Of all the questions that will be answered by the 2020 election, one matters above the others: Is Trumpism a temporary aberration or a long-term phenomenon? Put another way: Will the changes brought about by Donald Trump and today’s Republican Party fade away, or will they become entrenched? Trump’s reelection seems implausible to many people, as implausible as his election did before November 2016. But despite the scandals and chaos of his presidency, and despite his party’s midterm losses, he approaches 2020 with two factors in his favor. One is incumbency: Since 1980, voters have only once denied an incumbent...