The Ohio plan had overwhelming bipartisan support in the state legislature when lawmakers approved it for the May ballot.

Ohio voters approved a ballot measure on Tuesday that changes the congressional mapmaking process, a move proponents say will rein in excessive partisan gerrymandering.

By passing the ballot measure, called Issue 1, Ohioans amended their state Constitution to create four different pathways to draw congressional districts. The new process will go into effect in 2021, when the next round of redistricting takes place.

The complex process still leaves redistricting primarily in the hands of lawmakers, but it also builds in significant safeguards intended to prevent one party from shutting the other out of the process and drawing a severely unfair map.

Ohioans adopted the measure at a time when there is a strong push to limit the ability state lawmakers have to draw a map that benefits the party in power. While both Democrats and Republicans across the U.S. have gerrymandered to their advantage throughout the decades, critics say that Republicans took it to an extreme level in 2011. By the end of June, the U.S. Supreme Court is expected to decide two cases that could, for the first time, set constitutional limits on how far lawmakers can draw district lines to benefit their parties.

“Ohioans never gave up on the fight to end to the manipulation of congressional districts for political advantage and today our efforts paid off,” said Catherine Turcer, executive director of Common Cause Ohio, who led the coalition supporting the measure.

The Ohio plan had overwhelming bipartisan support in the state legislature when lawmakers approved it for the May ballot. It reaches a middle ground between giving lawmakers control over redistricting and letting independent commissions draw congressional lines ― a step some states have taken in an effort to limit political redistricting.

Under the new process, lawmakers still have initial control of mapmaking, but they can only pass a map with a 60 percent supermajority in the legislature, including the votes of 50 percent of the minority party. If lawmakers can’t agree on a map, the process goes to a seven-member bipartisan commission, which can only adopt a plan with the support of two members of the minority party in the legislature.

Should the commission fail to agree on a plan, lawmakers will get a second chance at redistricting. They’ll still need a 60 percent supermajority to pass a new congressional map, but they would need the support of just one-third of the lawmakers in the minority.

If lawmakers still can’t reach an agreement, they will be allowed to pass a congressional plan by a simple majority vote, but it would only be in effect for four years instead of the usual 10. According to the measure, a map passed under these conditions can’t “unduly favor or disfavor” one particular party or its incumbents, and lawmakers can’t “unduly split” localities. Legislators would also have to provide a justification for their map ― making an illegal one easier to challenge in court.

In an effort to try and prevent “cracking” voters and diluting one party’s voting power, map drawers will have to preserve 65 of Ohio’s 88 counties in a single congressional district. Of the remaining districts, they can split 18 counties into two districts and five counties into three districts.

The bipartisan support for the measure was notable because Republicans currently benefit from gerrymandering in Ohio. They controlled the redistricting process in the state in 2011 and have consistently won 12 of the state’s 16 congressional seats while winning around 50 percent of the vote. The Brennan Center for Justice estimates that congressional gerrymandering in Ohio is responsible for two to three additional GOP seats in Congress.

“The redistricting reform passed today in Ohio makes it much harder for politicians to rig elections through gerrymandering and gives me hope that we can restore fairness to our elections in states around the country,” said former U.S. Attorney General Eric Holder, who chairs the National Democratic Redistricting Committee, which supported the measure. “Ohio has proven that when citizens work together to demand their elected officials support a fair redistricting process, positive change is possible.”

Richard Gunther, a professor emeritus at Ohio State University, told HuffPost “the only reason” Republicans supported the ballot initiative was because activists had already gained 225,000 signatures to get a different measure on the ballot. This measure would have given the bipartisan commission complete control over the redistricting process.

However, some in the state were uncomfortable with the recently passed ballot proposal, saying it would still allow lawmakers complete control of the redistricting process. The Ohio chapter of the American Civil Liberties Union, for example, did not take a position on the ballot measure, and its senior policy director expressed concern the process would encourage deals that are advantageous to politicians but not in the best interests of their constituents.

The ACLU worries that the minority party can still be coerced into agreeing to a map that egregiously benefits the party in power, despite this complex process.

That majority party, the group says, could still put forth a plan, knowing the minority would never agree, with the intention of getting to the final step where they could enact the plan with a majority vote anyway.

This story has been updated with comment from Eric Holder and Catherine Turcer.