The assassination of John F. Kennedy affected American politics in one tangible way and in one way that is less tangible.
The tangible effect of Kennedy’s assassination was to push the United States in a more liberal direction. President Lyndon B. Johnson, who succeeded Kennedy, was not necessarily more liberal than Kennedy. However, he was more able to get legislation through Congress. This was partly because of his legislative skills, honed through years in the Senate, and partly because sympathy for Kennedy made people more likely to support Kennedy’s liberal agenda. Because of these factors, Johnson was able to get liberal policies on civil rights and on his “Great Society” program through Congress where Kennedy would probably not have succeeded. This helped push the Democrats in a more liberal direction and brought about a conservative backlash that started with Nixon and, arguably, is still with us today.
The less tangible effect of the assassination was to start the US on the road to disillusion with the government. Some people say that Kennedy’s death led to a loss of “innocence” on the part of the American people. Kennedy had seemed like a transcendent figure, one who brought an aura of romance and “Camelot” to American politics. He inspired people to want to be involved in politics. His death punctured that ideal and, some say, helped lead to our more cynical ideas about politics. (Myself, I question whether this ideal would have survived if Kennedy had not died, but there is no way to prove this one way or the other.)
In this way, JFK’s assassination is said to have pushed the US towards a more liberal government in the short run, but it also led to greater cynicism and a conservative backlash in the longer term.