(Jim Young/Reuters)

On Monday, a federal district court struck down Wisconsin’s state legislative districts, saying they were unconstitutionally gerrymandered to favor Republicans. The ruling is the first in three decades — and the second in the nation’s history — to strike down a district plan on partisan, rather than racial, grounds. And if other courts follow suit, the design of state legislative districts in more than a dozen other states could meet a similar fate.

Numerous times since the civil rights era, courts have struck down how partisan state legislatures set up legislative districts on the grounds that those decisions diminished the political power of racial minorities. But courts have been far less likely to intervene in cases where states organized voting districts to boost one party, preferring to not get involved in explicitly political matters.

This was why the decision Monday by the federal District Court for the Western District of Wisconsin was so significant. The court found that the Republican-held legislature had intentionally manipulated district lines to favor Republican candidates. And that manipulation was so extreme, it found, that it violated the “one person, one vote” principle and the Democratic voters’ rights to fair representation required by the Constitution's promises of equal protection.

In short, by drawing district lines that pack Democrats into a small number of districts or spread them in small numbers across many districts, the court ruled that a Republican legislature was able to gain narrow victories in a disproportionately large number of districts.

The defense had argued that any apparent bias was due to political geography — Democrats are highly concentrated in cities, so naturally are packed into districts in a way that disadvantages them. And in any case, they argued, Supreme Court precedent indicated partisan gerrymandering wasn’t up for court review.

Before this case, the existence of gerrymandering like this was widely recognized. But what wasn’t clear before was the best way to measure it — which many judges, including justices on the Supreme Court, have said is necessary to determine whether a gerrymander is unconstitutional.

The plaintiffs in this case proposed such a measurement, dubbed the “efficiency gap” by professors Nicholas Stephanopoulos of the University of Chicago Law School and Eric McGhee of the Public Policy Institute of California.

Essentially, the efficiency gap compares the number of one party’s votes that are, to use the professors’ term, “wasted,” compared to the other’s. Consider a legislative district where the Republican candidate received 1,000 votes and the Democrat got 700. Here, 299 Republican votes are wasted — the outcome of the election would have been the same whether the Republicans earned 701 votes or 1,000. Similarly, 700 Democratic votes are wasted, as the result wouldn’t change if the Democrats had received no votes at all.

The election outcome in that district alone certainly wouldn’t be considered unfair. But if a similar pattern appears across the state — more Democratic than Republican votes being wasted — it’s possible the map was drawn to systemically favor Republican candidates. If far more Democratic votes are “wasted” than Republican, Republicans could earn significantly more seats than Democrats, even if they had similar vote totals. In other words, Democrats would be less efficient at turning their votes into legislative seats — hence the term “efficiency gap.”

By that measure, Wisconsin’s state legislature’s gerrymandering appears unusually egregious. In 2012, its efficiency gap — the difference between the parties’ wasted votes divided by the total number of votes — was 13 percent in favor of the Republicans. The average state legislature’s bias was a third of that.

The plaintiffs argued that a 7 percent bias should be considered unconstitutional. Professor Simon Jackman of the University of Sydney found that was the point above which “you can be confident that you’ve seen … something that’s going to persist over the life of the plan.”  That is, with a bias above 7 percent, it’s exceedingly likely — more than a 95 percent chance, a common threshold used to determine statistical significance in academia — that the plan will never favor the other party in a subsequent election.

But breaching the 7 percent threshold is far from unique to Wisconsin. In the 2012 election, Stephanopoulos and McGhee determined efficiency gaps for 38 state legislature district maps. (It’s not possible to make the calculation if there’s no election, or if there are too many uncontested seats, among other reasons). Of those 38, 15 states’ efficiency gaps exceeded 7 percent. All but two of these were in favor of Republicans.

That said, striking down those 15 maps is far from automatic. For starters, the plaintiff must also prove that legislators intended to advantage their party in drawing the lines, which the Wisconsin plaintiffs did using expert testimony and other evidence about the legislative process. The plaintiff must also argue that there’s no other legitimate reason — such as a state’s underlying political geography — that the efficiency gap would be so large.

And the court did not explicitly endorse the 7 percent threshold the plaintiffs advocated. So, if and when a threshold is set, more or fewer states could be subject to having their map overturned. “They didn’t need to set a threshold in this case. But whatever you set it at, Wisconsin is far outside,” said Ruth Greenwood, a litigator on the plaintiff’s side.

But the plaintiffs expect more states to face legal challenges in the coming years. A case is already pending in North Carolina, where the efficiency gap is being used to argue its congressional districts are gerrymandered to favor Republicans.

Stephanopoulos, who is involved in that case as well, has advocated that a bias of two congressional seats should be the threshold to determine whether a gerrymander is unconstitutional. (An efficiency gap is put into “seat” terms by multiplying the percentage gap by the total number of congressional seats held by the state. Stephanopoulos considers this measure optimal for congressional plans since large efficiency gaps in small states should be viewed as relatively inconsequential.)

In 2012, North Carolina, among six other states, passed this threshold, all in favor of Republicans.

Gerrymandering critics say North Carolina may just be the beginning. “I think [the decision] is unlikely to prompt a massive flood of lawsuits, but it wouldn’t shock me if there were a few more,” Stephanopoulos said.

“Other states are vulnerable to lawsuits now,” Greenwood agreed, “And there’s also the possibility you can threaten lawsuits and get settlements.”

But they say more significant change could come following a ruling by the Supreme Court, which is the next level of appeal. Wisconsin has already announced it plans to appeal.

The court, which expressed interest in a measure like the efficiency gap in a 2006 opinion, could set the nationwide standard. And that may not be such a long shot. Gary King, a Harvard professor who studies gerrymandering, says a measure based on the vote-seat relationship such as the efficiency gap, “is the only real standard that has any support.”

Ahead of 2021 redistricting, the stakes are especially high. A Supreme Court ruling, because of its nationwide impact, “would mean there would be a limit on how extreme the gerrymanders could be,” Greenwood said.

Correction: An earlier version of the first graphic in this story misstated the number of wasted votes in the depicted scenario. The graphic has been updated.