FILE - In this Aug. 31, 2018 file photo, supporters of Missouri's redistricting ballot measure hold signs behind former state Sen. Bob Johnson as he serves as their spokesman during a press conference outside the Cole County Courthouse in Jefferson City, Mo. Voters approved Constitutional Amendment 1 on the Nov. 6 ballot. It is unique among redistricting reforms adopted by a number of states in recent years because it requires Missouri's state legislative districts to be drawn using a new mathematical formula to try to achieve "partisan fairness" and "competitiveness" after the 2020 Census. (AP Photo/David A. Lieb, File)

FILE - In this Aug. 31, 2018 file photo, supporters of Missouri's redistricting ballot measure hold signs behind former state Sen. Bob Johnson as he serves as their spokesman during a press conference outside the Cole County Courthouse in Jefferson City, Mo. Voters approved Constitutional Amendment 1 on the Nov. 6 ballot. It is unique among redistricting reforms adopted by a number of states in recent years because it requires Missouri's state legislative districts to be drawn using a new mathematical formula to try to achieve "partisan fairness" and "competitiveness" after the 2020 Census. (AP Photo/David A. Lieb, File)

JEFFERSON CITY, Mo. (AP) — The votes won’t be cast for another four years, yet Democrats already appear likely to gain seats in Missouri’s Republican-dominated Legislature in 2022.

The reason: a one-of-its kind redistricting initiative approved by voters in the recent midterm elections.

Missouri’s initiative marks a new frontier in a growing movement against partisan gerrymandering that has now notched ballot-box victories in eight states over the past decade.

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Other states have created independent commissions and required bipartisan votes to redraw legislative and congressional districts. Missouri will be the first to rely on a new mathematical formula to try to engineer “partisan fairness” and “competitiveness” in its state legislative districts; the Legislature will continue drawing the state’s congressional districts.

An Associated Press analysis of the new Missouri formula shows it has the potential to end the Republicans’ supermajorities in the state House and state Senate and move the chambers closer to the more even partisan division that is often reflected in statewide races. But the size of the likely Democratic gains remains uncertain, partly because the formula has never been put to a test.

“Missouri’s engaged in an experiment,” said Sam Wang, director of the Princeton University Gerrymandering Project, which uses math to measure partisan advantages in redistricting.

“Democrats have a fighting chance in a way that they didn’t before,” Wang added. But “a lot of it depends on what they do with it.”

After the 2010 census, Republicans nationwide controlled more state legislatures and governor’s offices than Democrats. They used that power to draw legislative and congressional districts that benefited the GOP.

Since then, advocates have been trying to reform the system to eliminate or greatly reduce partisan gerrymandering, which has been used by both parties over the years to draw political boundaries in ways that give the dominant party a disproportionate hold on power. They have succeeded in making the process less partisan in a number of states, mostly through ballot initiatives approved by voters.

Nov. 6 was the latest example of the trend, when voters in Colorado, Michigan and Utah joined Missouri in approving new redistricting systems.

All states must redraw their congressional and state legislative districts after the 2020 census. Those new maps generally will kick in for the elections two years later. Although the criteria vary by state, most require districts to contain similar populations, keep communities together when possible and give minorities a chance to elect candidates of their choice.

The constitutional amendment approved by Missouri voters in November keeps those criteria. But it also requires a new nonpartisan state demographer to base state House and Senate districts on the votes cast in the previous three elections for president, governor and U.S. senator — races that are decided by voters statewide and are not affected by gerrymandering.

The districts must come as close as practical to achieving “partisan fairness” as measured by a formula called “the efficiency gap.”

That formula was created earlier this decade by Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California, and University of Chicago law professor Nick Stephanopoulos. It compares the statewide share of the vote a party receives 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.

Although the efficiency gap has been cited in court challenges to politically gerrymandered districts in Wisconsin and elsewhere, no other state has made it a legally required test for redistricting.

California, which adopted an independent redistricting commission for the 2010 census, prohibited political favoritism in drawing districts but included no test to determine if that occurs. Since then, Democratic dominance has increased in both the state’s congressional delegation and its state legislative chambers.

McGhee, who was not involved in Missouri’s initiative, said the formula will constrain what mapmakers can do to inflate partisan majorities.

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.

In the November elections, Republican candidates received an average of 57 percent of the two-party vote across Missouri’s 163 House districts, yet Republicans won 71 percent of the seats. That gave them a 116-47 majority over Democrats.

That equates to an 8 percent efficiency gap favoring Republicans — or an additional 13 GOP seats beyond what would be expected from the total votes, according to an AP analysis.

The Republican majorities in Missouri’s two legislative chambers remained unchanged after this year’s elections despite the fact Democrats fielded their highest number of candidates since 2002.

Stronger Republican fundraising and messaging make it difficult for Democrats to win legislative races in parts of Missouri, said Rep. Peter Merideth, who led the Democrats’ House Victory Committee.

But “certainly, the way that they’ve drawn the districts is part of it,” he added.

Like many states, Missouri’s urban centers of St. Louis and Kansas City have high concentrations of Democratic voters while the rural areas are overwhelmingly Republican. That results in a lot of lopsided state legislative races, even though statewide races often are highly competitive.

By requiring legislative districts to be based on the votes for top statewide races, Democrats are likely to gain influence. The new redistricting formula takes into account nine statewide elections from 2012 to 2020. In the seven that already have been held, Republicans received 51 percent of the vote to Democrats’ 49 percent.

To achieve partisan fairness, the new legislative maps would have to be drawn so Democrats would be projected to win about 48 percent of the seats. That means some urban Democratic voters likely would have to be mixed together with rural Republican voters in elongated districts that could look more gerrymandered than the current ones.

But it still might not be possible to obtain perfect partisan fairness. Democrats topped out at around 44 percent of the House seats in a simulated map prepared for the group Missourians First, which opposed the redistricting ballot measure.

“Because rural Missouri is so Republican, you actually cannot draw to that standard” in the constitutional amendment, said Scott Dieckhaus, a Republican consultant who managed the Missourians First campaign. “So you basically just draw as many competitive seats as you can.”

Even then, Democrats could fall short of projected gains. That’s because the formula, by including the results of Democratic victories in the governor’s and U.S. Senate races in 2012, may understate the Republican shift that has occurred in the state since then.

“It may look like on paper you’ve suddenly created a plan that’s only going to elect 55 percent Republicans or something, but in reality, it will produce as many Republicans as the existing plan,” McGhee said.

Democratic consultant Sean Nicholson, who directed the Clean Missouri campaign for the ballot measure, said he believes Democrats could gain seats under the plan but are unlikely to win control of either chamber in the immediate future.

“The goal is to have a Legislature that more closely matches what voters are saying they want in these statewide races,” Nicholson said.

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