The breakdown of Joe Biden's impressive 14 point lead over Trump in the latest Sienna poll, which earns an A+ from FiveThirtyEight:
Increasingly I believe Biden's margin will continue to grow in other polls as well, since Trump never changes; he's a static character with one temperament: obnoxiousness. That explains his narrowing base — the macrocosm of obnoxiousness. Trump is the only incumbent in modern history who panders exclusively to a shrinking minority of voters, which strikingly reveals itself in the numbers above.
Adding to his campaign of exclusion are, more pointedly, his bumbling, desultory, and now oblivious response to the covid pandemic, which accounts for the economy's cratering, and his racist thus politically incompetent reactions to the Black Lives Matter movement. As the Times notes from the Sienna poll's internals, "white voters under 45 are overwhelmingly supportive" of the movement. And white voters were all he had to begin with.
So why, over the next four months, would general polling change when Trump doesn't?