Isn't it sad that Mamdani's own party is likely to do far more damage to his agenda than Trump or the GOP ever could...

If you look at the political dynamics at play that really are universal that won't happen. While the Democrat leadership did not welcome or embrace him, they didn't stand in his way or try to block him either. Just like with Bernie Sanders, they will gladly accept him into the fold to the extent his policies make sense (not all of them do) and he diligently and genuinely works to implement them.