“At some point, a pattern of the same conduct indicates willfulness and intentionality,” said John Q. Barrett, a law professor at St. John’s University and former associate special counsel in the Iran-Contra affair.

The fact that it would be legal for Trump to demand that Mueller be fired, or that it is legal for the president to dismiss an FBI director, would not necessarily stifle an obstruction case. There could still be obstruction if the president were using his official powers for a corrupt purpose; the crucial factor is whether he had “corrupt intent.” For example, prosecutors have discretion to decline to press charges in any particular case, but if they have taken money to do so, they have still broken the law.

“It’s not an absolute impediment that the president can fire the FBI director at will,” said Bruce Green, a law professor at Fordham University and former associate counsel in the Iran-Contra affair. “Otherwise lawful acts can be the basis of an obstruction charge if committed with corrupt intent.”

Since Mueller himself is not a witness to the attempt to fire him, it would not create a conflict of interest for the special prosecutor to include the incident as part of a hypothetical obstruction case, particularly if Mueller concludes that a sitting president is not prosecutable and refers the case to the House of Representatives for potential impeachment.

“Federal prosecutors’ offices do not recuse themselves in situations like this,” said Green. “For example, where a witness commits perjury in an investigation overseen by a particular federal prosecutor, that prosecutor can pursue a perjury prosecution.”

In February 2017, Trump reportedly attempted to pressure then-FBI Director James Comey to end the investigation into former National-Security Adviser Michael Flynn. Trump also reportedly pressured Director of National Intelligence Dan Coats, who oversees U.S. intelligence agencies, to urge Comey to end his investigation into Flynn—attempting to use the CIA to stifle the FBI investigation into Watergate was among the articles of impeachment against Nixon. Flynn pled guilty in early December to misleading federal investigators about whether he had contact with Russian officials about lifting newly imposed sanctions levied as retaliation for Russian interference in the 2016 election.

The official White House explanation for firing Flynn was that he had lied to Vice President Pence. But former acting Attorney General Sally Yates testified that she told McGahn, the White House counsel, that Flynn had lied about his contacts with Russia weeks before Flynn was actually fired.

In May, Comey was fired, just two months after publicly confirming the investigation into Russian interference. The White House initially said that Comey was fired because he harmed the integrity of the FBI by publicly releasing “derogatory information” about Hillary Clinton during the investigation into her private email server. Trump later said he fired Comey because of the “Russia thing.” After doing so, Trump told Russian officials in the Oval Office that firing Comey had relieved “great pressure” on him.