If you talk to leading progressives these days, you'll be sure to hear this message: The Democratic Party should embrace the economic populism of New York Mayor-elect Bill de Blasio and Massachusetts Sen. Elizabeth Warren. Such economic populism, they argue, should be the guiding star for Democrats heading into 2016. Nothing would be more disastrous for Democrats.

While New Yorkers think of their city as the center of the universe, the last time its mayor won a race for governor or...