Long after the dust settles in Iowa — and New Hampshire, and even the 2016 campaign itself — one question will remain: Why, after decades of supporting the liberal and conservative establishments, did the white middle-class abandon them? Wherever Donald J. Trump and Bernie Sanders end up, their candidacies represent a major shift in American politics. Since World War II our political culture has been organized around the needs, fears and aspirations of white middle-class voters in ways that also satisfied the interests of the rich and powerful. That’s no longer true.

As we know, the rich are now quite a bit richer. In itself, this need not disrupt the old political consensus. More decisive is the fact that the white middle class is in decline, both economically and culturally.

This story of decline is often told in racial and ethnic terms: White America is being displaced by a multicultural America, and especially on the right, voters are retreating to racist posturing. There may be some truth to this story, but for the most part it’s a huge distraction.

In fact, the real cleavage is not interracial, but intra-racial: The populism we’re seeing stems entirely from the collision of whites who flourish in […]