The AP’s analysis indicates that Republicans won about 16 more U.S. House seats than would have been expected based on their average share of the vote in congressional districts across the country. In state House elections, Republicans’ structural advantage might have helped them hold on to as many as seven chambers that otherwise could have flipped to Democrats, according to the analysis.
The AP examined all U.S. House races and about 4,900 state House and Assembly seats up for election last year using a statistical method of calculating partisan advantage that is designed to flag cases of potential political gerrymandering. A similar analysis also showed a GOP advantage in the 2016 elections.
The AP used the so-called “efficiency gap” test in part because it was one of the analytical tools cited in a Wisconsin gerrymandering case that went before the U.S. Supreme Court in 2017 and is part of a North Carolina case scheduled to be argued on Tuesday before the court. In that case, justices will decide whether to uphold a lower court ruling that struck down North Carolina’s congressional districts as an unconstitutional political gerrymander favoring Republicans.