On Monday, the Supreme Court agreed to hear an appeal this fall of a lower court ruling that struck down Wisconsin’s Republican-drawn state Assembly map as an unconstitutional partisan gerrymander. Over the past three decades, the high court has repeatedly held that partisan gerrymandering could in theory run afoul of the constitution, but it has never struck down any maps on these grounds because it could not decide upon a standard for when to do so. If the high court upholds this ruling, it would establish a sweeping precedent that could lead to a wave of lawsuits against widespread partisan gerrymanders nationally.

Republicans aggressively gerrymandered Wisconsin after they gained unified control of state government in the 2010 midterm wave. Their Assembly lines were so effective that Republicans won a commanding majority in the chamber in 2012 even as Obama won Wisconsin by 7 points and Democratic legislative candidates won more votes statewide than Republicans did. As shown in the map at the top of this post, Republicans maintained a lopsided 64-35 majority in 2016 even as Donald Trump barely won the state.

While it has regularly invalidated maps for improper racial gerrymandering, the Supreme Court, as noted above, has never struck down a map for excessive partisanship despite 31 years of precedent that partisan gerrymandering could theoretically be unconstitutional. In a 2004 case on the same topic, Justice Anthony Kennedy, as the deciding vote, refused to strike down the map in question on the basis that it represented an unfair partisan gerrymander. However, Kennedy effectively opened the door for future challengers to come up with a new standard that could satisfy the court’s perennial swing justice.

The plaintiffs in Wisconsin have sought to overcome that problem by proposing one such mathematical test called the “efficiency gap” that examines how many votes get “wasted” in each election, which we have explained in detail here. Under this test, if one party routinely wins landslide victories in a minority of seats while the other party wins much more modest yet secure margins in the vast majority of districts, that could signify a gerrymander that has gone so far as to infringe upon the rights of voters to free speech and equal protection. Although this test is imperfect, it provides one of many tools a court could use to judge a map’s partisan distortion.