What if? scenario. JFK runs for President in 1964?

First of all, despite his faults, JFK would most certainly be re-nominated. He appeared a strong leader to the public, and knew how to be popular.

Also, he would almost definitely take Lyndon Johnson, because he needed Southern support.

Presumably, Republicans would nominate Goldwater again. Goldwater wins only his native Arizona and the South because of his opposition to the Civil Rights legislation. Because the public sees both Goldwater and Kennedy as strong leaders, but JFK also promises change and "The Great Society." Goldwater loses by a landslide because he is seen as an out-of-touch reactionary.

JFK introduces major reforms like LBJ would - the Sixties take off...

Meanwhile, Vietnam escalates. Because JFK wants to look strong, especially after the Bay of Pigs, I'll let you guess what happens next...

What makes you think he would be allowed to run a second time?

In the few months before his murder he did some things that wouldn't make that a certainty: Bay of Pigs, Vietnam, Mafia. As he was murdered, most of the blame fell on Johnson.

Supposing he wasn't popped, he would have to do some serious explaining.

most likely i would have been the only person that would be born either way*