Alan I. Abramowitz examines redistricting and competition in congressional elections at Sabato’s Crystal Ball, and argues that “The ability of House incumbents to attract votes across party lines and thereby insulate themselves from competition is now much more limited than it was in the 1970s and 1980s.” Abramowitz, author of “The Great Alignment: Race, Party Transformation, and the Rise of Donald Trump,” argues further:
The declining personal advantage of incumbency has had a dramatic impact on competition in House elections, especially in districts in which the division of the presidential vote is close to the national average….partisan turnover has fallen slightly in the least competitive districts and remained fairly stable in moderately competitive districts. However, partisan turnover has increased sharply in the most competitive districts — those where the division of the presidential vote is closest to the national average. During the 1980s, the rate of turnover in these districts averaged only 5%. During the most recent decade, however, the rate of turnover in these districts averaged close to 18%.
As Abramowitz concludes,
Based on presidential voting patterns, a much larger proportion of U.S. House districts strongly favor one party, and a much smaller proportion are closely divided than 50 years ago. However, gerrymandering is not the major reason for this trend. Partisan polarization has increased dramatically in U.S. states and counties, whose boundaries have not changed. Moreover, despite the growing partisan divide evident in presidential voting, the competitiveness of House elections has changed very little over the past 5 decades because the personal advantage of incumbency has declined sharply during this period.
The declining ability of incumbents to attract support across party lines means that incumbents in marginal districts — those in which the presidential vote is close to the national average — are now much more vulnerable than they were in the 1970s and 1980s. Incumbents in moderately competitive districts remain somewhat vulnerable as well. Whether that vulnerability turns into actual party turnover depends mainly on the strength of national tides. Large-scale party turnover of House seats tends to occur when there is a strong tide favoring one party. Even if redistricting results in a decline in the number of closely divided districts based on presidential voting, a strong national tide favoring Republicans in 2022 could still result in a large number of seats flipping from Democratic to Republican control. The number of seats switching parties in 2022 and succeeding elections will depend more on the strength of national tides than the number of marginal districts.
While gerrymandering is still a force in state politics across the U.S. and Democrats have to keep at it as long as Republicans do it, Democrats would be wise not to expect too much gain or loss because of redistricting, and to allocate resources according to political attitude trends in each district.