  Congressional district lines in many U.S. states are drawn by partisan
actors, raising concerns about gerrymandering. To separate the partisan effects
of redistricting from the effects of other factors including geography and
redistricting rules, we compare possible party compositions of the U.S. House
under the enacted plan to those under a set of alternative simulated plans that
serve as a non-partisan baseline. We find that partisan gerrymandering is
widespread in the 2020 redistricting cycle, but most of the electoral bias it
creates cancels at the national level, giving Republicans two additional seats
on average. Geography and redistricting rules separately contribute a moderate
pro-Republican bias. Finally, we find that partisan gerrymandering reduces
electoral competition and makes the partisan composition of the U.S. House less
responsive to shifts in the national vote.
