Republicans in the U.S. House of Representatives will hold a slim 220-215 majority when the next Congress gets underway next month after Democrats picked up a seat in California, according to Edison Research on Wednesday.

To estimate competitiveness, Spotlight PA uses a measure from the nonpartisan map analysis organization Dave’s Redistricting App. It classifies a district as competitive when candidates consistently win or lose by a margin of 10 or fewer points based on composite election results.