As the late Sen. John McCain’s departing call to national unity reverberated across America last week, Donald Trump’s prediction of violence should Democrats prevail in November’s midterm elections seemed both discordant and, well, weird. The president issued his dire warning on Monday — two days after McCain’s death — to a gathering of evangelical pastors at the White House. Trump warned that if they didn’t rally their parishioners to turn out and vote Republican, Democrats “will overturn everything that we’ve done, and they’ll do it quickly and violently.” What sort of apocalyptic vision guides our commander in chief? What level...