The map, wrote for the majority of a divided three-judge Federal District Court, “was designed to make it more difficult for Democrats, compared to Republicans, to translate their votes into seats.”
Paul Smith, a lawyer for the voters who challenged the map, said it was time for the Supreme Court to act.
“Partisan gerrymandering of this kind is worse now than at any time in recent memory,” Mr. Smith said. “The Supreme Court has the opportunity to ensure the maps in Wisconsin are drawn fairly, and further, has the opportunity to create ground rules that safeguard every citizen’s right to freely choose their representatives.”
Wisconsin’s attorney general, Brad Schimel, said he was “thrilled the Supreme Court has granted our request” to hear the appeal. “Our redistricting process was entirely lawful and constitutional,” he said.
The case is part of a larger debate over political gerrymandering. Some critics, like Arnold Schwarzenegger, a Republican and the former governor of California, rather than politicians. Prominent Democrats, including former President Barack Obama and his attorney general Eric H. Holder Jr., are pushing to Republicans made after the 2010 census when the next census is taken three years from now.
In Wisconsin, the redistricting took place after Republicans had gained complete control of the state government for the first time in more than 40 years. Lawmakers promptly drew a map for the State Assembly that helped Republicans convert very close statewide vote totals into lopsided legislative majorities.
In 2012, Republicans won 48.6 percent of the statewide vote for Assembly candidates but captured 60 of the Assembly’s 99 seats. In 2014, 52 percent of the vote yielded 63 seats.
In the past, some justices have said the court should stay out of such political disputes. Others have said partisan gerrymanders may violate the Constitution.
The fate of the case is very likely to turn on the vote of Justice Anthony M. Kennedy, who has taken a middle position, leaving the door to such challenges open a crack, though he has never voted to sustain one.
Not long after the court agreed to hear the case, it suggesting the court was quite likely to be closely divided when it hears arguments next fall.
The order granted a request to stay the district court’s decision while the Supreme Court considers the case. The court’s four liberal members — Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — dissented. Justice Kennedy was in the majority.
In 2004, Justice Kennedy wrote in on a gerrymandering case that he might consider a challenge if there were “a workable standard” to decide when such tactics crossed a constitutional line. But he said he had not seen such a standard.
The challengers in the new case, Gill v. Whitford, No. 16-1161, say they have found a way to distinguish the effect of partisanship from the many other factors that influence how districts are drawn.
The proposed standard tries to measure the level of partisanship in legislative maps by counting “wasted votes” that result from the two basic ways of injecting partisan politics into drawing the maps: packing and cracking.
Packing many Democrats into a single district, for instance, wastes every Democratic vote beyond the bare majority needed to elect a Democratic candidate. Cracking, or spreading Democratic voters across districts in which Republicans have small majorities, wastes all of the Democratic votes when the Republican candidates win.
In , , a law professor at the University of Chicago and a lawyer for the plaintiffs, and devised a formula to measure partisanship. The difference between the two parties’ wasted votes, divided by the total number of votes cast, yields an efficiency gap, they wrote.
The gap in Wisconsin was 13.3 percent in 2012 and 9.6 percent in 2014, according to the formula. The Wisconsin voters who sued to challenge the Assembly map argued that gaps over 7 percent violated the Constitution. That number was meant to capture the likelihood that the gap would endure over a 10-year election cycle, but critics say it is arbitrary.
Adopting it could transform American elections. , then a political scientist at Stanford and an expert witness for the plaintiffs, found that a third of all redistricting plans in 41 states over a 43-year period failed the 7 percent standard. Elections in 2012 and 2014 in Florida, Indiana, Kansas, Michigan, Missouri, North Carolina, New York, Ohio, Rhode Island, Virginia, Wisconsin and Wyoming featured efficiency gaps of more than 10 percent, he found.
Judge Ripple did not ground his opinion in the efficiency gap, relying instead on a more conventional legal test that considered discriminatory intent, the map’s partisan effects and whether they were justified by other reasons. But Judge Ripple did say that the efficiency gap corroborated the majority’s conclusions.
In urging the Supreme Court to reverse the ruling, the Republican National Committee said the efficiency gap “is a tool that advances the partisan interests of the Democratic Party.”
If Democrats lack electoral power, it is because of geography rather than gerrymandering, the brief said. Democrats are often concentrated in cities, effectively diluting their voting power, while Republicans are more evenly distributed across most states, the brief said.
Judge Ripple acknowledged that how voters are distributed explains at least part of the gap. “Wisconsin’s political geography, particularly the high concentration of Democratic voters in urban centers like Milwaukee and Madison, affords the Republican Party a natural, but modest, advantage in the districting process,” Judge Ripple wrote, for instance.
But partisan gerrymandering amplified the Republicans’ advantage, he wrote.