Prepared a piece for the forthcoming Handbook of American Political Geography. Some tidbits deserve emphasis:

Researchers and policymakers must hold in their minds two truths: that geographic polarization, across urban-rural divisions, is close to as high as it has ever been in the country’s history, and that Americans are not completely isolated geographically from people who disagree with them politically.

In a 50-50 country, segregation would have to be quite extreme to truly bring cross-partisan proximity to zero. So despite areas of high isolation, many voters do live in places with large numbers of out-partisans, and as we move beyond the neighborhood level to larger geographies we see more cases of mixed partisan composition.

Still the urban-rural divide is strong, and the data suggests that partisan segregation rapidly increased starting around the 1970s through the 2000s, and continuing to increase more modestly up through 2020.

But as the Handbook piece demonstrates, recent electoral data suggest a plateauing or perhaps a reversal of these long-term trends. Whether this persists or is a blip in a steadier time-series may have to wait for more time to pass and data to be accumulated. But the evidence on sources of political segregation, both recent and more long-term point to the importance of political realignments changing the American map without people sorting residentially. So what political realignments of the Trump era could even out the American map? From the piece, on college education and racial realignments:

This demographic realignment challenges urban-suburban polarization, previously falling along the income gradient, effectively liberalizing the suburbs relative to cities, reducing polarization along this dimension. Republican gains with minority voters can erode Democratic strong points and upend urban-suburban divides.

The most interesting demographic here to me is one where voters are realigning but also where the mass of the distribution is imbalanced:

However, the American electoral map is headed towards an age cliff. The distribution of age in the electorate is such that the largest voting bloc is at or approaching senior status. The Baby Boomer generation dwarfs younger generations. Thus, a large aging group of voters anchors the political geography of the electorate, in that they are unlikely to move because of their age and are either set in their ways or trending Republican in a manner that increases geographic polarization. But as this generation dies off, they are replaced by a younger, more mobile electorate. This distributional shift could produce a shock to levels and trends of geographic polarization.

So the current American political map is held down, to an extent, by an immobile aging electorate, declining residential mobility more generally, and -- until maybe recently -- polarized politics with lower rates of party switching than in past eras.