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

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