By Michael Wines, New York Times

A panel of three federal judges said Monday that the Wisconsin Legislature’s 2011 redrawing of state Assembly districts to favor Republicans was an unconstitutional partisan gerrymander, the first such ruling in three decades of pitched legal battles over the issue.

Federal courts have struck down gerrymanders on racial grounds, but not on the grounds that they unfairly gave an advantage to a political party — the more common form of gerrymandering. The case could now go directly to the Supreme Court, where its fate may rest with a single justice, Anthony Kennedy, who has expressed a willingness to strike down partisan gerrymanders, but has yet to accept a rationale for it.

Should the court affirm the ruling, it could upend the next round of state redistricting, in 2021, for congressional and state elections nationwide, most of which is likely to be conducted by Republican-controlled legislatures that have swept into power in recent years.

“It is a huge deal,” said Heather Gerken, a Yale Law School professor and an expert on election law. “For years, everyone has waited for the Supreme Court to do something on this front. Now one of the lower courts has jump-started the debate.

“If this were to be a nationwide standard, 2021 would look quite different,” she said, “especially for the Democrats.”

Several election-law scholars said the ruling was especially significant because it offered, for the first time, a clear mathematical formula for measuring partisanship in a district, something that had been missing in previous assaults on gerrymandering.

The 2-1 ruling (pdf) by the U.S. District Court for the Western District of Wisconsin said that the Legislature’s remapping violated both the First Amendment and the Equal Protection Clause of the 14th Amendment because it aimed to deprive Democratic voters of their right to be represented.

“Although Wisconsin’s natural political geography plays some role in the apportionment process,” the court wrote, “it simply does not explain adequately the sizable disparate effect” of Republican gains in the state Assembly after the boundaries were redrawn.

The two judges who ruled in favor of the plaintiffs, Kenneth Ripple and Barbara Crabb, were nominated to the bench by Presidents Ronald Reagan and Jimmy Carter, respectively. Judge William Griesbach, who was nominated by President George W. Bush, dissented.

The boundaries of both federal and state legislative districts are redrawn every 10 years after the census to ensure that each district contains roughly the same number of people, a standard the Supreme Court set with its one-person-one-vote ruling in 1962.

But both Republican and Democratic majorities in statehouses often remap districts to favor themselves, either by cramming opposition voters into a single district or by dividing them so they are the majority in fewer districts, tactics called “packing and cracking.”

Courts have generally agreed that some partisan advantage in redistricting is tolerable, in part because voters themselves are not spread equally across a state or district by party. But the plaintiffs in the case, 12 state Democrats represented by the Campaign Legal Center, had argued that the Wisconsin remapping was among the most sharply partisan in the nation.

Their lawsuit said that in the 2012 elections for the Assembly, Wisconsin Republicans won 48.6 percent of the two-party vote, but took 61 percent of the 99 Assembly seats.

A key question in Monday’s ruling, as in past challenges to redistricting, was whether that division was unacceptably partisan, a question that previous courts have stumbled over.

“Nobody has come up with a standard to measure constitutionality — how to distinguish between malevolent, evil partisanship that’s manipulative, versus the natural advantage one party might have as a result of where voters happened to live,” said Edward Foley, the director of the Election Law Project at Ohio State University’s Moritz College of Law.

In Monday’s ruling, the court was swayed by a new and simple mathematical formula to measure the extent of partisan gerrymandering, called the efficiency gap. 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 truly efficient gerrymander spreads a winning party’s votes so evenly over districts that very few votes are wasted. A review of four decades of state redistricting plans concluded that any party with an efficiency gap of 7 percent or more was likely to keep its majority during the 10 years before new districts were drawn.

In Wisconsin, experts testified, Republicans scored an efficiency gap rating of 11.69 to 13 percent in the first election after the maps were redrawn in 2011.

Some experts said the efficiency gap gives gerrymandering opponents their most promising chance yet to persuade a majority of the Supreme Court to limit partisan redistricting.

“It does almost exactly what Justice Kennedy said he was looking for back in the ‘80s, a clear threshold for deciding what is acceptable,” said Barry C. Burden, the director of the Elections Research Center at the University of Wisconsin-Madison.

Under procedural rules, cases like redistricting lawsuits that are heard by three-judge panels in District Court are appealed directly to the Supreme Court, skipping the U.S. Court of Appeals. But it remains unclear whether Wisconsin will file an appeal or accept the ruling and limit its effect to a single state.

Were the Supreme Court to hear the case, the effect could be profound, regardless of the decision. Republicans have more than doubled their control of state legislatures since 2010, and with gains from this month’s election they now control both legislative chambers in a record 32 states — 33 if Nebraska, which has a nominally nonpartisan, unicameral legislature, is included.

Nicholas Stephanopoulos, a University of Chicago law professor and the lead lawyer for the plaintiffs, said on Monday that a number of state redistricting plans, including those in Virginia, North Carolina and Michigan, have efficiency gap scores rivaling those of Wisconsin.

To Learn More:

William Whitford, et al., v. Beverly R. Gill, et al. (U.S. District Court for Western District of Wisconsin) (pdf)

Wisconsin Gerrymandering Case Going to Trial (by Molly Willms, Courthouse News Service)

Republicans under Pressure as Redistricting Ruled Unconstitutional in Florida and Challenged in Wisconsin (by Steve Straehley, AllGov)