Gerrymandering and geographic polarization have reduced electoral competition

TL;DR

This study analyzes how geographic polarization and gerrymandering have decreased electoral competition in the US by examining redistricting plans and their effects on partisan bias and district competitiveness.

Contribution

The paper introduces a large ensemble of simulated redistricting plans to disentangle the effects of geography and gerrymandering on electoral outcomes.

Findings

Geographic polarization increased between 2010 and 2020.

Gerrymandering bias in favor of Republicans decreased from 16 to 10 seats.

Over 25% reduction in highly competitive districts due to polarization and gerrymandering.

Abstract

Changes in political geography and electoral district boundaries shape representation in the United States Congress. To disentangle the effects of geography and gerrymandering, we generate a large ensemble of alternative redistricting plans that follow each state's legal criteria. Comparing enacted plans to these simulations reveals partisan bias, while changes in the simulated plans over time identify shifts in political geography. Our analysis shows that geographic polarization has intensified between 2010 and 2020: Republicans improved their standing in rural and rural-suburban areas, while Democrats further gained in urban districts. These shifts offset nationally, reducing the Republican geographic advantage from 14 to 10 seats. Additionally, pro-Democratic gerrymandering in 2020 counteracted earlier Republican efforts, reducing the GOP redistricting advantage by two seats. In total, the pro-Republican bias declined from 16 to 10 seats. Crucially, shifts in political geography and gerrymandering reduced the number of highly competitive districts by over 25%, with geographic polarization driving most of the decline.