The Supreme Court will hear a case on political gerrymandering that could reshape the way states draw their congressional districts for decades to come.

Last year a federal court found that Wisconsin's congressional districts, created by the state's Republican lawmakers and signed into law by Gov. Scott Walker (R), were unconstitutionally drawn to disadvantage Democratic voters.

The first piece of evidence for this is in the imbalance between the statewide popular vote and the partisan makeup of the Wisconsin Assembly: in 2012, Republican candidates earned 48.6 percent of the statewide popular vote. But because of how they had drawn district boundaries the prior year, they won more than 60 percent of the Assembly seats.


In large part, at least, through the magic of gerrymandering. The best way to understand it is to look at the chart below. It's a simplified grid of 50 voters distributed geographically into a tidy rectangle. Sixty percent of the voters belong to a hypothetical “blue” party, while 40 percent belong to the “red.”

You'd assume that for a legislature to reflect this split, it would similarly show a 60-40 blue-red split, right? But the diagram shows how you can divide those voters up in various ways to give either party a huge advantage. You can even give the reds a majority in the legislature if you get really creative, despite their minority in the population at large. And that's exactly what happened in Wisconsin, according to the people challenging the districts.

Here's the thing, though: political gerrymandering is largely a question of intent. You can draw some outrageously unfair districts and then contend that you didn't mean to. That any apparent partisan imbalance is purely an accident, or because of forces outside your control.

That's what Wisconsin Republicans have pleaded before the courts. They've said that the geography of the state's population disadvantages Democrats in a way that can't be helped: Democrats tend to cluster in big cities such as Milwaukee and Madison. That makes it harder for people drawing districts to avoid either splitting them up or packing them all together.

As Wisconsin Republicans see it, the lopsided vote totals are incidental smoke, not fire. But the plaintiffs in the Wisconsin case have brought the statistical fire.

Their argument centers on the idea of the “wasted” vote: to win an election in our winner-take-all system you need 50.1 percent of the vote. Any margin above that is wasted. Similarly, all votes cast for the opposing side are wasted, since those people's preferred candidate doesn't make it into office.

This notion was first articulated by a pair of academics in a 2015 paper. In any given election, there are going to be a lot of wasted votes. In a perfectly fair (theoretical) world, those wasted votes would kind of cancel out between the parties. But savvy politicians can draw district lines so that their adversaries waste many more votes.

“A gerrymander is simply a district plan that results in one party wasting many more votes than its adversary,” Nicholas Stephanopolous and Eric McGhee wrote in their paper. “The gerrymandering party enjoys a political advantage not because of its greater popularity, but rather because of the configuration of district lines. The parties do not compete on a level playing field.”

In the 2012 Wisconsin assembly elections, Democrats wasted hundreds of thousands more votes than Republicans, leading to the lopsided result. Some of that is likely a result of random chance -- Stephanopolous and McGhee wouldn't expect the typical state to enjoy perfect wasted vote parity.

But most of the gap is unexplainable by chance alone. Stephanopolous and McGhee say that if the difference between the parties' wasted votes accounts for more than 8 percent of total votes, it's likely the result of an unconstitutional gerrymander.

In Wisconsin in 2012, that difference amounted to over 13 percent of all votes cast. Stephanopolous and McGhee say that's way more than what can be explained by geography alone, as the state's Republicans contend in court.

The federal court agreed, striking down the districts and ordering the state to draw new ones. “A Republican advantage in political geography, although it exists, cannot explain the magnitude of Act 43’s [the act creating the districts] partisan effect,” the court wrote in its decision.

Beyond that, the plaintiffs demonstrated that in creating the districts, Republicans discarded several alternate plans that would have led to less lopsided outcomes: “the plan’s drafters created and passed on several less burdensome plans that would have achieved their lawful objectives in equal measure,” the court found.

In the end, the court ruled that “Wisconsin’s modest, pro-Republican political geography cannot explain the burden that Act 43 imposes on Democratic voters in Wisconsin.” The state subsequently appealed to the Supreme Court and that's where we find ourselves today.

Will the nation's highest court find Stephanopolous and McGhee's math as compelling?

Stay tuned.