Walter Shaub points out that Nixon’s downfall established the principle that a president may not select his investigator.

Now with the Senate’s likely confirmation of William Barr as attorney general, Trump may succeed in destroying this principle. Barr’s nomination is before the Senate only because Trump fired Attorney General Jeff Sessions for refusing to stop special counsel Robert Mueller’s investigation. (Sessions technically resigned, but a “resignation” requested by the president is Washington-speak for “fired.”)

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Barr is infinitely more qualified than acting attorney general Matthew Whitaker, but Barr and Whitaker have something in common: They both auditioned for the job by making sure Trump knew they opposed the special counsel investigation. Whitaker made his views known in television appearances and op-eds, and Barr sent the Justice Department an unsolicited 20-page memorandum challenging the scope of the investigation.

Which is not, many commentators have pointed out, a routine or normal thing to do. Lawyers don’t just send 20 page memos of unsolicited expensive legal opinion to presidents or the DOJ when the mood strikes them. That’s not a thing. It’s a not-thing. The fact that it’s a not-thing makes it suspect.

Barr has also displayed his partisanship in the media. In an October 2016 Washington Post opinion piece headlined, “James Comey did the right thing,” Barr defended Comey for releasing information about an investigation of presidential candidate Hillary Clinton shortly before the election. Trump later fired Comey. Then, on May 11, 2017, Trump admitted on television that the firing was motivated by the investigation of his campaign. The next day, Barr raced to Trump’s defense with a new opinion piece condemning Comey for having released information about the Clinton investigation.

Oh really; I didn’t know that. Worse and worse.

But the problem is bigger than Barr. Confirming any Trump nominee for the attorney general position, without requiring the nominee to commit to recusing from the special counsel investigation, would put an end to the principle that presidents may not choose their investigators. The Senate majority put this principle on life support when it confirmed an FBI director after Comey’s firing. If it confirms a replacement for Sessions without demanding recusal, the Republican majority will pull the plug on the patient.

Maybe Mueller and the US attorneys will yet outwit the conspirators against them, but Barr’s confirmation without a commitment of recusal would change the presidency. It would set a dangerous new precedent that presidents are free to fire law enforcement officials for investigating them, and to choose their replacements.

To protect the rule of law, the Senate must demand, as a condition of confirmation, that Barr agree to recuse himself from the special counsel investigation.

Will the Senate demand that? No. Mitch McConnell will see to it that they don’t.