How big is the House playing field?

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.

Of course, this kind of crude, back-of-the-envelope math doesn’t always translate to actual wins and losses. Candidates and campaigns matter too. A wave election in 2018 wasn’t enough to oust GOP Rep. John Katko from his blue-leaning upstate New York CD. And in Wisconsin, Democratic Rep. Ron Kind managed to hold onto his conservative CD in good Democratic years and bad ones. But, those ‘unicorns’ have become even rarer in recent years as fewer and fewer House incumbents are able to separate themselves from the party brand.