On Monday, a federal district court ruled that Wisconsin’s hyperpartisan gerrymander violates the U.S. Constitution. The three-judge panel’s 2–1 decision marks a turning point in the way the judiciary evaluates gerrymandering: Never before has a federal court invalidated a gerrymander for providing an unfair advantage to one political party. If the Supreme Court affirms the decision—and the ruling appears to be catered toward the idiosyncratic constitutional philosophies of Justice Anthony Kennedy, who could be the deciding vote—it will overhaul congressional maps across the country and fundamentally alter representation in the United States.

Gerrymandering, the practice of drawing districts to benefit one party, has been used by the majority party in America for more than 200 years. But recent technological advances have allowed parties to create extraordinarily specific gerrymanders that keep the minority party out of power indefinitely. Republicans have spent an immense amount of money developing and utilizing these resources; Democrats are only just beginning to catch up.

The courts have struck down race-based gerrymanders as a violation of equal protection—but until now, they have consistently held that nakedly partisan gerrymanders do not by themselves pose a resolvable constitutional problem. That’s because of a muddled Supreme Court decision in 2004 called Vieth v. Jubelirer. In Vieth, five justices agreed that partisan gerrymanders are likely unconstitutional. But Justice Kennedy refused to actually strike them down. Kennedy wrote that extreme gerrymanders may unconstitutionally burden the “representational rights of voters,” but that there was not yet any “manageable standard” by which to assess whether a gerrymander ran afoul of the Constitution. He hoped such a standard might “emerge in the future,” leaving the door open to a future challenge.

That standard appears to have emerged. In an opinion written by Judge Kenneth Ripple, a Reagan appointee, the federal district court concluded that Wisconsin’s gerrymander violated voters’ right to freedom of association and equal protection under the First and Fourteenth Amendments. The Constitution prohibits a redistricting scheme, Ripple wrote, when it is intended to impede the effectiveness of a citizen’s vote based on her political affiliation; does, in practice, dilute that vote; and cannot be justified on other legitimate grounds.

How can courts determine whether a gerrymander excessively dilutes votes? Simple, Ripple explained: They can use a mathematical formula called the efficiency gap. As the New York Times explains it:

The formula divides the difference between the two parties’ “wasted votes”—votes beyond those needed by a winning side, and votes cast by a losing side—by the total number of votes cast. When both parties waste the same number of votes, the result is zero—an ideal solution. But as a winning party wastes fewer and fewer votes than its opponent, its score rises.

A fair redistricting scheme will create few wasted votes and thus an efficiency gap near zero. The more partisan the gerrymander, the higher the efficiency gap; a review of gerrymanders over the last four decades revealed that an efficiency gap of 7 percent will entrench the majority until new districts are drawn. The current Wisconsin gerrymander results in an efficiency gap of up to 13 percent.

The efficiency gap—and the bright line of 7 percent—appears to be precisely what Kennedy was hoping for when he opined in Vieth on the need for a clear standard. Should the efficiency gap be adopted, judges could examine the record for evidence that the legislature intended to dilute votes for the minority party and demand a legitimate justification for the advantage provided to the majority party. If there is none, the map must be invalidated and redrawn.

In recent years, Republicans have used gerrymanders to secure control of both legislative chambers in 32 states, as well as Nebraska’s unicameral legislature, and to entrench their majority in the House of Representatives. This unfair consolidation of political power at the expense of Democratic voters is surely troubling to Kennedy, who cares deeply about representational equality. The Wisconsin ruling proves that we now have the tools to reverse these outrageous gerrymanders and restore representational rights to all voters. If Wisconsin appeals Monday’s decision to the Supreme Court, the justices will probably feel compelled to settle this issue for good. And thanks to advances since Vieth, Kennedy may well provide the fifth vote to bust gerrymanders across the country.