A political mapmaking process controlled by Ohio Republicans proved nearly impenetrable to Democrats' efforts during the 2018 elections, an Associated Press analysis has found, delivering results that allowed the GOP to retain sizable majorities even in the face of an upswing in Democratic votes.
The analysis found that Ohio Republicans won at least three more U.S. House seats and seven more state House seats than would have been expected based on the average share of the votes that Republicans received. Both gaps grew compared with 2016, when the same analysis showed Republicans winning nearly two more U.S. House seats and five more Ohio House seats than expected based on their share of the votes.
The AP used a mathematical formula for calculating partisan advantages that is designed to flag potential cases of political gerrymandering, a practice where the party in power alters voting districts to its advantage in federal and state legislative races. The 2018 analysis again placed Ohio's "efficiency gap" near the top for both state and federal legislative races.
The 2018 analysis found Republicans won 52 percent of the votes in Ohio House races yet 62 percent of the seats. Republican candidates for Ohio's U.S. House seats won 52 percent of the votes but 75 percent of the state's 16 congressional seats. Republican support was down nearly 4 percentage points from 2016 for state House races and almost 6 percentage points in U.S. House races, the analysis showed.
The finding comes as a federal court panel weighs its decision in a lawsuit by Democrats alleging that unconstitutional partisan gerrymandering has disadvantaged Democratic voters in Ohio congressional districts. After arguments earlier this month, the panel has not yet ruled. Ohio's current map was enacted in 2011 by a Republican governor and Republican-led Legislature.
Since then, Ohioans have voted to change the redistricting process and make it less partisan.
Voters overwhelmingly approved new rules for drawing state legislative districts in 2015 and changed the method for drawing congressional districts during last May's primary. The redistricting reforms have had bipartisan support.
For its analysis, the AP scrutinized all U.S. House races and about 4,900 state House and Assembly seats up for election last year using an "efficiency gap" statistical method that is designed to calculate partisan advantage. It found that the GOP may have won as many as 16 more U.S. House seats than expected based on the average vote share in congressional districts across the country. A 2016 analysis similarly showed a GOP advantage nationally.
The formula was developed by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, who wrote an academic paper about it earlier this decade with University of Chicago law professor Nick Stephanopoulos. Their model has been cited in a Wisconsin redistricting case that went before the U.S. Supreme Court in 2017 and is part of a North Carolina case scheduled to be argued Tuesday before the court.
The efficiency gap formula compares the statewide average share of the vote a party receives in each district with the statewide percentage of seats it wins, taking into account a common political expectation: For each 1 percentage point gain in its statewide vote share, a party normally increases its seat share by 2 percentage points.
