RALEIGH — North Carolina’s congressional and state House districts are among the most Republican-skewed in the country despite voter preferences that are relatively evenly split, according to an Associated Press analysis.

The measurement, known as the “efficiency gap,” has separately gained attention as a key argument in a pending Supreme Court case from Wisconsin that alleges partisan gerrymandering. It’s also cited by groups challenging the design of North Carolina’s congressional map, though the Republican defendants argue the measure shouldn’t be used as a legal standard.

In the nationwide AP analysis, North Carolina had the highest efficiency gap — or greatest Republican advantage — among the roughly two-dozen largest states that determine the vast majority of Congress.

An attorney leading one of the challenges against North Carolina’s congressional map, Anita Earls, described the efficiency gap in an interview as “an easily applied and really easily understood measure for how much partisanship is too much.”

“While I think it is possible to minimize the issue of partisanship, it’s also true that inevitably if you have legislators drawing the map there will be some partisan consideration,” said Earls, the executive director of the Southern Coalition for Social Justice. But the efficiency gap, she says, uses “data to make sure the districts are within a normal range of fairness.”

However, an expert witness for the Republicans sued by Earls’ group, political scholar and journalist Sean Trende, cast doubt on whether the efficiency gap can serve as a single measure for judging elections around the country.

“Gerrymandering has no particular hallmark; it is inherently a fact-intensive inquiry,” Trende wrote in an April report for the North Carolina case. “This is especially true if we are going to measure it in terms of elections, which are frequently beset by unpredictable effects that have long-lasting consequences.”

The Associated Press used a version of the efficiency gap formula — developed by a law professor and a public policy researcher— to analyze 2016 results of U.S. House and state House or Assembly elections. The efficiency gap formula compares the statewide average share of the vote a party receives in each district with the statewide percentage of seats it wins, taking into account a common political expectation: For each 1 percentage point gain in its statewide vote share, a party normally increases its seat share by 2 percentage points.

In North Carolina, about 2.4 million votes overall were cast for Republicans in the state’s 2016 congressional races, compared to about 2.1 million Democratic votes. Yet the congressional delegation has a much more lopsided 10-3 split in favor of Republicans. The study calculated that the GOP won at least two more seats than what would be expected based on its share of the statewide vote.

Among state House races, North Carolina’s efficiency gap score ranked in the top 10 most skewed of states analyzed. The election gave Republicans a 74-46 majority in that legislative chamber. State Senate races weren’t analyzed.

North Carolina’s legislative and congressional maps, drawn by Republicans after the 2010 Census, have faced protracted litigation. In late 2016, a federal court struck down 28 state House and Senate districts as illegally race-based, and the U.S. Supreme Court upheld the ruling earlier this month. The case is being returned to federal court to determine a timetable for redrawing the map.

Separately, two of the state’s congressional districts were struck down as racial gerrymanders in another ruling upheld by the Supreme Court. A redrawn map was used in the 2016 elections; that replacement map was the one analyzed in the AP’s efficiency gap study.

The current map is being challenged by voters who allege that it’s so skewed toward Republicans that it violates the Constitution. The lawsuit led by Earls uses its own efficiency gap analysis among other arguments.

But lawyers for Republican legislators argue that theories similar to the efficiency gap have previously been rejected by courts. In a legal filing, they criticize the efficiency gap as “a theory of recovery developed by law professors and scholars that has no foundation in the text of the United States Constitution.”