The Republicans have 36 seats ranked R+20 or above, while the Democrats have 65 seats ranked D+20 or above.
That, in a generalized sense, is what the Democrat's problem in the House is all about.
your D+20 limit is completely arbitrary, though.
The Republicans have 146 seats ranked R+10 or above, while the Democrats have 122 seats ranked D+10 or above.
The Republicans have 189 seats ranked R+7 or above, while the Democrats have 150 seats ranked D+7 or above.
IMO these breakdowns are much better demonstrations of the Democratic Party's problems in the House. The problem isn't that Democrats self-segregate into urban areas at a greater rate than Republicans self-segregate into rural areas, because if you were to draw fair non-partisan maps nationwide, the number of R+20 seats would increase and the number of D+20 seats would probably decrease.
When you start getting into the districts where the GOP has a PVI advantage somewhere in the teens, you see many districts that would be R+20 or more but they awkwardly stretch and go out of their way to take in juuuust enough Democratic voters to keep the Democratic base too diluted so they can force as many GOP districts as possible
That's the real problem - gerrymandering. all other concerns are secondary. The 2018 House elections are meaningless compared to the extreme importance of the 2018 gubernatorial and state legislative elections . That is how the Congressional majority of the next decade will be determined. We don't even need to make our own gerrymanders - the Democratic party wins when the maps are drawn fairly
R+10 is almost exactly what you'd want to have your districts at, somewhere between R+7 and R+13 probably. Anything with a PVI above 15 is pretty much just wasted votes. The Republicans probably designed R+10 districts wherever they could afford to do so, and I'm sure the incumbents approved as well.
