Specifically, Republicans won in 18 House districts that President Biden carried in 2020. By contrast, Democrats won just 5 House districts that President Trump won in 2020. That's a nearly 4-1 margin. What makes this phenomenon even more amazing is that 11 of these districts were either in heavily "blue" New York (6) or California (5). As Kyle Kondik of the University of Virginia points out: "Overall, Republicans won 5 double-digit Biden districts, while Democrats won just 1 double-digit Trump district."
In the 2002 midterms, there were 58 such 'crossover districts'—32 went to Republicans and 26 to Democrats. In 2022, only 18 districts crossed-over. Despite this downward historical trend, Republicans are getting better and better at winning these dwindling districts. In fact, in the last seven elections, the GOP has won more of these "split districts" than Democrats.
So, while a myriad of factors led to the slim Republican margin in the House, crossover district voting was clearly a major contributor.