Confronted with two historically unpopular presidential candidates, the American electorate in 2016 delivered a shock to the political system. Less noted, amid the drama of Donald Trump's victory, was the substantial share of the vote won by minor parties and independent candidates--one of whom, Libertarian Gary Johnson, put in the best third-party performance since Ross Perot's 1996 Reform Party bid. Even more surprising, at the state-level minor-party...