Gerrymandering and geographic polarization have reduced electoral competition

2025-08-21

AI Summary

This study investigates how geographic polarization and partisan gerrymandering jointly erode electoral competitiveness and descriptive representation in U.S. congressional elections. Employing large-scale redistricting simulations, we generate thousands of legally compliant alternative district plans per state, then apply counterfactual analysis and partisan bias metrics to disentangle and quantify the distinct, time-varying contributions of structural (geographic) and intentional (manipulative) factors. Our contribution is the first systematic decomposition of these mechanisms—tracking their evolution across the 2010–2020 decade. Results show that intensifying urban–rural political sorting drove geographic polarization, reducing competitive districts by over 25%. Concurrently, pro-Republican partisan bias declined from 16 to 10 seats, indicating that redistricting practices partially offset structural polarization. This underscores the critical role of institutional design—particularly redistricting rules—in mitigating the democratic consequences of deepening geographic cleavages.

Technology Category

• Game Theory and Economic Paradigms: Fair Division
• Knowledge Representation and Reasoning: Preferences
• Computer Vision: Bias, Fairness & Privacy

Application Category

• Social Networks and Social Media: Media and governance, opinion dynamics, filter bubbles, polarization
• User Modeling, Personalization and Recommendation: Fairness-aware retrieval and ranking
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs

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.

Problem

• Analyzing effects of gerrymandering and geographic polarization
• Measuring partisan bias through simulated redistricting plans
• Assessing decline in competitive districts due to polarization

Innovation

• Generated ensemble of alternative redistricting plans
• Compared enacted plans to simulated legal redistricting
• Analyzed geographic polarization and gerrymandering effects separately