A gentrified cocoon of progressive privilege has cost Democrats the middle class. Democrats for over a century were associated with the American middle class. Working-class voters once believed that Democratic-inspired intervention into the economy — minimum-wage laws, overtime pay, Social Security, Medicare, workers’ compensation — protected their interests better than unfettered free-market capitalism. Republicans often had trouble selling the argument that an unleashed economy and new technology would relegate poverty to a relative, not absolute, condition — something like suffering with a cheap, outdated iPhone 4 while the better-off could afford an iPhone 6. Why, then, have Democrats lost the...