Our new book is titled Divided We Stand: The 2020 Elections and American Politics. Among other things, it discusses state and congressional elections.
The good news for Democrats is that (at this point) there are only 21 districts where Biden's margin was fewer than seven points. Even if we expand that universe to include districts Biden carried by 8-10 points, that universe of potentially vulnerable Democratic-held seats expands only slightly.
However, the good news for Republicans is that they currently hold eight of those 21 seats that Biden carried by less than seven points. In other words, it would make some of the most vulnerable GOP-held districts's, like Rep. Don Bacon's Omaha-based NE-02 (Biden +6) and Rep. Dave Schweikert's Phoenix-based AZ-01(Biden +1.4), tougher for Democrats to pick off. But, these are "holds," not flips, which lowers the ceiling for GOP gains.
Another factor that may cap GOP gains is that Democrats don't have to defend many Trump-won districts. Right now, Democrats represent only six districts (AZ-02, IA-03, WI-03, PA-08, ME-02, and OH-09) that Biden did not win. In 2018, Republicans were defending four times as many districts that Hillary Clinton had carried in the 2016 election.
Finally, while Democrats picked up 40 seats in the 2018 election, they did not flip any CD in which Trump took 54 percent of the vote or more. In other words, the Blue Wave only breached the already most vulnerable portions of the GOP seawall. There aren't all that many districts held by Democrats that Biden carried with less than 54 percent; just 24 so far. This week, many of the newly added targets to the NRCC list are in districts that Biden carried with 56 to 59 percent of the vote.