One overlooked factor in forecasting the House popular vote is that Democratic districts have fewer eligible voters on average because they have far more immigrants. Here's my estimate of each district's 2018 CVAP (Citizen Voting Age Population) vs. Trump's margin in 2016. 
There are several factors, including gerrymandering, that cause Democrats to underperform in the number of *seats* they win in the House relative to their number of *votes*. But this factor works in the opposite direction and counterbalances them to some degree.
Incidentally, this is entirely about the *citizen* population. Democratic districts have about the same (actually a tiny bit larger) *overall* populations on average. It used to be that GOP areas were faster-growing, but that isn't true anymore.
