to recuse himself from overseeing the Russian Federation investigation.
The letters also followed a decision by Barr not to appear himself at a Judiciary Committee hearing over differences in its rules for questioning, and Barr's decision not to give Nadler's committee the full, unredacted report by special counsel Mueller.
In a letter to the House Judiciary Committee earlier this month, White House counsel Pat A. Cipollone said McGahn does not have the legal right to comply with its subpoena for 36 types of documents - most related to Mueller's almost two-year probe of Russian interference in the 2016 election.
The Judiciary Committee still plans on meeting even if McGahn doesn't show up. That vote must still go to the full House of Representatives.
Members of Congress may use lawsuits to challenge the denials they've encountered from the administration in accessing witnesses and documents, but those cases could take months or years to resolve.
Our democracy is built on an understanding that one branch of government won't systematically obstruct another.