Mr. Trump’s direction of the prosecutions of his perceived enemies — notably James Comey, the former F.B.I. director, and Letitia James, the New York attorney general — has illustrated the folly of Chief Justice Roberts’s rationale. Between the court’s dissolving potential liability for bogus fraud investigations and Mr. Trump’s neutering of executive branch rules governing the independence of the Justice Department, the court has seemingly left the president free to target anyone he wishes.