Donald Trump’s lawyers have signaled he won’t agree to a voluntary interview with special counsel Robert Mueller. If Mr. Mueller insists, he will have to subpoena the president. To enforce a subpoena, the special counsel would have to go to court and meet a highly exacting standard, showing what he wants and why he needs it. He would be unlikely to succeed, given that Mr. Trump already has cooperated extensively with the investigation, producing 1.4 million documents and making dozens of White House staffers available for interviews.

The leading precedent is a 1997 opinion, In re Sealed Case, by the U.S. Circuit Court of Appeals for the District of Columbia. The case involved the independent counsel investigation of former Agriculture Secretary Mike Espy, who was accused of receiving unlawful gifts. The independent counsel sought to obtain sensitive documents produced in the course of an internal White House inquiry. These materials involved the preparation of a report to then-President Clinton himself. Although Mr. Clinton had directed that most of the materials be provided, he asserted executive privilege to withhold some items.

At issue in particular was information regarding whether Mr. Clinton should discipline or fire Mr. Espy, who did resign. To justify producing such sensitive materials involving “the exercise of [the president’s] appointment and removal power, a quintessential and non-delegable presidential power,” the court required the independent counsel to demonstrate with “specificity” why he needed the materials and why he could not get them, or equivalent evidence, from another source. (Mr. Espy was acquitted in 1998.)

Mr. Mueller’s initial charge was to investigate Russian interference in the 2016 presidential election and possible collusion between the Trump campaign and the Russian government. But his investigation has expanded to cover whether Mr. Trump has obstructed justice. The president’s critics say his obstructive acts include urging then-FBI Director James Comey to “go easy” on former national security adviser Mike Flynn, subsequently firing Mr. Comey, and his public criticism of Mr. Mueller, Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein.

There are significant factual disputes about these episodes, but all involve the president’s exercise of his core constitutional powers as chief executive, including the power to appoint and remove high-level executive-branch officials, to supervise the performance of their duties (as in the Espy case), and to determine law-enforcement priorities. We have argued in these pages that the president cannot obstruct justice by exercising the discretionary powers of his office, especially in determining whether and why to fire high-level presidential appointees like Mr. Comey. According to the two leaked letters from Mr. Trump’s lawyers to Mr. Mueller, they take essentially the same view.