None of this is to suggest the president has absolute immunity from criminal obstruction-of-justice laws. He simply cannot be prosecuted for an otherwise lawful exercise of his constitutional powers. The cases of Richard Nixon and Bill Clinton —the latter impeached, and the former nearly so, for obstruction of justice—have contributed to today’s confusion. These were not criminal charges but articulations of “high crimes and misdemeanors,” the constitutional standard for impeachment.

And in neither case was the accusation based on the president’s exercise of his lawful constitutional powers. If a president authorizes the bribery of a witness to suppress truthful testimony, as Nixon was accused of doing, he can be said to have obstructed justice. Likewise if a president asks a potential witness to commit perjury in a judicial action having nothing to do with the exercise of his office, as Mr. Clinton was accused of doing.

Although neither man could have been prosecuted while in office without his consent, either could have been after leaving office. That’s why President Ford pardoned Nixon—to avoid the spectacle and poisonous political atmosphere of a criminal trial. In Mr. Trump’s case, by contrast, the president exercised the power to fire an executive-branch official whom he may dismiss for any reason, good or bad, or for no reason at all. To construe that as a crime would unravel America’s entire constitutional structure.