(AP) — Frustrated by partisan gerrymandering, voters in a growing number of states have taken the pen and computer away from lawmakers who have traditionally drawn U.S. House and state legislative districts and instead entrusted that responsibility to others.

In the past decade, eight states have overhauled their redistricting procedures to lessen the potential of partisan manipulation, including four that adopted ballot measures last fall. More could consider redistricting changes during the 2020 elections — the last before the U.S. Census initiates another round of mapmaking for over 400 U.S. House seats and nearly 7,400 state legislative seats.

The current movement began in California for the 2010 Census, when voters approved ballot initiatives creating an independent citizens’ commission to handle redistricting. Measures touted as redistricting reforms also have passed in Florida, New York, Ohio and — most recently— in Colorado, Michigan, Missouri and Utah.

In Ohio, the effort was bipartisan. Republicans joined with Democrats to back a pair of successful ballot measures that will require minority-party support to enact new congressional and state legislative districts for the next decade.

Ohio’s congressional delegation has remained at 12 Republicans and four Democrats ever since GOP officials redrew the maps after the 2010 Census, a 75-25 percent tilt that is out of line with the statewide vote for the two major parties. In November, Republican congressional candidates in Ohio won 52 percent of that vote while Democrats won 48 percent.

The Associated Press used a so-called “efficiency gap” test to analyze the 2018 elections. It’s one of the same analytical tools cited in a North Carolina gerrymandering case for which the U.S. Supreme Court is hearing arguments on Tuesday. The test showed Ohio’s pro-Republican leaning ranked just behind North Carolina’s in the 2018 congressional elections, and its state House districts also showed a GOP advantage.

“We’ve been living under that rigged system for the entire decade,” said Ohio Democratic Party Chairman David Pepper.

Yet one of the supporters of Ohio’s new redistricting procedures is Republican Secretary of State Frank LaRose, who worked as a state senator to refer the measures to the ballot. LaRose said he hopes the new process leads to more competitive elections — even if that puts Republicans at risk of losing seats.

“I also see this in some ways as tough love for my party,” LaRose said. “I believe that Republican candidates are likely to win based on their ideas and based on the quality of their solutions for governing. But I think that when we rely on something other than that to win an election, it weakens us.”

Voters in Missouri went a step further last fall, becoming the first state to insert a version of the efficiency gap test into its constitution. Under the new measure, a nonpartisan state demographer will use the 2020 Census data to draw districts for the state House and Senate that achieve “partisan fairness” and “competitiveness.”

The Missouri measure will not apply to congressional districts, which will continue to be drawn by the Legislature, currently controlled by Republicans.

Republicans have maintained a 6-2 advantage over Democrats in Missouri’s congressional delegation ever since the current districts were enacted in 2011, when a few Democrats joined with Republican lawmakers to override a veto by the Democratic governor.

The AP analysis shows that Missouri Republicans won one more congressional seat than would have been expected in 2018 based on their average share of the votes. That swing district was in suburban St. Louis, where Republican U.S. Rep. Ann Wagner withstood a close challenge from Democrat Cort VanOstran.

Though he acknowledged shortfalls in fundraising and name identification, VanOstran said: “I think that Missouri is a victim of gerrymandering.”

Yet independent commissions don’t always do away with partisan advantages, some of which can arise naturally when Democratic or Republican voters choose to live in high concentrations in certain neighborhoods or cities.

The AP’s efficiency gap analysis shows California Democrats won four more congressional seats than would have been expected based on their district average share of the vote in the 2018 elections. That helped boost Democrats’ overwhelming majority in California’s congressional delegation to 46-7 over Republicans. The AP’s analysis showed California had a more neutral result when Democrats won a 39-14 majority over Republicans in the 2016 elections.

“There’s no doubt that the commission produced a map that tilts a little bit Democratic,” said Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California who developed the efficiency gap model. But “looking at average results over time, it’s not consistently Democratic. It flips around; it’s variable in that sense.”

Other states that use independent or bipartisan commissions to draw state legislative or congressional districts include Arizona, Hawaii, Idaho, Montana, New Jersey and Washington. In Iowa, nonpartisan legislative staff create the redistricting maps, which then go to the Legislature for an up-or-down vote.