An Optimization Case Study in Analyzing Missouri Redistricting

Every 10 years, U.S. states redraw their congressional and state legislative district plans. This process decides the political landscape for the subsequent 10 years. Prior to the 2021 redistricting cycle, Missouri enacted new criteria for state legislative districts. The Missouri League of Women Voters (LWV-MO) contacted the authors to analyze the potential impact of these new criteria on the map-drawing process. We apply recombination (a spanning tree method) within a local search optimization framework to analyze the interplay between political geography, constitutional requirements, and political fairness in Missouri. We use this framework to produce district plans that satisfy the new criteria and prioritize different aspects of fairness. The results, quantified by several measures of fairness, reveal an inherent Republican advantage in Missouri because of the state’s political geography and constitutional requirements. We conclude that Missouri’s political geography and constitutional requirements prevent the optimization framework from substantially improving political fairness in state legislative plans. In contrast, the framework can substantially improve political fairness in Missouri congressional plans, which are not subject to the new requirements. The LWV-MO used this work to advocate for fairness and transparency in their testimonies for the Missouri redistricting commission’s public hearings.