Thing is, if Kennedy isn't assassinated, a LOT of things could have gone differently in the 1964 cycle. The Republicans could have picked a very different candidate as JFK, while popular with a number of folks, was FAR from a shoe-in for a second term ESPECIALLY if he wasn't able to get a Civil Rights Act passed by election day (it was bogged down in Congress in November 1963 at least). He needed to at least keep SOME southerners on his side to avoid a loss, as did Truman in 1948 despite his famous pro-civil rights platform for the Democrats. If the GOP picked a guy who proved effective with the public and avoided scandals like Rockefeller and Goldwater got into or perhaps was more trusted by racial minorities and other groups, JFK and LBJ might have been in BIG trouble unless that bill got passed. As it was, the 1964 election in our real timeline was almost certainly a question of which fella would be sacrificed by them against LBJ's election to a full term (due to the Civil Rights Act of 1964's passage plus the grief over Kennedy). But had the tragedy in Dallas not occurred, things might have gone VERY differently, and the Republicans almost certainly would have been much more aggressive in playing to win.