The Supreme Court agreed Monday to hear arguments in a Wisconsin case about partisan redistricting and gerrymandering, taking on a longstanding question that could change the way states draw congressional and legislative districts.

The justices have never fully answered when partisan gerrymanders — or maps that benefit one political party to the detriment of another — could be unconstitutional. The Supreme Court hasn’t weighed in on the issue in more than a decade and could be sharply divided.

In the meantime, state lawmakers have used political affiliation as a way to draw electoral maps, and an increasing chorus of political groups blames partisan gerrymandering for a growing divide in Congress and state legislatures.

“This is a historic opportunity to address one of the biggest problems facing our electoral system,” said Wendy Weiser, director of the Democracy Program at the Brennan Center for Justice. “Gerrymandering has become so aggressive, extreme and effective that there is an urgent need for the Supreme Court to finally step in and set boundaries.”

The court is expected to hear arguments during the next term that starts in October.

In the Wisconsin case, a divided panel of three judges last year struck down the state’s 2011 state assembly maps as drawn with discriminatory intent — “to entrench the Republican party in power” for the remainder of the decade.

The panel, in a 2-1 vote, found the Republican-controlled legislature enacted a plan that dilutes the voting strength of Democratic voters statewide, impeding their ability to translate their votes into legislative seats — and no political geography of the state or other state interest explains the discriminatory effect.

That redistricting plan violated the First and Fourteenth Amendment rights of Wisconsin voters, the panel ruled, ordering the state to redraw district lines by November. The dissenting judge said that Wisconsin might have been politically motivated but the state complied with traditional redistricting principles that the Supreme Court has previously upheld.

On Monday, the Supreme Court in a separate order stopped the redrawing of Wisconsin’s state legislative districts until the justices decide the case, but showed how divided it could become. The liberal wing of the court — Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — would not have blocked the order and let the redistricting proceed.

“I am thrilled the Supreme Court has granted our request to review the redistricting decision and that Wisconsin will have an opportunity to defend its redistricting process,” Wisconsin Attorney General Brad Schimel, a Republican, said in a statement. “As I have said before, our redistricting process was entirely lawful and constitutional, and the district court should be reversed.”

Case history

Lawyers for Wisconsin had urged the Supreme Court to summarily reverse the three-judge panel’s decision even without hearing the case. The electoral results under the Wisconsin map were remarkably similar to court-drawn maps in past decades, the state’s brief states.

“This is a reflection of Wisconsin’s political geography, where Democrats concentrate in urban areas like Madison and Milwaukee, as well as incumbency advantage,” the state wrote.

The only way the Wisconsin legislature could have attained the election results sought in the lawsuit would have been to “engage in heroic levels of nonpartisan statesmanship by abandoning Republicans’ advantage under court-drawn plans, including by adopting a plan under which incumbents were more likely to lose their seats,” the state argued in the brief. 

The voters who are challenging the plan, led by the nonpartisan Campaign Legal Center, argue that the map allowed Wisconsin’s state Assembly to not resemble the electorate in a closely divided swing state that has supported presidential candidates from both parties and currently has one Democrat and one Republican representing it in the U.S. Senate. 

In 2012, Republicans won a supermajority of 60 seats — out of 99 — while losing the statewide presidential vote, the challengers wrote in a brief. In 2014 and 2016, Republicans extended their statehouse advantage to 63 and then 64 seats even though the statewide vote remained nearly tied.

“Republicans thus wield legislative power unearned by their actual appeal to Wisconsin’s voters,” the challengers wrote.

At the center of the challenge is a new way to measure the effect of partisan gerrymandering — a formula called the efficiency gap. In part, it seeks to tally the number of “wasted votes” in districts that would give an advantage to one party: for example, packing Democrats into one district where a Democrat is elected in a landslide while Republicans in nearby districts win by narrow margins.

The Supreme Court has sought a way to establish a standard for determining when partisan gerrymandering occurred, and it is unclear whether this Wisconsin formula will satisfy the justices.

The efficiency gap is also part of a similar case in North Carolina that challenges that state’s congressional districts. That case, scheduled for a trial in June, could be put on hold as the Supreme Court considers the Wisconsin case.

“The threat of partisan gerrymandering isn’t a Democratic or Republican issue; it’s an issue for all American voters,” Trevor Potter, president of the Campaign Legal Center and a former Republican chairman of the Federal Election Commission, said in a written statement.

“Across the country, we’re witnessing legislators of both parties seizing power from voters in order to advance their purely partisan purposes,” Potter said. “We’re confident that when the justices see how pervasive and damaging this practice has become, the Supreme Court will adopt a clear legal standard that will ensure our democracy functions as it should.”

The case is Gill v. Whitford, Docket No. 16-1161.