The memorandum then identifies the fatal offense of any FBI leader—the usurpation of the authority of the Justice Department itself. In a power grab, Mr. Comey had announced the ultimate prosecutorial decision, namely that Mrs. Clinton would not be prosecuted. The FBI director had no authority to do that. That was not all. Mr. Comey, the memo went on, “compounded the error” by holding a press conference releasing “derogatory information about the subject of a declined criminal investigation.” This was all way outside the foul lines of Justice Department professionalism.

Succinctly, but with devastating effectiveness, the Rosenstein memorandum demonstrates Mr. Comey’s egregious violations of long-settled Justice Department practice and policy. Mr. Rosenstein draws from the director’s testimony before Congress and his unprecedented letter to Congress days before the election. He addresses Mr. Comey’s argument that had he failed to insert himself once again into the presidential campaign—as voting was already under way in many states—it would have constituted “concealment.”

Balderdash, the deputy attorney general concludes, albeit in more polite language. Prosecutors, to say nothing of FBI directors, are not to set out a confidence-shattering bill of particulars with respect to any potential defendant’s conduct, and certainly not a presidential candidate in the heat of a national campaign.