In 2004, Justice Kennedy wrote in a concurring opinion on a gerrymandering case that he might consider a challenge if there were “a workable standard” to decide when such tactics crossed a constitutional line. But he said he had not seen such a standard.

The challengers in the new case, Gill v. Whitford, No. 16-1161, say they have found a way to distinguish the effect of partisanship from the many other factors that influence how districts are drawn.

The proposed standard tries to measure the level of partisanship in legislative maps by counting “wasted votes” that result from the two basic ways of injecting partisan politics into drawing the maps: packing and cracking.

Packing many Democrats into a single district, for instance, wastes every Democratic vote beyond the bare majority needed to elect a Democratic candidate. Cracking, or spreading Democratic voters across districts in which Republicans have small majorities, wastes all of the Democratic votes when the Republican candidates win.

In a recent article, Nicholas O. Stephanopoulos, a law professor at the University of Chicago and a lawyer for the plaintiffs, and Eric McGhee devised a formula to measure partisanship. The difference between the two parties’ wasted votes, divided by the total number of votes cast, yields an efficiency gap, they wrote.

The gap in Wisconsin was 13.3 percent in 2012 and 9.6 percent in 2014, according to the formula. The Wisconsin voters who sued to challenge the Assembly map argued that gaps over 7 percent violated the Constitution. That number was meant to capture the likelihood that the gap would endure over a 10-year election cycle, but critics say it is arbitrary.

Adopting it could transform American elections. A 2015 report from Simon Jackman, then a political scientist at Stanford and an expert witness for the plaintiffs, found that a third of all redistricting plans in 41 states over a 43-year period failed the 7 percent standard. Elections in 2012 and 2014 in Florida, Indiana, Kansas, Michigan, Missouri, North Carolina, New York, Ohio, Rhode Island, Virginia, Wisconsin and Wyoming featured efficiency gaps of more than 10 percent, he found.