Maryland’s Brian Frosh and 17 of his fellow state attorneys general dispatched a letter last week to Acting U.S. Attorney General Matthew Whitaker that ought to be required reading in the White House as well as the Justice Department. That’s not too much to ask, as it’s just four paragraphs long. The dozen and half top law enforcement officers simply point out that Mr. Whitaker should not be supervising Special Counsel Robert Mueller’s investigation of Russian interference in the last election.

Why? That’s the simplicity of it. Mr. Whitaker has expressed strong and highly partisan views in various media outlets not just lambasting the investigation but specifically suggesting various back-door methods of undermining it, such as reducing the special counsel’s budget or limiting his authority. The whole point of appointing a special counsel is to be above reproach. After playing by those rules for 18 months by leaving the matter to Deputy Attorney General Rod Rosenstein, it’s ridiculous to compromise the inquiry in its final hours.

“By all appearances, Deputy Attorney General Rosenstein has ably supervised the Special Counsel’s investigation from its outset,” the AGs wrote in their Nov. 8 missive. “He should continue to do so, as Mr. Mueller’s work must proceed free from interference or supervision that would appear to many Americans to be biased.”

What the attorneys general didn’t mention but likely all would acknowledge is that Mr. Whitaker is in a particularly weak position. There are doubts about the constitutionality of his own appointment (from none other than George T. Conway III, spouse of White House counselor Kellyanne Conway, as spelled out in a recent New York Times op-ed he co-authored with a former acting solicitor general). And there are more fundamental reservations about Mr. Whitaker’s background: His resume is thin. He has never faced Senate confirmation required of top Justice officials. He is chiefly from a political background (as a former Republican candidate for elected office as recently as four years ago) and only briefly worked as a U.S. attorney.

And then there’s the whole “I know him” and “I don’t know him” back and forth from President Donald Trump himself. In a denial worthy of Peter, the president claimed not to know Mr. Whitaker two days after appointing him acting attorney general — despite having assured Fox News weeks earlier, “I know him.” The records show the two men have been in meetings together at least a dozen times. In the law enforcement business, the president’s abrupt change-of-story is called suspicious behavior. In the bible, it would simply be time to cue the rooster.

Now, it’s entirely possible (if clearly not probable) that Mr. Trump asked Jeff Sessions to step down and appointed Mr. Whitaker in his place temporarily simply out of disgust with Mr. Sessions, whom he has been publicly ridiculing for months, and the president bypassed Mr. Rosenstein because he was uncomfortable with him in the top post. If that’s the case, it’s yet another argument to recuse. By not interfering with the Mueller probe, Mr. Whitaker allows the president and his family the opportunity to be exonerated. Take action now, and the findings are tainted. Mr. Trump would only be giving fuel to the House Democrats and a potential impeachment push.

Americans don’t need a law degree — or one in ethics — to understand that someone under investigation should not have the authority to tell his investigator what to do — even secondhand. That’s the whole point of having a special counsel in the first place, to have someone removed from the political process follow the facts where they lead. During his many years as a U.S. attorney in Maryland, Mr. Rosenstein developed a reputation as a straight shooter who took public corruption seriously but was not especially political. As we’ve observed before, a Republican who prosecuted plenty of Democratic officeholders in Maryland can surely be trusted to supervise an investigation that involves Republicans in Washington. Putting aside his recent gag-inducing observation that Mr. Whitaker is a “superb choice” to be acting attorney general, Mr. Rosenstein is nobody’s lackey. It just that he’s working for a president who only trusts lackeys. Mr. Whitaker must recuse himself for Mr. Trump’s own sake.