The White House announced onMonday that President Donald Trump has instructed former White House counsel Don McGahn to ignore a congressional subpoena for testimony.

To justify the move, the White House pointed to a new Justice Department memo that says Congress cannot force a presidential adviser to testify.

But legal experts were quick to point out that the memo didn't account for one key possibility: McGahn choosing to testify.

"T here's nothing in [the memo] that remotely supports the idea that McGahn can somehow be 'blocked from testifying," one law professor said.

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The White House announced on Monday that President Donald Trump instructed the former White House counsel Don McGahn not to comply with a congressional subpoena for testimony related to the Russia investigation.

In a statement, White House press secretary Sarah Sanders pointed to a new memo from the Justice Department's Office of Legal Counsel "stating that, based on long-standing, bipartisan, and Constitutional precedent, the former Counsel to the President cannot be forced to give such testimony, and Mr. McGahn has been directed to act accordingly."

Indeed, the memo said, "Congress may not constitutionally compel the President's senior advisers to testify about their official duties."

But legal experts were quick to point out the opinion doesn't account for one important possibility: McGahn choosing to testify before lawmakers.

Read more: White House officials asked Don McGahn to publicly say Trump didn't obstruct justice after they saw Mueller's report

'McGahn does NOT have to comply with Trump's instruction'

"The key here is what the OLC opinion does not say — that McGahn can be barred from voluntarily testifying before Congress," Steve Vladeck, a professor at the University of Texas School of Law, wrote on Twitter. "All it says on that front is that he can't be punished for refusing to testify (Part IV), not that the President has any means of stopping him from doing so."

"Whether or not the OLC opinion is correct that McGahn can't be punished for refusing to testify (which is the weakest part of the analysis, in my view), there's nothing in these 15 pages that remotely supports the idea that McGahn can somehow be 'blocked from testifying,'" Vladeck added.

Eric Columbus, who served as senior counsel to the deputy attorney general under former President Barack Obama, echoed that view, tweeting, "McGahn does NOT have to comply with Trump's instruction."

The OLC memo said, "Coercing senior presidential advisers into situations where they must repeatedly decline to provide answers, citing executive privilege, would be inefficient and contrary to good-faith governance."

But "executive privilege has no force against a former employee who wants to testify," Columbus tweeted.

Read more: The FBI's former top lawyer said Mueller's obstruction findings are 'alarming' and show a 'pattern of corruption'

He also said McGahn cannot assert attorney-client privilege in refusing to testify because the Court of Appeals for the District of Columbia Circuit has ruled that there is no attorney-client privilege between the White House counsel and the president.

But McGahn does have an incentive to comply with Trump's order

But several observers said McGahn has incentive to comply with Trump's order, even if he isn't legally required to. McGahn is a prominent player in Republican political circles, and defying the White House could not only put his own career at risk but also prompt the president and his allies to urge others to boycott McGahn's law firm, Jones Day.

Indeed, McGahn has walked a tightrope since the special counsel Robert Mueller's final report in the Russia investigation was released. Although prosecutors did not make a decision on whether Trump was guilty of obstruction of justice, the document laid out an extensive road map of evidence against the president, much of which came from McGahn when he testified for more than 30 hours to Mueller's office.

Trump and his lawyers promptly attacked McGahn's credibility and accused him of falsifying his testimony. Although McGahn's lawyer said in a statement that what his client told Mueller was true — and backed up by corroborating evidence — he emphasized that "Don, nonetheless, appreciates that the President gave him the opportunity to serve as White House Counsel, and assist him with his signature accomplishments."

Renato Mariotti, a former federal prosecutor in Chicago, highlighted how McGahn is likely to comply with Trump's order.

The probable solution, he added, is that "Democrats will have to hold him in contempt and then go to court to compel his testimony."