In digging through the latest @pewresearch polling and comparing it to their 2016 voter survey there's no evidence that he's slipped significantly with his base, or gained support from anyone who didn't support him in 2016. Even so...1/ https://cookpolitical.com/analysis/national/national-politics/four-years-later-its-still-all-about-base
He’s trailing Biden by a wider margin than he did Clinton. Why? A big reason: voters who supported third-party/other candidates in 16. According to Pew, those voters are now breaking 55% to Biden and 39% to Trump. https://cookpolitical.com/analysis/national/national-politics/four-years-later-its-still-all-about-base
These non-Trump/non-Clinton voters don’t get as much attention as Obama-Trump or Never-Trumpers, but they matter a lot. In ‘16, they were 6% of the vote. https://cookpolitical.com/analysis/national/national-politics/four-years-later-its-still-all-about-base
What this suggests is that Trump’s best path to Electoral College victory is to turn out voters who didn’t vote in ‘16 - increase the pool of voters instead of trying to pick-off former Hillary or 3rd party voters. https://cookpolitical.com/analysis/national/national-politics/four-years-later-its-still-all-about-base
A big thanks to @CarrollDoherty and the folks @pewresearch for their help on this. https://cookpolitical.com/analysis/national/national-politics/four-years-later-its-still-all-about-base