Tom MacArthur’s town-hall meeting should have been uneventful. It was May and MacArthur, a Republican congressman, had come home to New Jersey to talk to constituents, having won re-election with a hefty majority less than a year earlier. Normally, at such an event, he might have expected to field a couple of dozen idiosyncratic questions, trot out some polite replies, wrap up after an hour or so, and go home for an easy night’s sleep. But it didn’t quite pan out like that. Six days earlier, the House of Representatives had voted to pass the American Health Care Act, which...