Trump’s pardon of Arpaio will likely rank among the most controversial exercises of the power. Other presidents have used it to absolve pirates, participants in tax rebellions, ex-Confederates, polygamous Mormons, socialists, anarchists, rumrunners, Puerto Rican nationalists, Vietnam War draft-dodgers, and Jimmy Hoffa. But only rarely has it been used to absolve a prominent political backer—much less one as divisive as Arpaio.