I'm not sure how WaPo ends up with +7/8 nationally after getting D+4 in 70 battleground districts that Trump won by *15* on average.
Because the biggest shifts seem to be concentrated in the more competitive districts. Solidly blue districts are not getting much more blue, and solidly red districts are probably not getting much less red. We are not dealing with a uniform national swing.
This is actually really good news, because it suggests Democrats could (emphasis on could, we won't know until Tuesday) win the House without the large national majority that seemed required a few months ago.
And we do see this at the senate level, where blue-state democrats like Gillibrand, Murphy or Warren do barely even better than purple-states like Casey, Brown or Stabenow.