A few things I'd like to put in, here, for whatever they might be worth, on the question of whether JFK would have been reelected:

1) I've heard that the last poll released before JFK's assassination had him beating Goldwater by eighteen points. And unlike Nixon in 1960, Goldwater would not have had a unified Republican Party backing him- the party still had a moderate wing then, and those moderates were horrified by Goldwater's opposition to the Civil Rights Act of 1964- he actually vowed to fight for its repeal if elected.

Another factor that likely would have worked in JFK's favor was that the voters would have had a full four years to see what a Catholic president might be like, and would have noted that none of the paranoid predictions made about that by Protestant extremists had come even remotely close to occurring.