A Republican ballot proposal seeking to reverse key parts of Missouri's recently adopted redistricting method could allow some of the most gerrymandered voting districts in the nation while still purporting to provide "partisan fairness," opponents of the measure said Thursday.

JEFFERSON CITY — A Republican ballot proposal seeking to reverse key parts of Missouri's recently adopted redistricting method could allow some of the most gerrymandered voting districts in the nation while still purporting to provide "partisan fairness," opponents of the measure said Thursday.

A proposed constitutional amendment placed on the November ballot by the GOP-led Legislature would change the criteria for redrawing state House and Senate districts in 2021 based on the results of this year's census.

Among other things, it would shift a requirement that districts be drawn to achieve "partisan fairness" and "competitiveness" from the top of the priority list to the bottom, placing it behind criteria for compact districts that keep communities intact. 

The fairness and competitiveness criteria were part of a 2018 constitutional amendment called "Clean Missouri" that won 62% of the vote. It made Missouri the first state to require use of a specific formula called the "efficiency gap" to measure partisan fairness.

Under that formula, a score near zero is considered politically neutral. The 2018 measure requires a nonpartisan state demographer to draft districts "as close to zero as practicable." The measure on this year's ballot does away with that demographer and allows an efficiency gap up to 15%, which could allow districts to heavily favor one party over another. 

"Instead of trying to create fair maps for Missouri voters, it enshrines extreme partisan gerrymandering," said Chris Lamar, a redistricting attorney at the Campaign Legal Center, a Washington, D.C.-based organization that has challenged voting districts in several states as illegal gerrymanders. 

Republican state Sen. Dan Hegeman has said the "partisan fairness" criteria in 2018 measure could result in elongated districts that unnaturally combine Democratic-leaning urban voters with Republican-leaning rural voters. 

Hegeman said Thursday that the goal of this year's measure, which he sponsored, "was to put people over politics by prioritizing districts that keep communities together."

The efficiency gap formula was created earlier this decade by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, and law professor Nick Stephanopoulos, who is now at Harvard University. They suggested state House district maps with an efficiency gap of at least 8% could be unlawful gerrymanders, if additional analysis shows a persistent advantage for one party. 

The U.S. Supreme Court has declined to adopt the efficiency gap as a measure of gerrymandering and instead left it up to state courts to decide such cases. 

Missouri's current state Senate districts were adopted by a bipartisan commission after the 2010 census. Its House districts were drawn by a judicial panel after a bipartisan commission failed to agree on a plan.

During the 2012 and 2014 elections, Missouri's state House districts averaged an efficiency gap of 9% in favor of Republicans, according to PlanScore, a website that calculates historical efficiency gap ratings for some states. 

An AP analysis found the Missouri House had an 8% efficiency gap favoring Republicans in the 2018 elections, the sixth highest nationally. The most extreme rating was a nearly 16% efficiency gap in Wisconsin, one of five states where Republicans retained their House majorities even though Democratic candidates received more total votes.

The AP's analysis showed Missouri's pro-Republican efficiency gap was 4% in the 2016 state House elections, right at the national median. 

The AP did not analyze the efficiency gap for state Senate elections, because senators are elected to staggered terms. 

A separate AP analysis of the redistricting formula approved by voters in 2018 shows it is likely to lead to Democratic gains in the 2022 elections while dropping Republican majorities closer to the more even partisan division often reflected in statewide races.