In neighborhoods across the country, partisan segregation is increasingly becoming the norm, research shows

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.

As new research has found, it’s not just that many voters live in neighborhoods with few members of the opposite party; it’s that nearly all American voters live in communities where they are less likely to encounter people with opposing politics than we’d expect.

Democrats and Republicans are effectively segregated from each other, to varying degrees by place, according to the Harvard researchers Jacob Brown and Ryan Enos. And at least over the past decade, they believe this partisan segregation has been growing more pronounced.

By living apart, opposing partisans might scorn aid for one another (with a term like “blue-state bailouts”) or become more likely to buy into myths about one another (like widespread voter fraud). Other processes like racial segregation, Mr. Enos added, have shown a tendency to accelerate.

This growing residential separation doesn’t necessarily mean that partisans are searching out cities — or neighborhoods, or even individual streets — where the neighbors are politically like-minded. Several forces have been pushing them apart, including broad changes in whom the two parties represent.

Over time, the Democratic Party has increasingly aligned with urban voters, and the Republican Party with voters outside of cities, deepening geographic polarization nationally.

Highly educated white voters are also shifting toward the Democrats as working-class white voters move toward Republicans. Educational realignment has geographic consequences, too, with the changes concentrated, respectively, in highly educated suburbs and more working-class towns and rural communities.

But other factors are also at play.

“If we get down to a very low level and we still see this sorting going on,” Mr. Enos said, “it probably means there’s something pretty fundamental going on here.”