Trump won in 2016 with 46.4% of the votes. Clinton lost with 48%.

Blue voters were not all that excited about the race because the polls said it was in the bag. Some may have stayed home because their favorite candidate lost in the primaries, some stayed home because they read so much negative press about Clinton.

Red voters were looking for racist revenge.

The dynamics of the upcoming race are completely reversed.

Red voters haven't got a primary race to get voters interested in the race. They can deny it all they want, but he's an embarrassment and a bumbling fool. There is NO positive press coverage of Trump because he never does anything positive. Some will sit home because they are ashamed of him. Some might actually worry about another potential 4 years of this...
He certainly hasn't won any new votes since he was elected so let's be very generous and predict that he will get 46.4% of the vote in 2020.

It won't be enough.

Because Blue voters want revenge on the racists.

Good coffee, good weed, and time on my hands...