Artificial Partisan Advantage in Redistricting

I propose a measure of artificial partisan advantage in redistricting. Redistricting is the process of drawing electoral district maps. Electoral outcomes depend on the maps drawn. The measure I propose is to compare the seat share won by a party to the share of the population that lives in counties won by this party. If a party has a larger share of seats than the share of the population in counties in which the party won most votes, then the drawing of the electoral maps conferred an artificial advantage to this party. This measure takes into account the geographic sorting of partisan voters and is simple to compute. Using U.S. election data from 2012 to 2018, I find an artificial partisan advantage of sixteen House seats to the Republican party. I argue that the artificial partisan advantage in the congressional maps of North Carolina, Utah, Michigan and Ohio is excessive. I thank Alex Tybl, Andrew Earle and Christian Cox for diligent research assistance, and Chris Ahlin, Stephen Ansolabehere, Jowei Chen, Jonathan Katz, Gary King, Alan Miller, Corwin Smidt, Nicholas Stephanopoulos, and participants at the 2019 PECA conference in Chicago, at the Big 10 Alliance’s SPARK conference in College Park, and at an internal seminar at Michigan State U. for their comments. I gratefully acknowledge the Institute for Public Policy Social Research for funding through the Michigan Applied Public Policy Research grant. I volunteered for the organization Voters Not Politicians, which led the 2018 successful campaign for constitutional reform on redistricting in Michigan. § Email: eguia@msu.edu 2 In some democracies, including the United States, representatives to a legislative assembly are elected by drawing electoral districts and electing in each district a single legislator to represent the district.1 In order to preserve an equal population across districts, the boundaries of these districts must change with population changes. Redistricting is the process of drawing maps that partition a given polity (a country, a state, etc.) into electoral districts. In the United States, redistricting typically occurs every ten years, following a decennial population census. Because electoral outcomes depend on how the district maps are drawn, those in charge of redistricting have incentives to draw maps that advance their own electoral goals. The practice of drawing district maps to favor one party or class is called “gerrymandering.” In 1986, the US Supreme Court held that maps that confer too much partisan advantage to one party are unconstitutional. However, the Court could not agree upon a test or measure of partisan advantage.2 In 2019, while the Court conceded that “excessive partisan gerrymandering” is “unjust” and “incompatible with democratic principles”, it ruled that “none of the proposed tests for evaluating partisan gerrymandering claims meets the need for a limited and precise standard” and hence that gerrymandering claims cannot be addressed in federal courts.3 The Court suggested that gerrymandering should be addressed instead by the States, litigating in 1 The United Kingdom uses this system, as do several other countries with a legacy of British rule, such as India and Canada. 2 Davis vs Bandemer 478 U.S. 109 (1986). The Court reached a similar ruling in Vieth vs Jubelirer 541 U.S. 267, 310-311 (2004) and in League of United Latin American Citizens vs Perry 548 U.S. 399 (2006), holding that a satisfactory measure might emerge in the future. 3 The quotes are from the summary of Rucho v. Common Cause, No. 18-422, 588 U.S. ___ (2019). 3 state courts under the guidance of state legislation, as in Florida (2015) or Pennsylvania (2018).4 Fourteen states (California, Colorado, Delaware, Florida, Hawaii, Idaho, Iowa, Michigan, Missouri, Montana, New York, Ohio, Oregon and Washington) include explicit provisions against partisan advantage in redistricting in their state constitutions. I provide a summary of these provisions and a link in the online appendix Traditional notions of partisan advantage (discussed below) measure the partisan asymmetry in how votes translate to seats without addressing a key question: can some (or all?) of the partisan advantage be explained by the geographic sorting of voters? Indeed, for any map with compact districts, a party with many of its voters concentrated into a small area obtains fewer seats than if its voters are better dispersed over much of the state. I propose a precise and limited notion of partisan advantage that accounts for the natural advantage due to population sorting, and measures only the artificial partisan advantage caused specifically by the chosen redistricting map. The measure of artificial partisan advantage compares the seats that a party obtains to the seats that the party would obtain if seats were assigned in proportion to the population residing in counties in which the party won the popular