On a late January afternoon, as press secretary Sean Spicer walked into the White House media briefing room, a tall, thin, bespectacled man poked his head in the doorway for a moment before turning around and heading back into the West Wing. Later that week, at another briefing, the man stayed longer, standing in the corner behind the podium, out of view of the array of television cameras. The reporters peppering Spicer with questions were unlikely to know it, but the wallflower watching over the proceedings happened to be the leading conservative intellectual to argue for the election of Donald...