“This is before and this is after. Before and after,” Cory Booker is saying on Snapchat. He’s just met a young fan with an Afro and is drawing the contrast to his own shiny bald head. It’s a routine he will repeat again later in the day and as he crisscrosses the country before the midterms. The New Jersey senator can’t say no to a selfie. If you are standing around looking idle, he’ll probably ask you to take one and upload the photo to his social media accounts for his millions of followers. Social media are Booker’s bread and...