Sources and Extent of Rising Partisan Segregation in the U.S. – Evidence from 143 Million Voters

Using two datasets tracking the location and party affiliation of every U.S. voter between 2008 and 2020 in states recording partisan registration, we find that geographic segregation between Democrats and Republicans has increased year over year at all geographic levels, from Congressional Districts to Census Blocks. Areas trending Democratic have a starkly different demographic profile than those trending Republican, so the confluence of demographics, partisanship, and geography is growing. We decompose the increase in geographic partisan segregation into different sources and show that it has not been driven primarily by residential mobility but rather by generational turnover, as new voters entering the electorate cause some places to become more homogeneously Democratic, and by party switching, as existing voters leaving the Democratic party cause other places to become more Republican. The groups that most contribute to the rise in partisan segregation are young people, women, and non-white voters in Democratic-trending areas; and white and older voters in Republican-trending areas.