I haven’t seen not one poll of him going up against Newsom. Probably because Newsom hasn’t declared he’s running.
What does Newsom "declaring" have to do with anything? He's obviously the candidate. They will protect him from exposure as long as they possibly can, so that people can be kept raving about "Biden" and "Brandon" and "Hunter's laptop" for as long as possible.
And if I know Republicans (and I do), when Biden announces in July 2024 that he is withdrawing, all the "FJB" nuts are going to give a sigh of relief and figure the game is over - when it's just beginning.