A campaign marked by Donald J. Trump’s apocalyptic extremes has turned even darker in its final days.With voting already underway in battleground states, the former president on Friday escalated his threats to prosecute and imprison a wide range of people involved in elections and politics. Hours later, on the hugely popular “The Joe Rogan Experience” podcast, he said that the “enemy within,” a phrase he has used to describe political opponents, poses a bigger threat to the nation than North Korea.For eight years, Democrats have warned that Mr. Trump’s political ambitions have fueled some of the nation’s deepest divides. But...