If Eisenhower goes for the third term, as he might, he'd beat JFK. Simple as that. JFK would likely still get the Democratic nomination, but he might not want it if he had to face Eisenhower. Which leaves Johnson or Humphrey, I imagine.
No Viet Nam? I doubt Eisenhower wants another war in Asia and I can't see him doing what Johnson did. Cuban Missile Crisis? That's a tough one but Eisenhower was a known quantity to the Russians, I doubt they'd screw with him.
In '64 I doubt Eisenhower goes for a fourth term so Nixon/JFK? Heh. JFK wins, probably easier then OTL. Now Goldwater or Rockefeller or Lindsay (NY Mayor) may run and may defeat Nixon in the primaries but again, I'd say JFK would beat them.
On the Democratic side… Johnson, perhaps. McCarthy but I doubt it. Humphrey again is quite possible. Regardless I think JFK could get the nomination, but it's been a few years. Maybe some of those rumours are out now and he's politically damaged.
No Viet Nam is changing the political culture quite a bit.
The Republican Party is quite different too. No Goldwater changes things. No exodus of Goldwater operatives to Reagan changes things.
So. Let's say JFK over someone in 1964. Civil rights? Might be the right time, might not. A later Cuban Missile Crisis isn't likely as ranges have improved. No Viet Nam to get involved in.
Gets murky after that, but I could probably muddle on for a bit.