Noah Feldman, a law professor at Harvard, argued before the pardon was issued that such a move “would express presidential contempt for the Constitution.”

“Arpaio didn’t just violate a law passed by Congress,” Professor Feldman wrote on Bloomberg View. “His actions defied the Constitution itself, the bedrock of the entire system of government.” By saying Mr. Arpaio’s offense was forgivable, Professor Feldman added, Mr. Trump threatens “the very structure on which is right to pardon is based.”

It was the first act of outright defiance against the judiciary by a president who has not been shy about criticizing federal judges who ruled against his businesses and policies. But while the move may have been unusual, there is nothing in the text of the Constitution’s pardons clause to suggest that he exceeded his authority.