Former FBI director James Comey published an op-ed last week asserting that President Trump has “eaten” Attorney General William Barr’s “soul.” Mr. Comey’s piece is a remarkable exercise in question-begging—a series of bald assertions that assume their own truth.

It begins with this rhetorical question: “How could Mr. Barr . . . start channeling the president in using words like ‘no collusion’ and FBI ‘spying’?” If you don’t already agree that Mr. Barr’s words are outrageous and borderline-criminal, Mr. Comey makes no effort...