Ruth Marcus at the Post explains why the head of state is not supposed to ask the top cop how he voted.

In my country — in our country — the ruler does not call in the head of the state police and demand proof of loyalty. That is because in our country the ruler is an elected, term-limited official, and the state police is, or is supposed to be, an independent, professionalized entity.

The importance of that distinction becomes starkly obvious when the elected official is incompetent and malign in every possible way. A head of state who has integrity and a conscience is less likely to try to make the head of the police into a personal servant, while a head of state who is incompetent and malign is far more likely to do that. We’re getting to see what happens when a head of state who needs that rule the most is the least inclined to pay attention to it. Trump needs the rule against meddling with the FBI because he’s exactly the kind of guy who will meddle with the FBI.

As Andrew Kent, Susan Hennessey and Matthew Kahn, writing in Lawfare, reminded us after President Trump took the extraordinary step of firing FBI Director James B. Comey — just the second time that had happened in the history of the bureau — the 10-year term was established in the wake of Richard Nixon’s abuses of the FBI during Watergate. The Lawfare post notes, “Nixon’s acting FBI Director and nominee for the permanent post, L. Patrick Gray, had resigned in 1973 after it was revealed that he was giving the White House daily briefings on the FBI’s Watergate investigation and that he destroyed documents relevant to the inquiry.” Now that was loyalty.

So that explains Mark Felt aka Deep Throat, I guess. I need to brush up on Watergate.

Trump, as we know from Comey’s testimony, pressed Comey to pledge similar fealty before firing him. And now we know, from The Post’s Ellen Nakashima, Josh Dawsey and Devlin Barrett, that Trump summoned Comey’s temporary successor, shortly after Comey’s firing, for a get-to-know-you session in which the pleasantries quickly turned sinister: In the dark wake of Comey’s ouster, Trump wanted to know whether Andrew McCabe had voted for him or for his opponent, Hillary Clinton. This was stomach-churningly inappropriate.

It’s inappropriate in the terms Marcus is talking about but it’s also stomach-churning on another level, a level I don’t know exactly how to name. It’s personally disgusting; it underlines how disgusting Trump is as a person. That kind of sky-blotting-out egotism is profoundly repellent. He might as well have asked McCabe, “Do you think I’m awesome?”

A reminder about the history of FBI directors: President Jimmy Carter named a Republican, William H. Webster, to head the FBI. President Bill Clinton named a Republican, Louis Freeh, to the job. President Barack Obama extended the term of then-FBI director Robert S. Mueller III, a Republican, for two years. Then he named another Republican, Comey.

Of course, for appropriate positions, the president gets to name political appointees and to take into account, when naming those political appointees, their personal politics.  But our country has a civil service that provides a continuing corps of expertise from administration to administration — what Trumpists demean as the “deep state.” Nowhere is the independence of that group more essential than in the arena of law enforcement; nowhere has that independence more rankled Trump.

Since the election of Trump has underscored our reckless and destructive hostility to expertise, we need that independence more than ever.

The reason Trump is not supposed to be involved with the Justice Department or the FBI involves the power these institutions wield to investigate and prosecute crimes, the imperative that this power not be used to punish political enemies or shield political allies, and the accompanying imperative that the public be able to trust in the department’s impartiality.

Thus the George W. Bush Justice Department brought charges against Sen. Ted Stevens of Alaska, a Republican. Thus the Obama Justice Department brought charges against former Democratic vice presidential nominee John Edwards. That both these cases were flawed does not undercut my point about the need for apolitical justice — it strengthens it. Now the Trump Justice Department has decided to seek a retrial in the case of Sen. Robert Menendez, the New Jersey Democrat originally indicted during the Obama administration. How can the public, given the behavior of this administration, be confident that this move is untainted by politics?

The question answers itself.