Or to put it another way, who is going to vote for Trump now that did not vote for him in 2016.
This is the one question I keep coming back to. To get re-elected, Trump either needs to increase his overall support OR depress turnout against him. Due to demographic shifts and the closeness of the 2016 election he'll lose if each major demographic has similar turnout and support.
Given his conduct and polling, it seems unlikely that he will have MORE support in November than he had in 2016. Which leaves voter turnout.
On one hand, he's the incumbent and turnout is typically much lower when an incumbent is on the ballot. That could be what gets him re-elected
OTOH, the GOP was pretty effective at limiting voting and reducing enthusiasm for HRC, which makes it that much harder to
further reduce support for Biden. There's also an unusually high number of people who are politically active at this moment in time.
The latest polls show Biden well ahead in all the states that Trump narrowly won in 2016 (MI, WI, PA & FL) and still behind in the close states that he lost (MI, NV, ME). No other states have had a dramatic 'flip', though several 'red' states he comfortably won are much closer than anyone predicted, and some polls have within 2 points (eg. TX and GA).