NYT: Internal 17-state RNC polling shows Trump losing re-election

[GeorgiaModerate]

It’s only April. And during huge crisis, Americans generally don’t change horses in the middle of it. Trump has historically underperformed in polls and yet still wins. See 2016. For all y’all who think this is going to be an easy democratic victory you might want to stop drinking the Biden koolaid and look at the reality. Biden is more or less in the same Clinton mold: aka running on a platform “I’m not Trump.” That is not going to win him the election.

You are generalizing about Trump's performance relative to polls based on a sample size of one election.  In addition, it's not a very good generalization. The national polling average was very close in 2016; it was just state polls that were off, due to failure to model the educational split in preferences that had developed.  Smart pollsters have adjusted for that.

As for "I'm not Trump", that may not have been a winning strategy for the highly disliked Clinton against unknown quantity Trump, but it's likely to be much more effective for likable Biden against demonstrably incompetent and corrupt Trump.  

David Frum had a good thread on the history of elections during/following severe economic downturns that ended with this summary (click to read the whole thread, which is interesting):

https://twitter.com/davidfrum/status/1254037903698800640

[GeorgiaModerate]

Another thought just occurred to me regarding the "Trump overperforms the polls" argument:

To the extent this is true, it's equally as likely that the cause is "Hillary Clinton underperforms the polls."  There's no way to choose between those two propositions based on a sample size of one election.

[GeorgiaModerate]

Aren't internals supposed to be biased in favor of the party that is conducting them? So perhaps Trump is down by even more?

The Party that commissions the poll controls wording and to some extent methodology.  Such polls are not comparable to those that have a track record. On the other hand they may be the only ones available. 

Also, an internal poll may be withheld if it's unfavorable to the sponsoring party (although sometimes it could be released for motivational purposes).