It seems odd to me that all three of Nixon's opponents were either assassinated or an assassination was attempted. Was he involved? I'm inclined to believe that he was, since his involvement in the Watergate Affair proves that he clearly wasn't the most ethical president in the history of the US.
You're talking about RFK and JFK and Wallace, right?
But no. Nixon was a corrupt politician, no doubt. But Kennedy and Nixon ran two dirty campaigns in 1960 and Kennedy was assassinated in 1963. In '63 it seemed to almost everyone that Nixon was a has-been.
RFK was assassinated while campaigning for the 1968 Democratic nomination, but there was no guarantee that he was going to be nominated -- Hubert Humphrey was leading him in the delegate race at the time of his assassination.
As for Wallace, he was also shot while campaigning in the primary. And the would be assassin had actually targeted Nixon first, but never had a clear shot -- he was just looking for notoriety.
Nixon also recorded all of his conversations in the West Wing and never once mentioned anything alluding to assassinations.