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The Supreme Court has long acknowledged that redistricting is first the province of the State Legislature while being willing to wade into disputes over it. In 1964, the court upheld the newly established rule of ‘‘one person, one vote’’ to end the practice of wildly uneven apportionment, which produced, for example, a map in Vermont with a State Assembly district for only 36 people and California State Senate districts that varied from 14,000 people to six million. Earl Warren, the chief justice who presided over an array of major decisions, including the order to desegregate schools, called it ‘‘the most important case of my tenure on the court.’’ Over the decades, lower courts have commonly overseen mapmaking to ensure that states draw districts roughly equal in population. Courts have also regularly monitored redistricting for racial bias. Beginning in the 1960s, the Supreme Court ruled that legislators can’t carve up maps to intentionally diminish the power of black voters, and in the 1980s, Congress amended the Voting Rights Act to make states redraw maps if they have a discriminatory effect. 

The Democratic plaintiffs who are challenging Wisconsin’s map in Gill v. Whitford, represented by the Campaign Legal Center, will argue to the Supreme Court next month that partisan gerrymandering, like racial gerrymandering, violates voters’ rights to be treated equally. They will also offer a second argument, based on the First Amendment, that comes from Justice Kennedy. He suggested in Vieth v. Jubelirer that gerrymandering could violate the right to freedom of expression and association, by ‘‘subjecting a group of voters or their party to disfavored treatment by reason of their views.’’

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Experts for the Gill plaintiffs used multiple metrics to show a high degree of bias in Wisconsin’s Assembly elections. In striking down the Assembly map, the three-­judge panel relied primarily on a metric called the efficiency gap, which measures ‘‘wasted votes,’’ as described by its creators, the University of Chicago law professor Nicholas Stephanopoulos and the political scientist Eric McGhee. Wasted votes are those cast for a losing candidate or above the number a winning candidate needed to prevail. The efficiency gap is low statewide when the number of wasted votes in a given election is similar for both parties, and it’s high when one side wastes votes at a far greater rate, because its voters are densely concentrated or thinly spread. In other words, the efficiency gap tracks packing and cracking.

Stephanopoulos and McGhee found that Wisconsin’s 2011 State Assembly map produced some of the highest efficiency gaps compared with election results in the state and in other states over the last four decades. ‘‘It’s very rare to have a map that’s this bad for this long,’’ Stephanopoulos told me. ‘‘And it’s really hard to flip.’’ The three-­judge panel in the Gill case found that as long as the 2011 map remained in place, ‘‘in any likely electoral scenario, the number of Republican seats would not drop below 50 percent.’’