An investigation wouldn’t necessarily lead to a prosecution. For one thing, investigators might conclude that Messrs. McCabe and Rosenstein were merely engaged in idle chatter. In this context, conspiracy requires both an agreement to defeat lawful government functions by dishonest means and an overt act in furtherance of that end. Canvassing cabinet members about their willingness to vote for the president’s removal—if that is what happened—would likely qualify. (Mr. McCabe has said Mr. Rosenstein believed two cabinet members would support the move, although he described this as “simply Rod thinking off the top of his head” and doesn’t think Mr. Rosenstein “actually sought support or talked to those people about it.”) Planning to record conversations with the president through an FBI wire, if substantiated, would also clear the bar for conspiracy.

Another challenge is that the chief witness would be Mr. McCabe, who has a credibility problem. He was fired from the FBI after the Justice Department inspector general concluded that he “lacked candor” in statements to investigators about his role in the bureau’s probe of Hillary Clinton. Still, now that his allegations have been publicly aired, they merit a prompt and vigorous investigation by the Justice Department. It would be bad enough if a conspiracy by government officials against American democracy went undiscovered, vastly worse if such a conspiracy is revealed and goes uninvestigated and unpunished.