The easiest way would be JFK lives, maybe his cheating ways get out or he overextends on civil rights, Rockefeller narrowly beats Goldwater and in doing so seriously pisses off the conservatives, Rockefeller narrowly beats JFK, Rockefeller has no better than average four years, Goldwater trounces him in the primaries, Goldwater (almost certainly) loses to generic Democrat.
Goldwater is always the steep underdog in a general election, with '64 vs. JFK with JFK scandal his only real (if slight) chance.