But the Supreme Court has long been tolerant of partisan gerrymandering — and some justices have thought that the court shouldn't even be involved. A finding otherwise would have a revolutionary impact on the reapportionment that will take place after the 2020 election and could come at the expense of Republicans, who control the process in the majority of states.
The court accepted a case from Wisconsin, where a divided panel of three federal judges last year ruled that the state's Republican leadership in 2011 pushed through a redistricting plan so partisan that it violated the Constitution's First Amendment and equal rights protections.
The issue will be briefed and argued during the Supreme Court term that begins in October.
The justices gave themselves a bit of an out, saying they will further consider their jurisdiction over the case when it is heard on its merits.
And the justices gave an indication of how divisive the issue might be. After granting the case, the court voted 5 to 4 to stay the lower court's decision, which had required that new state legislative districts be drawn this fall. Wisconsin had argued that would create unnecessary work should the Supreme Court ultimately overturn the lower court's decision and allow the Republican plan to stand.
The liberal justices — Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — went on record saying they would have denied the stay, meaning that the court's five conservatives granted it. Justice Anthony M. Kennedy, who probably holds the key to the case, voted for the stay.
The court's action comes at a time when the relatively obscure subject of reapportionment has taken on new significance, with many blaming the drawing of safely partisan seats for a polarized and gridlocked Congress. Former president Barack Obama has said that one of his post-presidency projects will be to combat partisan gerrymanders after the 2020 Census.
Both parties draw congressional and legislative districts to their own advantage. A challenge to a congressional plan drawn by Maryland Democrats is making its way through the courts.
But Republicans have more to lose because they control so many more state legislatures. The Republican National Committee and a dozen large Republican states have asked the court to reverse the decision of the federal court in Wisconsin court decision.
That state's legislative leaders asked the Supreme Court in their brief to reject any effort that "wrests control of districting away from the state legislators to whom the state constitution assigns that task, and hands it to federal judges and opportunistic plaintiffs seeking to accomplish in court what they failed to achieve at the ballot box."
But the dozen plaintiffs — voters across the state — said the evidence laid out in a trial in the Wisconsin case showed that "Republican legislative leaders authorized a secretive and exclusionary mapmaking process aimed at securing for their party a large advantage that would persist no matter what happened in future elections."
In the election held after the new district maps were adopted, Republicans got just 48.6 percent of the statewide vote, but captured a 60-to-39 seat advantage in the State Assembly.
The Supreme Court has been reluctant to tackle partisan gerrymandering and sort through arguments about whether an electoral system is rigged or, instead, a party's political advantage is because of changing attitudes and demographics, as Wisconsin Republicans contend.
The justices last took up the topic in 2004 in a case called Vieth v. Jubelirer, which involved a Pennsylvania redistricting plan. The case split the court five ways, with the bottom line being that the justices could not agree on a test to determine when normal political instincts such as protecting your own turned into an unconstitutional dilution of someone else's vote.
Four justices — only Justice Clarence Thomas remains of the group — said it was not the court's business to make such decisions. Four others — only Ginsburg and Breyer remain — said such challenges could be heard by the court, but they disagreed on the method.
Kennedy was in the middle. He joined the first group in deciding the specific case against the challengers of the Pennsylvania plan, but he left the door open for future cases.
Kennedy said he could envision a successful challenge "where a state enacts a law that has the purpose and effect of subjecting a group of voters or their party to disfavored treatment." What was elusive, Kennedy said, was "a manageable standard by which to measure the effect of the apportionment and so to conclude that the state did impose a burden or restriction on the rights of a party's voters."
In the Wisconsin case, plaintiffs urged the use of a measure called the "efficiency gap" to determine how Republican mapmakers hurt Democrats with the main tools of gerrymandering: "packing" and "cracking." These refer to packing like-minded voters, such as supporters of the same party, into a limited number of districts or cracking their influence by scattering them across districts in numbers too small to make an impact.
Under the approach, developed by two University of Chicago professors, every voter packed into a district above the threshold needed to elect a candidate from his party creates a "surplus" vote. And someone in a cracked district who votes for a candidate who is unable to win is a "lost" vote. Surplus and lost votes are considered wasted votes.
The efficiency gap measures the difference between the wasted votes of the two parties in an election divided by the total number of votes cast.
The Wisconsin federal court in Wisconsin was not so definitive. It acknowledged the efficiency gap, but only as one of several theories the court said corroborated its findings that the Republican leadership had a discriminatory intent, that its plan had a discriminatory effect and that the state had no legitimate reason for drawing the districts in the way it did.
The state contends that while Wisconsin is a purple state in national elections, its geography favors Republicans in legislative elections. Democratic voters are clustered in cities such as Milwaukee and Madison, while Republican voters are more evenly spread across the state. Any method of drawing districts will favor Republicans, they say. The case is Gill v. Whitford.