Less than you think. A treasure trove of data has just been released by Cook Political Report on the partisan lean of states and congressional districts, with a lot fantastic maps, tables and other graphics. Among their findings is the following:
"It's become fashionable to blame gerrymandering for polarization and Congress's ills. In truth, redistricting is only responsible for a fraction of the decimation of swing seats over the past few decades. The bigger factor? Voters' self-sorting.
In many minimally altered districts, the electorate has simply become much more homogeneous. For example, the boundaries of West Virginia’s 2nd CD have barely changed since 1997, but its PVI score has shifted from EVEN to R+20 as its voters have moved away from national Democrats. Likewise, Albuquerque’s migration to the left has bumped the PVI score of New Mexico’s 1st CD from R+1 to D+9.
The Cook PVI illustrates how voters’ natural geographical sorting from election to election, much more than redistricting and gerrymandering, has driven the polarization of districts over the last two decades. Our 12 unique sets of PVI scores over the past 25 years give us a powerful tool to isolate and quantify the impacts of sorting and redistricting on the makeup of House districts.
We've released new PVI scores in seven odd-numbered years following presidential elections: 1997, 2001, 2005, 2009, 2013, 2017 and 2021. We've also recalculated scores in five even-numbered years following redistricting: 2002 (after the 2000 census), 2004 (after mid-decade redistricting in Texas), 2012 (after the 2010 census), 2016 (after new court-ordered maps in Florida, North Carolina and Virginia) and 2020 (after new court-ordered maps in North Carolina and Pennsylvania).
On balance, redistricting wasn’t as much of a factor in the House’s polarization as the most vocal opponents of gerrymandering might think. Of the net 86 “swing seats” that have vanished since 1997, 81 percent of the decline has resulted from areas trending redder or bluer from election to election, while only 19 percent of the decline has resulted from changes to district boundaries."