Howard Schultz had every reason to be surprised when his recent announcement that he might run for president in 2020 was met with fury by fellow liberals. It was not that Schultz is comparatively “moderate” by the standards of the Democratic Party, or even his self-description as a “centrist,” that inspired the hostility. Joe Biden, who these days (amazingly) passes as a moderate, is currently leading in opinion polls, and Schultz’s moderation is largely confined to fiscal policies that interest few Democratic activists. On the hot-button social issues of the moment, the retired Starbucks CEO is safely orthodox-liberal. No, Schultz’ error was in telling his “60 Minutes” interlocutor that he intended to run not as a Democrat but as an independent. In that sense, Schultz was being both shrewd and resourceful. Despite the fact that most Americans tend to vote along sharply defined partisan lines, and very closely divided lines, in recent elections, there is a tradition of looking slightly askance at the parties themselves. We’re all centrists, when asked. While the figures are flexible, the number of people who regard themselves as “independents” (42 percent) is nearly twice the number of people who identify either as Republicans (27 percent) or Democrats (29 percent). And while in Europe political affiliation is often bound up with social class or family tradition, in America we proudly declare that we vote for the candidate not the party. Except...