The news organization scrutinized the outcomes of all 435 U.S.
In 2012, the efficiency gap of Pennsylvania's congressional districts was the largest in the nation, the lawsuit said.
The AP analysis was based on a formula developed by University of Chicago law professor Nick Stephanopoulos and Eric McGhee, a researcher at the nonpartisan Public Policy Institute of California.
Stephanopoulos and McGhee computed efficiency gaps for four decades of congressional and state House races starting in 1972, concluding the pro-Republican maps enacted after the 2010 Census resulted in "the most extreme gerrymanders in modern history." The 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.
So a party that receives 55 percent of the statewide vote could expect to win 60 percent of the legislative seats.