Lee Harvey Oswald only got the job in the Texas School Book Depository after Ruth Paine the kindly Quaker who allowed his wife Marina and their two daughters to live at her home heard from a neighbour who's son worked there that there was vacancy. The day before the assassination the route of the motorcade was published in a local paper which Oswald read at work. That evening was a Thursday and because he didn't drive he asked the son of Paine's neighbour for a ride over to the house which was surprising because he usually went on a Friday night. The next morning he took a ride to work carrying a paper package with "curtain rods" and JFK was shot by Oswald from the 6th floor window that afternoon.
JFK was struck in the back of the neck by the first shot but remained upright because he was wearing a back brace. If his wife Jackie had the presence of mind to pull him down or Secret Service Agent Clint Hill had got to the limosine faster JFK would have survived the neck wound. Instead of flooring the accelerator Agent William Greer had turned around to look and let the limo slow allowing Oswald to fire the fatal shot that wounded Kennedy in the head.
If Kennedy had lived public sympathy and outrage would have secured his reelection. Oswald's trial would have inspired a crackdown on the extreme left and renewed focus on the political inflitration of Marxism in American society. Oswald would have achieved celebrity status unless he was still shot by an outraged Jack Ruby during his prison transfer and achieved a following among disaffected young leftists who were pro Castro and opposed to American involvement in Latin America and South East Asia.
Kennedy had uneasily agreed to the assassination of the South Vietnamese leader earlier that month and had resisted commiting American combat troops to South Vietnam. If Kennedy had kept out of Vietnam there might have been no hippy movement draft dodgers Woodstock would never have happened there would have been no peace movement and we never would have heard of a host of pop culture figures in music film and other entertainment. The Manson murders the Kent State shootings and Patty Hearst kidnapping among other events would never have happened either
However is likely Kennedy would have been forced to give into pressure to go into Vietnam unlike Johnson who in our timeline eagerly committed troops after the Gulf of Tonkin incident gave him his casus belli. The steady deterioration of the military situation for the South Vietnamese as the 1960s progressed would probably have forced Kennedy's hand.
Kennedy's popularity would have suffered as casualties mounted as Johnson's had perhaps with health consequences that could no longer be hidden by a compliant press. Bobby Kennedy would not have run for President on a peace platform nor been assassinated in 1968.
Kennedy was open to detente with the Soviet Union after the Cuban Missile Crisis however the escalation in Vietnam would have made that more difficult and it would likely have been delayed as it was in our timeline until the 1970s and 1980s.
Kennedy did not have the skill of Johnson to ram through Civil Rights legislation nor did he share Johnson's Great Society welfare state reforms. Indeed he contemplated dropping Johnson as his running mate in 1964 due to allegation of corruption. A host of reforms would never have happened and this may have led to the dominance of black militancy over moderate black leaders like Martin Luther King. King would never have been assassinated instead fading in popularity and relevance. Right wing Democrats resisted Kennedy sensing weakeness and in a second term with violence exploding they would have taken full advantage and maintained segregation in the Deep South.
Kennedy's extra marital affairs might have been exposed further discrediting him
Nixon could have run in 1968 and won seeking to calm domestic strife and bring peace with honor in Vietnam.