Judge T.S. Ellis has expressed skepticism about the scope of special counsel Robert Mueller’s investigation. “What we don’t want in this country is . . . anyone with unfettered power,” Judge Ellis, who is to preside over the trial of former Trump campaign manager Paul Manafort, told prosecutor Deputy Solicitor General Michael Dreeben May 4. “So it’s unlikely you’re going to persuade me that the special prosecutor has unlimited powers.”

Judge Ellis is right to be skeptical. Mr. Mueller’s investigation has crossed a constitutional...