Rothschild pointed to a toxic stew of age-old antisemitism; the worst incentives of today’s social media; and the right’s unending obsession with Bill and Hillary Clinton. Donald Trump pops up in the documents, including some released on Monday, but that’s typically not a problem for his hard-core supporters. As with most conspiracy theories, adherents create their own reality. “There’s nothing Trump can do that will lose these people’s loyalty,” Rothschild said. “Anything that is inconvenient, that alters their worldview, they’ll just ignore it, or they’ll make up some bizarre justification for it.” This interview has been edited for length and...