This is, of course, a question that cannot be answered in an objective way. There is no way to tell for sure what impact any particular event had on American political culture. My own view is that these assassinations had little, if any, impact on American political culture.
It is possible that the assassinations have had some impact on our political culture. It is possible to argue that they hurt the American liberal movement. From this point of view, the assassinations of the two Kennedys deprived the liberal movement of two of its most charismatic and talented politicians. The death of Dr. King took away the best hope for racial unity and reduced white people’s willingness to support black demands.
I, personally, do not subscribe to this theory. After John F. Kennedy was killed, President Johnson stepped in and passed a very liberal agenda. I would argue that it was the problems associated with his policy in Vietnam and his Great Society that helped to destroy liberalism more than anything else. It is not at all clear that Robert Kennedy would have been able to win the election in 1968 given the baggage of Johnson’s legacy.
While some would argue that these assassinations harmed American liberalism, I would argue that they had little or no long-term effect on our political culture.