The episode raises the question of the obligations of a White House counsel when he realizes that he is the lawyer for a fundamentally dishonest president who is ready to violate the criminal law to achieve self-interested or political ends. The counsel in these circumstances may have to consider what it means for him to remain in this post for a president who considers him a “lying bastard” for refusing to follow an unlawful order.

Mr. McGahn perhaps stayed on in the belief that the larger objectives of the administration, like moving judicial nominations and achieving deregulation, were well worth pursuing. But that is a judgment more about the administration’s policy imperatives than the working conditions required for the maintenance of the rule of law in the presidency.

The choice Mr. McGahn faced was unprecedented. He was not, for example, in the position of John Dean, White House counsel to Richard Nixon, who did testify against the president in the Watergate affair but who was an original party to the wrongdoing that ended that presidency. There has never been a suggestion that Mr. McGahn ever encouraged or participated in unlawful activities.

In fact, Mr. McGahn acted appropriately and admirably to resist involvement in the president’s scheme to commit obstruction and cooperated truthfully and at length with Mr. Mueller’s investigation. The special counsel declared him a “credible” witness with no discernible motive to lie or exaggerate, and accepted his account over the president’s denials.

But should a future White House counsel have a clear obligation to alert the Department of Justice when the president attempts to obstruct justice? Federal law mandates that department and agency employees alert the attorney general to “any information” that relates to “violations of federal criminal law” involving government officers and employees. The code of ethics for government service requires reporting of “corruption” to the authorities. The application of these requirements to the president’s White House counsel poses unique and difficult issues, but they need to be confronted.

And what does the discovery of the president’s lawless instincts — or the president’s belief that his lawyer is a “lying bastard”— mean for the counsel’s continued service? A lawyer in this situation could conclude that he should not serve such a president, or that, lacking a relationship of mutual trust and confidence, he cannot function in the role. Mr. McGahn was stuck working with a president who, pressuring his counsel to end an investigation and fake records about a presidential order, also admonished him for taking notes. “Lawyers don’t take notes,” he told Mr. McGahn, and Mr. Trump urged on him the role model of Roy Cohn, counsel to Senator Joe McCarthy.

This is unexplored ground. In the government of a president like Mr. Trump, a White House counsel faces unprecedented challenges, and we might reflect on options for answering them beyond the threat or act of resignation.