A Close-Up Picture of Partisan Segregation, Among 180 Million Voters

The broad outlines of America’s partisan divides are visible on any national map. Republicans typically dominate in most Southern and Plains states, and Democrats in Northeastern and West Coast ones. Democrats cluster in urban America, Republicans in more rural places.

But keep zooming in — say, to the level of individual addresses for 180 million registered voters — and this pattern keeps repeating itself: within metro areas, within counties and cities, even within parts of the same city.

Democrats and Republicans live apart from each other, down to the neighborhood,...