A lot of the individuals I study, like the late Senators Henry Jackson and Daniel P. Moynihan stayed with the Democratic party, but from time to time endorsed Republican candidates and platforms, despite having significant difference of opinion with the establishment Republican manifesto, if it were to be described in such a way.
There is a somewhat justified feeling that that era of liberals have been pushed aside in favor of those more sympathetic to the New Left, but the reality is more difficult than that. Take notice with how Robert Kennedy transformed himself from a McCarthy fan and into a champion of the New Left against the war in Vietnam.
Do we know what path John Kennedy would have chosen? No. Do we know what path he likely would have chosen in the short-run? More-so.