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Here’s how to fix partisan gerrymandering, now that the Supreme Court kicked it back to the states.

We analyzed different approaches -- and found one reform that really works.

On Thursday, the Supreme Court released its much-anticipated decision in Rucho v. Common Cause, saying that if a legislature draws districts that disproportionately favor one party — usually called “partisan gerrymandering” — it’s a “political question” that the federal courts can’t fix.

Those who oppose partisan gerrymandering must therefore fight it in the states, where reformers have made significant gains. Last year, the Pennsylvania Supreme Court overturned a Republican-drawn state map and replaced it with one drawn by an independent expert, leading Democrats to gain three House seats in 2018. Voters in five states approved ballot initiatives to adopt independent redistricting. Several other states are considering similar measures.

But it is not clear whether independent districting will lead to politically neutral maps, in part because so little research has been done. To understand the consequences of redistricting reform, we have spent the last several years compiling a comprehensive data set on how all 50 states handled redistricting of the 99 state legislative chambers after the 2010 Census. Here’s what we found.

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Reformers have tried for large and smaller changes in redistricting, aimed at reducing partisan bias

In the 1960s, the Supreme Court first ruled that states must revise their legislative maps after every census to ensure they met the “one person, one vote” standard. Since then, reformers at the state level have advanced many types of measures aimed at reducing partisan influence in redistricting.

Some significantly change the process, as when California voters in 2010 installed an independent and nonpartisan redistricting commission to draw new maps.

Others involve more modest changes, such as rules that require maps to meet what has been called “fair districting” criteria. In the 1970s, for example, several states added new “district compactness” requirements to their constitutions that required lawmakers to avoid drawing irregularly shaped districts. More recent reforms have included rules that prohibit the splitting of political or “community” boundaries across district lines.

Do redistricting rules prevent maps that are biased toward one party?

Using data published by Justin Levitt, we considered the effects of three of the most common types of districting criteria: district compactness requirements; rules that require districting authorities to preserve “communities of interest”; and rules requiring that districts respect political boundaries such as counties or municipalities.

None of these rules made a difference in the level of partisan bias we found in the districts drawn before and after they were in place. Of course, although such rules don’t prevent maps that tilt toward one party or another, they may serve other purposes — such as preserving the bond between an elected official and a community of citizens.

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Does an independent redistricting commission prevent partisan bias in district maps?

To understand this, we grouped each of the 99 state legislative plans redrawn after 2011 into four categories, based on who drew the maps: Republicans, Democrats or politicians from both parties (working in a legislature or political committee); or nonpartisan outsiders, including those working in an independent commission like California’s Citizens Redistricting Commission, or a court-ordered remedial map.

To measure bias, we used the standard of symmetry — that is, whether a map treats parties similarly under similar circumstances. For example, if Party A were to receive 65 percent of the seats with 55 percent of the vote in one election, a symmetrical map would also award Party B the same share of seats if they were to receive 55 percent of the vote in a different election. As we explain in great detail in our previous work, this method allows us to estimate how biased a plan is in favor of one party over another.

When we compared the maps used before 2011 with the maps used after 2011, we found starkly different outcomes depending upon which party controlled redistricting. Not surprisingly, the maps drawn by a single party — either Democrats or Republicans — became less symmetrical after redistricting. Plans drawn by Republicans generally became more favorable to the Republican Party, while the plans drawn by Democrats increasingly favored Democratic politicians. This didn’t happen to the same degree. Republicans drew more bias into their plans, in part because Democratic voters tend to live disproportionately in cities, which makes it easier for Republicans to draw efficient gerrymanders.

By contrast, in the maps drawn by bipartisan or nonpartisan actors, the level of partisan bias didn’t change much after 2011 — with one key difference. The nonpartisan-drawn maps tended to be more symmetrical on average after redistricting. In other words, they tended to treat both parties similarly. This suggests that nonpartisan bodies have successfully neutralized partisan bias, as intended.

Independent districting works, but politicians will resist it

Although our analysis is only preliminary, our findings suggest that independent redistricting is likely to succeed in neutralizing partisan influence. But in many of the states where reformers have won key victories, lawmakers have resisted implementing those reforms.

For example, last year, Missouri voters approved a ballot measure that tasks a nonpartisan state demographer with redistricting — but Republican legislators are suing to block it from going into effect. Similarly, last fall Michigan’s voters passed a constitutional amendment creating an independent redistricting commission — but in a lame-duck session, lawmakers passed regulations that, critics said, would constrain the commission’s power.

The battle to end partisan influence in districting has moved to the states — and it’s far from over.

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Alex Keena is an assistant professor of political science at Virginia Commonwealth University and co-author of “Gerrymandering in America: The House of Representatives, the Supreme Court, and the Future of Popular Sovereignty” (Cambridge University Press, 2016)

Michael Latner (@mlatner) is a political science professor at California Polytechnic State University and a Kendall Voting Rights fellow at the Union of Concerned Scientists’ Center for Science and Democracy.

Charles Anthony Smith is a professor of political science and law at the University of California at Irvine.

Anthony McGann is a professor of government and public policy at the University of Strathclyde.