Without actually dying and being martyred, I find it hard to imagine JFK’s legend/mystique is quite as powerful as it was. Especially after he’s been back in the public spotlight as a senator for six years. Hell, given all the health problems he already had, it would be somewhat surprising if he even made it this far in good enough shape to run again.
But just rolling with it, this is more or less what I think would be JFK’s best case scenario:
Reagan would simply be too tough to beat that year. And a guy who was president 20 years earlier simply wouldn’t be as special as the mythical figure JFK became after his death.
I do think, however, that if JFK was wounded in the assassination attempt but stayed in office, he would have won a massive landslide in 1964. Perhaps even bigger than LBJ’s.