In 2016, there are 14 Republican presidential candidates for whom Ronald Reagan is both the benchmark for conservative values and the lodestar of conservative ideas. There’s also one who wrote, in the second to last year of Reagan’s presidency, that he had been “so smooth, so effective a performer” that “only now, seven years later, are people beginning to question whether there’s anything beneath that smile.” The gadfly was Donald Trump, writing in his book The Art of the Deal. But it wasn’t just a glancing blow; to promote the book, Trump launched a political campaign that tore into Reagan’s...