If the question is "If RFK had not been assasinated, would he have defeated Nixon?", I think the answer is "no". Kennedy was advocating a withdrawal from Vietnam. While the press was on board with this and the war protesters wanted it, the American public was not in favor of this strategy. Neither Humphrey nor Nixon advocated this in the general election and the vote was very close. If the Democrats had nominated a candidate advocating withdrawal he would have been soundly trounced. In fact, in the next election, the Dems did nominate a withdrawal candidate, McGovern, and he was soundly trounced.
If the question is "If RFK had not been assasinated, but had beaten Nixon, what would have happened?" Nixon, who felt he had been beaten by dirty tricks in the election of 1960, and who was willing to engage in dirty tricks (unlike in 1960) would have had Kennedy's sex life exposed for all the world to see. I just can't see Kennedy being elected. But, once again I am begging the question. If Kennedy had won he would have withdrawn from Vietnam and the communists would have murdered thousands earlier than they actually did. The US would have been demoralized and the Reagan revolution would have kicked in 12 years earlier than it did.