The reason Mr. Trump replaced Mr. Sessions with Mr. Whitaker seems clear. When The Daily Caller, a conservative news website, asked Mr. Trump last week for his thoughts about the man now running the Justice Department, the president volunteered, “As far as I’m concerned, this is an investigation that should have never been brought. It should have never been had. It’s something that should have never been brought. It’s an illegal investigation.”

Mr. Whitaker is an avowed antagonist of Mr. Mueller — he has called the investigation a witch hunt, said Mr. Mueller’s team should not investigate Mr. Trump’s finances and suggested that an attorney general could slash the special counsel’s budget.

As if concerns about the Constitution, the law and Mr. Whitaker’s judgment weren’t enough, the broader picture that has emerged about Mr. Whitaker is even more disturbing. He has expressed skepticism toward Marbury v. Madison, the landmark case that established the concept of judicial review; he would support the confirmation of federal judges who hold “a biblical view of justice”; he may have prosecuted a political opponent for improper reasons when he was a federal prosecutor in Iowa; and then there’s the fiasco of his business involvement with a company accused of scamming customers that is being investigated by the F.B.I.

Justice Department regulations governing the day-to-day operations of the special counsel’s office allow for Mr. Whitaker to be read in on many of its inner workings, including that the acting attorney general be given “an explanation for any investigative or prosecutorial step” that Mr. Mueller decides to take. So there is nothing to keep Mr. Whitaker from being the president’s eyes and ears inside the most closely guarded investigation in the history of American politics.