On Tuesday, the Supreme Court will hear oral arguments in a case that has the potential to put firm limits on partisan gerrymandering — and dramatically change the way states draw legislative boundaries.

The case, Gill v. Whitford, challenges the 2011 Wisconsin state assembly map. Those districts were drawn by the Republican state legislature in Wisconsin, and packed Democrats into a smaller number of districts to maximize Republican odds. The lawsuit argues that the map is an unconstitutional effort to help Republicans retain power.

That kind of gerrymandering, the suit alleges, violates Democrats’ constitutional rights in two ways: under the First Amendment freedom of association, since they’re being disenfranchised based on the party they chose to join, and under the 14th Amendment’s equal protection clause, because Democrats are effectively entitled to less representation than Republicans.

A divided three-judge panel of the US District Court for Wisconsin ruled in 2016 against the Wisconsin map, concluding that the plaintiffs are correct and that the map’s gerrymandering is unconstitutional. Kenneth Ripple, the author of that opinion, wrote, "We conclude … that the evidence establishes that one of the purposes of [the district map] was to secure Republican control of the Assembly under any likely future electoral scenario for the remainder of the decade, in other words to entrench the Republican Party in power."

The panel stayed the map and ordered the legislature to redraw it. As part of agreeing to hear the case, the five conservatives on the Supreme Court (John Roberts, Neil Gorsuch, Samuel Alito, Clarence Thomas, and Anthony Kennedy) stayed the District Court ruling, effectively removing the near-term requirement that Wisconsin redraw its map. The four liberals (Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan) dissented.

The Supreme Court’s history with gerrymandering

Gerrymandering is as old as the American republic, and has been done for a variety of reasons. Historically, districting meant to reduce the power of black voters has been very common. But partisan gerrymandering has also been a powerful force. Typically, the party in control of a state legislature will try to draw congressional and state legislative districts in such a way as to maximize their own odds.

For instance, Democrats in charge of the Maryland legislature have divvied up Democratic base voters in the DC suburbs and Baltimore into large a number of districts, while concentrating Republicans in more rural parts of the state in a smaller number of districts. Republicans in Ohio and Pennsylvania have done the reverse, concentrating urban voters in a few heavily Democratic districts.

While the Supreme Court has ruled on many aspects of the districting process — banning state legislative districts with unequal populations and banning districts intended to disenfranchise black voters — it has issued muddled opinions on the question of whether partisan gerrymanders are unconstitutional. There’s extensive case law on racial gerrymanders, which has established that racial discrimination in districting is subject to strict scrutiny by courts.

But discrimination on the basis of party is not the same as racial discrimination. The Court has agreed to no firm standard as to which political considerations are and are not allowed in creating congressional and legislative districts, and in 2004’s Vieth v. Jubelirer a plurality opinion by the right wing of the Court argued that no such standard is even possible.

But the Court’s four liberals dissented, and Anthony Kennedy filed a concurrence arguing that it was possible the Court could develop such a standard in the future.

Since then, there’s been a lot of academic energy around trying to develop such a standard. University of Chicago law professor Nicholas Stephanopoulos and political scientist Eric McGhee devised one promising option, which notes that gerrymandering forces the losing party to "waste" votes by placing all its voters into a small number of districts where the party gets a landslide, rather than spreading out those voters so they can have more impact.

Stephanopoulos and McGhee argue that fair districting requires a roughly equal number of wasted votes for each party, and that districting schemes where one party is wasting many more votes are unconstitutional. They call their metric the “efficiency gap,” calculated by taking the difference between the number of “wasted votes” for each party, and dividing that difference by the total number of votes. (Stephanopoulos explains the math in more detail in this Vox piece.)

The efficiency gap is key to the plaintiffs’ arguments in Gill v. Whitford. They proposed setting a threshold of 7 percent: If a districting plan produces a larger gap than that, if one party is getting a wasted-vote advantage of more than 7 percent of the total vote, then it’s getting a huge leg up, which will continue for a long time. As Yale Law School dean Heather Gerken noted in a Vox piece following the initial district court decision, a gap above that amount indicates that the disadvantaged party “would have almost no chance of taking control of the legislature during the 10-year districting cycle.”

By contrast, the Wisconsin plan created efficiency gaps of 13 percent and 10 percent in 2012 and 2014, respectively. Those are truly massive advantages enjoyed by the Republican Party.

By taking up the case, the Supreme Court is essentially promising to rule on the merits of the efficiency gap as a means of determining whether an improper partisan gerrymander has happened — and, if one has occurred, on whether that violates either First or 14th Amendment protections.

The key question, as always on this Supreme Court, is where Kennedy will land. His 2004 concurrence indicated an openness to quantitative measures of partisan skew, and the efficiency gap and similar measures were to some extent devised to answer that demand of his. However, he sided with the Court’s conservatives in staying the lower court ruling, which might indicate a lack of sympathy with the plaintiffs and a willingness to let the map slide.