The great, gradual migration of the human population from the countryside to the city has transformed the world, but we’ve barely begun to reckon with its political implications. Over generations, urbanization has sorted us on the traits — ethnicity, education level, personal temperament — that draw us toward cities or keep us away.

The logic of our electoral institutions has always sorted the bulk of American voters into one of two major parties. What’s new is that the sorting dynamic of urbanization now accounts for partisan sorting, too. Democrats have become the party of the multicultural city, Republicans the party of the monocultural country — the party of urbanization-resistant white people.

Such a clean partisan break along density lines has thrown our democracy into a crisis of legitimacy and dysfunction. Our federal system of democratic representation has drifted dangerously out of sync with the geographic distribution, demographic makeup and outsize economic role of America’s urbanized population.

Our politics is cracking up over the density divide. Big cities and their distinctive interests are suffering a density penalty and need more visibility in our scheme of representation.