More races for the U.S. House next year will start with one party holding a significant advantage because the process of redrawing congressional district lines is whittling down the number of politically competitive seats.

Nearly half the states that elect more than one House member have finished adjusting their districts, which is generally done after each 10-year census. While states that hold just over half of the remaining House seats are still at work, one trend is clear: State lawmakers, who in most cases draw the maps, have created more districts where voters skew heavily toward one party, eliminating many districts where voters are more evenly divided in their political preferences.