Donald Trump was meeting privately in mid-September with one of his oldest friends, Steve Wynn, when the casino mogul and Republican mega-donor delivered the former president a blunt warning: You’re off message, and it isn’t helping. Trump had been distracted, in Wynn’s view. The former president at the time was promoting a conspiracy theory that Haitian immigrants were eating people’s cats and dogs in Ohio, among other things. To drive home his point, Wynn showed Trump polling and suggested...