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, to a degree that raises provocative questions about how closely lifestyle preferences have become aligned with politics and how even neighbors may influence one another.

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. That means, for example, that in a neighborhood where Democrats make up 60 percent of the voters, only 50 percent of a Republican’s nearest neighbors might be Democrats.

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.

The maps above — and throughout this article — show their estimates of partisanship down to the individual voter, colored by the researchers’ best guess based on public data like demographic information, voter registration and whether voters participated in party primaries.

We can’t know how any individual actually voted. But these maps show how Democrats and Republicans can live in very different places, even within the same city, in ways that go beyond the urban-suburban-rural patterns visible in aggregated election results.

“We know that with groups in general, when they’re separated, bad things happen,” Mr. Enos said. That has proved true with racial segregation, and religious and ethnic divides — patterns of separation that make it easier to demonize one another, and harder to share resources or power.

“The question with political parties is whether those are sufficiently like those other groups that we should worry about that happening,” he said.

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. The closer we look among all these dots, however, the harder it gets to explain these patterns.

For each individual voter, tied to an address, the researchers looked at their thousand nearest voters, weighting those next door more heavily than those a mile away. Drawn this way, about 25 million voters — urban Democrats especially — live in residential circles where at most only one in 10 encounters is likely to be with someone from the opposite party. Democrats in parts of Columbus, Ohio, and Oklahoma City live this way. So do Republicans in the reddest parts of Birmingham, Ala., and Gillette, Wyo.

Even when Democrats and Republicans are more equally represented in the same ZIP code or census tract, Mr. Brown and Mr. Enos still find traces of segregation. That means the two groups don’t appear randomly jumbled together. A Republican in a more mixed Denver suburb is still more likely to live close to other Republicans than mere chance would suggest.

“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.”

Educational shift, geographic switch

So what can explain these patterns?

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 in highly educated suburbs and more working-class towns and rural communities. None of these voters have to move to effectively “sort” on a map; rather, their preferences change in place (in ways that may show up in their voting behavior before voters update their party registration).

“Party coalitions have shifted in a direction that aligns really well with spatial differences in a way it didn’t use to,” said Greg Martin, a Stanford political scientist who has also studied these trends.

Racial segregation also feeds partisan clustering, given that African-American voters in particular are overwhelmingly Democratic and also residentially segregated (metro Milwaukee’s map of partisan segregation, for one, resembles its map of racial segregation). But Mr. Brown and Mr. Enos find that racial segregation alone doesn’t explain the levels of partisan separation they find.

Lifestyle preferences that seemingly have little to do with partisanship are also increasingly correlated with it. If you like city living and use transit, you’re more likely a Democrat; if you prefer large-lot houses and pickup trucks, you’re more likely a Republican. And so voters of the same party might choose to live in the same places for such features, not necessarily to be around one another, and it would produce partisan clustering.

Yet even when Republicans and Democrats live in the same city, or in the same part of town — essentially the same kind of place — they still appear separated from each other to a degree. Much of that is probably about housing. Even within the same census tract, there may be pockets of apartment buildings (more likely home to Democrats) and streets with single-family homes (more likely home to Republicans).

But housing can’t explain the full effect, either. Across 98 percent of census tracts nationwide, Democrats and Republicans live with at least some segregation. That’s true even within suburban neighborhoods southwest of Kansas City, Mo., where the residents are almost all white and homeowners, and the houses are all single-family.

That leaves a more intriguing question: Are people really paying attention to the politics of their neighbors and acting on it in some way?

“I do think that something new is happening at the neighborhood level around partisan politics,” said Nancy Rosenblum, a political theorist at Harvard who has written a book about neighbors. Interactions between neighbors have long been distinctly nonpartisan, she said, grounded in values like reciprocity — I’ll lend you my leaf blower, you watch my kid.

But she fears that a more malignant kind of politics is seeping all the way down into neighborhoods: “The most interesting question to ask here is: How deep does it go? And the test for how deep it goes for me is: How do neighbors in neighborhoods behave during disasters?”

That’s when we normally see neighborly reciprocity really come through, she said. “If we look at Covid — and we consider Covid a national disaster — you see something change,” she said. “And this is really very discouraging. It could make you weep.”

Now even masks are freighted partisan signals.

Local influencers

There is little evidence that people choose where to live with politics in mind. Other concerns tend to take precedence, like finding a house that’s affordable and near a preferred school, the political scientists Clayton Nall and Jonathan Mummolo have argued.

And neighborhoods contribute just one piece of anyone’s social circle, along with co-workers, friends and family, to say nothing of the political influences of partisan news and social media.

But there is some evidence that local environments matter. Mr. Brown, a doctoral candidate in government at Harvard, has also looked at what happens to voters who stay at the same address over time, as the partisan makeup of the community shifts around them. As a neighborhood becomes more Democratic or Republican over time, he finds, voters become more likely to change their party registration to match.

In a neighborhood that has gone from slightly more Republican to slightly more Democratic, for example, that increases a nonpartisan voter’s likelihood of registering as a Democrat by several percentage points. That’s modest, but Mr. Brown said that it’s “a sizable change in something that for the most part is pretty stable.”

He also finds in surveys that voters are more likely to display their partisanship — wearing clothing with a message, putting out a yard sign or bumper sticker — when the people around them share their politics.

Other research shows that yard signs can increase candidates’ vote shares, and that neighbors may influence political donations. Ricardo Perez-Truglia, a political economist at the University of California, Berkeley, found that people gave more in politically like-minded areas when he sent them a letter reminding them their neighbors could look up their donations. And donors to Barack Obama in 2008 were likely to give more generously in 2012 if they relocated in the intervening years to a more heavily Democratic community.

“This is just one example of many other contexts in which this could be going on,” Professor Perez-Truglia said. “If you know that everyone else is of your same party, you don’t have anything to lose. You can be very vocal; nobody’s going to disagree with you.”

These studies together suggest that as places become more politically homogeneous, people there are more likely to conform and to publicly signal their partisanship. Maybe no one says, “I want to move here because of all these Biden yard signs.” But perhaps one neighbor is swayed by the people who put them up, and another neighbor concludes, “This isn’t the place for me.”

Many partisan signals are not so subtle these days. They come from billboard-size Trump yard signs that stand proud even outside of election years. Other signs — “love is love,” “no human is illegal,” “science is real” — implicitly reproach anyone who doesn’t share those values. And it has become easier over time for entrepreneurs to make and market such messaging, said Donald Green, a political scientist at Columbia who has studied yard signs.

“It’s very easy to tell who’s who,” he said, describing the equally divided Hudson Valley community where he has a home, and where signs opposing a state gun control measure are common. “If you see ‘Repeal the SAFE Act’ — I saw four just driving to the Post Office just now — you know, you just absolutely know.”

It’s also possible that partisan segregation is increasing because these patterns are feeding one another. Voters with similar taste in housing and who are realigning by party right now happen to be clustered in space, and they’re nudging each other along as they go, in a kind of self-reinforcing cycle.