Last month, as President Trump made broad claims about his power to pardon, I noted that he “may find out that something can be both legal and, simultaneously, an impeachable offense.” Last night, as the president issued a pardon to former Maricopa County, Ariz., sheriff Joe Arpaio, who was convicted of criminal contempt of court, some commentators argued that this was exactly the case. Harvard Law professor Noah Feldman, for example, wrote after Trump’s belligerent Phoenix rally speech that such a pardon would represent an “assault on the federal judiciary, the Constitution and the rule of law itself” for which...