Michele Obama is the only one consistently beating Trump in polls. Not that I want Obama v2, but that’s where it stands currently.
Done properly, a replacement would get a big bump with the nomination, but that’s not much to count on.
Jon Stewart would probably wipe the floor with Trump if anyone can figure out how to draft him.
To be fair, in normal times I would want him doing just that. He has no experience in government or campaigning. But right now, beating Trump is all that matters.