Donald Trump says he will appoint a special prosecutor expressly to put Hillary Clinton in jail. You’d expect this kind of thing from a tin-pot dictator, but not from an American president. It’s not like we haven’t seen special prosecutors hired (and fired) for political reasons. Whitewater and Watergate come to mind. Neither went well. This one wouldn’t either, given the FBI’s closely reasoned arguments against pressing charges. Trump’s threat, made during a campaign, to use elected office to punish his political opponent is unprecedented in modern times. It offers the clearest proof yet of his unfitness for the presidency.