One of Donald Trump’s more dubious constitutional notions is that the attorney general serves to protect the president, rather than serving as a lawyer for the American people. He fired the office’s previous holder, Jeff Sessions, for failing to provide him personal legal cover in the Russia investigation.

Now he has selected William Barr as Mr. Sessions’ replacement, and one is compelled to ask exactly what President Trump will expect from his new nominee. Because Mr. Barr previously served as attorney general, under George H.W. Bush, many observers assume the Senate might give him an easy confirmation. In fact, it should do the exact opposite.

“Where’s my Roy Cohn?” That’s the phrase attributed to President Trump at the tail end of a reported presidential rant about Mr. Sessions’ decision to recuse himself from overseeing the Russia investigation. Cohn, a top aide to Senator Joseph McCarthy, was Mr. Trump’s personal lawyer and fixer throughout the 1970s, a man with a reputation for pushing legal limits and employing all manner of dirty tricks to protect his client; he ran afoul of the law himself, resulting in his disbarment in 1986.

Mr. Trump’s contempt for Mr. Sessions centered around his recusal and unwillingness to shield the president from the Russia investigation. In longing for a Roy Cohn replica, the president made clear that he was seeking not a skilled and experienced attorney who would help him navigate choppy legal waters but rather a legal bruiser who would stop at nothing to protect him from scrutiny. It is no stretch to think that the ability of the next attorney general to do just that might have been on the president’s mind when making his selection.