Far less scrutiny has been devoted to the challenge Mr. Mueller poses to the authority of the legislative branch. In this case, ironically, the challenge stems less from the aggressiveness of the special prosecutor than from the meekness of Congress. In between their public tributes to Mr. Mueller’s sterling character, too many in Congress seem to worry more about how they might be affecting his investigation than about what his investigation might be doing to theirs.

One small snapshot: Mr. Mueller, an unelected appointee, had the Trump memos written by former FBI Director James Comey even as the FBI was refusing to release them to the elected representatives of the American people.

When Mr. Mueller was appointed back in May, Sen. Lindsey Graham rightly noted that though he respected the decision, the appointment will “really limit what Congress can do, and it’s going to really limit what the public will know about this.” Alas, the South Carolina Republican went on to say that “we in Congress have to be very careful not to interfere in his lane.”