2016: D's-->65m; R's-->62m; Non-voters 108m.
1) 10m R supporters don't vote in 2020 and 2) 15m nonvoters from 2016 (just 14% of nonvoters) find the motivation to get to the polls and cast an anti-Trump vote.
Possible for 2020: D's-->80m; R's 52m, Non Vosters 103m.
80 / (80+52) x 100% = 61% for the D's.
Let's shift another 5m votes of both groups.
D's --> 85m; R's-->47 ; nonvoters --103
85 / (85+47) x 100% = 64%
70% is a stretch, but it would only require 40m of the nonvoters to vote (37% of the non-voters) No one knows how pissed off this large demographic will be until Nov3 .
And Mr. Trump could lose another 5m votes in the next couple of weeks.