Anthony Salvanto was imprisoned in a windowless office on Nov. 8, 2016. His phone was confiscated; guards escorted him to the bathroom. And all he had to look at were numbers, numbers and more numbers. It wasn’t some sadistic punishment. It was Election Day, and Salvanto, head pollster for CBS News, was one of the few experts allowed see the exit-poll data as it rolled in. Hillary Clinton held a lead all day, as most polls had predicted. But Salvanto saw a radical shift brewing. Clinton’s advantage was narrow, and many of the voters who hadn’t yet cast ballots were...