20%, though this is primarily because of the map moreso than the idea that Democrats will struggle in midterms forever. The party of a president with ~40% approvals is not going to have a good midterm, and not that it matters but I'd give the Democrats a greater than 50% chance of winning the popular vote in the House.
With Democrats extremely concentrated in relatively few 75-95% districts, popular vote in the House is of minimal importance, IMHO..
P.S. As last election has shown popular vote becomes useless even in Presidential election too.
Distribution of votes is far more important...