As he fell further behind in polls and battled allegations of sexual misconduct in recent days, Donald Trump moved to darker corners. He sketched out conspiracies involving global bankers, casually threatened to jail his political opponent, and warned in increasingly specific terms that a loss by him would spell the end of civilization. The distrust of U.S. institutions that Trump has nurtured among his core supporters is readily apparent. One North Carolina man predicted in an interview that the military would probably assassinate Hillary Clinton if she’s elected president. A woman at an Iowa town hall for Trump’s running mate,...