One by one, they had trickled out of the conference room on the 25th floor of Trump Tower. It was Friday, October 7, 2016, two days before the second presidential debate, and the Republican nominee’s brain trust had spent the morning running a carefully simulated rehearsal session. New Jersey Gov. Chris Christie, playing the role of Hillary Clinton, was stationed adjacent to his opponent at a conference table; Republican National Committee Chairman Reince Priebus, acting as the moderator, was positioned directly across from Donald Trump. The rest of the observers—press secretary Hope Hicks, campaign chief executive Steve Bannon, campaign manager...