Is partisan gerrymandering constitutional? And if not, how is it to be measured? Those are the questions at the heart of one of the most consequential Supreme Court cases of the year, which the justices will hear next week. How the court answers those questions in the case, Gill v. Whitford, has the potential to fundamentally change how we build our representative democracy.

Later this fall, FiveThirtyEight is launching an audio documentary series about the challenges of reforming the redistricting process in America. Traditionally, state lawmakers redraw the maps that determine the races in which you vote after the census every 10 years. Reformers want to change who draws the maps and/or the criteria for drawing them. One of the episodes of our series focuses on the gerrymandering case before the Supreme Court. Rather than keep it in our pocket until after the case is heard, we wanted to share it with you ahead of oral arguments, which are on Oct. 3. So here it is! Listen here or subscribe to the FiveThirtyEight Gerrymandering Project podcast feed.

The Case. After the 2010 midterm elections, the party in charge of Wisconsin’s government flipped. A democratically controlled State Assembly, Senate and governor’s office became controlled instead by the GOP. Soon after, it was time to redraw the district lines. Republicans came up with new maps that Democrats allege diluted their supporters’ voting power. The question for the court is whether the Republicans violated the Constitution by redrawing the district maps in their favor — a process known as partisan gerrymandering.

The Court. Conservative judges usually argue that it is not the court’s place to dictate the decision-making of a state legislature. Liberal judges maintain that the process has gotten out of hand to the point of violating the Constitution, and it is therefore the responsibility of the court to step in.

The decision is likely to come down to Justice Anthony Kennedy. He is a conservative judge, but his views on this matter don’t exactly align with either side of the aisle. In a 2004 opinion, he expressed alarm at the practice of extreme partisan gerrymandering but said the court should hold off on outlawing it. He also left the door open to ruling differently in the future, as long as lawyers provided a suitable way to measure when gerrymandering goes too far. He said that without that measurement, the court runs the risk of becoming too involved in a decision-making process that is legally mandated to state legislatures.

The Big Question. So, how exactly do you measure partisan gerrymandering? Reformers have scrambled to come up with an answer ever since Kennedy’s challenge. The solution presented in the current case proposes three questions to determine whether a map violates the Constitution:

Was the intent of the new map to benefit one party over the other?

Does the map significantly discriminate against one party over a sustained period of time? The plaintiffs’ lawyers point to multiple ways to measure this in their case, but their primary measure has been the efficiency gap. We explain in full, using vodka tonics, in the podcast.

Is there any reason other than partisan gerrymandering that one party is continuously at a disadvantage? For example: Are voters of one party naturally concentrated in a small number of districts?

To hear the story of how the case came to be and the arguments both sides are making, click on the play button above. And we’ll be back with our full redistricting series in a couple of months.

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