(Ricky Carioti/The Washington Post)

A unanimous panel of federal judges on Friday declared Ohio’s Republican-drawn congression­al map unconstitutional, adding to a growing number of states where partisan gerrymandering has been outlawed.

That decision and a similar one last month in Michigan could be seen as signals from the lower courts to their superiors. The Supreme Court is deciding whether judges even have a role in such disputes.

While the high court regularly polices redistricting plans for racial gerrymandering, it has never found lawmakers’ partisan efforts to preserve power so extreme that their actions violate the constitutional rights of voters. The justices’ decision is expected by the term’s end in June.

But with the ruling in Ohio, federal courts in five states have struck down maps as partisan gerrymanders. The decisions will either guide the Supreme Court to find there is a way for judges to identify extreme partisanship — or make the rulings short-lived.

Often, lower courts will hesitate when the Supreme Court is considering an issue. But last month, the unanimous panel that found that some of Michigan’s legislative and congressional maps were unconstitutional seemed anxious to send a message to the high court.

“Judges — and justices — must act in accordance with their obligation to vindicate the ­constitutional rights of those harmed by partisan gerrymandering,” Judge Eric L. Clay of the U.S. Court of Appeals for the 6th Circuit wrote in the Michigan case.

The Ohio court took a similar approach in Friday’s decision.

“We join the other federal courts that have held partisan gerrymandering unconstitutional and developed substantially similar standards for adjudicating such claims,” the panel said in its unanimous ruling.

“We are convinced by the evidence that this partisan gerrymander was intentional and effective and that no legitimate justification accounts for its extremity.”

The judicial decisions have been markedly bipartisan. Both the unanimous panels in Michigan and Ohio consisted of judges chosen by Democratic and Republican presidents, and a scathing decision about Maryland’s redistricting plan was written by Judge Paul V. Niemeyer, a conservative nominated to the U.S. Court of Appeals for the 4th Circuit by President George H.W. Bush.

The judges in the Ohio case said the district lines were drawn by Republican legislators to entrench Republican control, and that violated the rights of Democratic voters. The maps drawn after the 2010 Census have consistently resulted in a 12-4 Republican advantage in Ohio’s congressional delegation.

The Ohio map “is so skewed toward one party that the electoral outcome is predetermined,” wrote Judge Karen Nelson Moore of the U.S. Court of Appeals for the 6th Circuit. “We conclude that the map unconstitutionally burdens associational rights by making it more difficult for voters and certain organizations to advance their aims, be they pro-Democratic or pro-democracy.”

The Ohio challengers were represented by the American Civil Liberties Union.

The Supreme Court in March heard arguments in similar cases from North Carolina, where judges found that Republicans had manipulated congressional maps to their advantage, and from Maryland, where Democratic lawmakers redrew a district that resulted in a loss for a longtime Republican congressman.

At the oral arguments, the conservative justices who make up the Supreme Court’s majority seemed skeptical that the court could find a manageable test for deciding when politics plays an unconstitutional role in drawing maps.

If technology has made thousands of potential maps available to state lawmakers who draw the districts, Justice Samuel A. Alito Jr. asked, “How do you determine whether that choice is constitutional?”

And the attorney representing the North Carolina Republican legislators who drew the map, Washington lawyer Paul D. Clement, said the reason it was difficult for courts to come up with tests for making such judgments is because it is clear the founders considered drawing the maps a job for the political branches, not the courts.

But others said the Ohio decision was another example that judicial oversight is required and workable.

“The Supreme Court has been looking for years for a manageable legal standard to limit partisan gerrymandering,” said Ruth Greenwood, co-director of voting rights and redistricting at the Campaign Legal Center. “Today’s decision in Ohio now marks the fourth federal court to use the three-part test we proposed.”

That test involves a finding that the map-drawing was intentional to benefit one party and hurt the other, that it was effective and durable, and that it cannot be justified by other factors, such as political geography.

The Ohio decision, like the one in Michigan, would require the state to draw new districts for use in the 2020 elections. Both states will ask the Supreme Court to stay the decisions, and that is a likely outcome while the justices decide the North Carolina and Maryland cases.

Ohio Attorney General Dave Yost said in a statement:

“Ohioans already voted to reform how we draw our congressional maps. This protracted opinion takes that decision out of the hands of the people and is a fundamentally political act that has no basis whatsoever in the Constitution.”

As Yost noted, people in Ohio and Michigan were among those in five states in 2018 who voted to reduce — or eliminate — the role of legislators in drawing district lines.

At the Supreme Court oral arguments, Justices Neil M. Gorsuch and Brett M. Kavanaugh noted those successes, and said that might provide a reason for courts to stay out of the fray.

But lawyers noted that in most states, voters do not have the right to initiate such constitutional amendments — only the legislature does.

And there’s been a development since oral arguments that casts doubt on Gorsuch’s faith that voters can fix the problem — if there is one — themselves.

Missouri is one of the states that passed such a referendum in 2018. It would, among other changes, give a nonpartisan state demographer the lead in drawing the maps.

But there’s an effort by Missouri legislators to undo the changes just six months after more than 60 percent of voters approved them. A constitutional amendment that would restore legislators’ role has already passed one house.

“The fights go on and on and on,” Greenwood said.