Trump says he alone can do it. His AG nominee usually agrees.

Mr. Barr’s argument derived from his broad view of executive power: The Constitution, he claimed, does not permit Congress to make it a crime for the president to exercise his executive powers corruptly — even if he were to fire a subordinate, pardon someone or use what Mr. Barr termed his “complete authority to start or stop a law enforcement proceeding” to cover up crimes by himself or his associates.

The claim that the framers of the Constitution empowered presidents to impede investigations for corrupt ends goes too far, many legal scholars say. But Supreme Court precedents offer few definitive guideposts, giving the attorney general broad latitude.

“The interpretive approach of Justice Department lawyers to the Constitution is very important because many separation-of-powers issues never wind up in court,” said Peter Shane, an Ohio State University law professor. “Barr’s method is not uniquely his, but it does represent a particularly aggressive school of executive power thought.”