Larry Sabato’s Crystal Ball: “Despite the growing proportion of districts that strongly favor one party based on presidential voting, the actual margins of House elections have changed very little over the past 50 years, and partisan turnover has fallen only slightly, if at all.”
“The explanation for these contradictory trends is that the personal advantage of incumbency has declined dramatically since the 1980s: 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.”

