In the memo, Mr. Barr wrote he sent it as a “former official” who hoped his “views may be useful.” He wrote he was concerned about the part of Mr. Mueller’s probe that, according to news reports in the Journal and elsewhere, has explored whether Mr. Trump obstructed justice in asking then-FBI director James Comeyto drop an investigation into former national security adviser Mike Flynn’s contacts with Russia, and by later firing Mr. Comey.

Mr. Barr’s memo, dated June 8 and sent to Deputy Attorney General Rod Rosenstein, argues that, based on the facts as he understands them, the president was acting well within his executive-branch authority…

“As I understand it, his theory is premised on a novel and legally insupportable reading of the law,” Mr. Barr wrote. “Moreover, in my view, if credited by the Justice Department, it would have grave consequences far beyond the immediate confines of this case and would do lasting damage to the Presidency and to the administration of law within the Executive branch.”