Most people take for granted that both Mr. Mueller and the new attorney general, William Barr, accept the current Justice Department legal position — reached in a 2000 opinion — that a sitting president cannot be indicted. In a June 2018 memo, Mr. Barr said that under “the Framers’ plan,” the “proper mechanism for policing the president’s” actions “is the political process — that is, the People, acting either directly, or through their elected representatives in Congress.”

Yet since 1973, the Justice Department has revisited its position five times on the question of indicting a sitting president and reached different conclusions. In fact, as executive assistant to President Richard Nixon’s attorney general, Elliot Richardson, I can speak to the circumstances that delivered that first opinion: The principal purpose of the 1973 Watergate-era legal opinion — which concluded that a sitting president cannot be indicted — was to aid in removal from office of a criminally tainted vice president, who, the memo concluded, could be indicted.

But it was not intended to set an ironclad precedent that would forever shape how a president might be treated.