I'd say that if RFK decided to stay out of the race and endorse McCarthy, the latter might actually have a chance at the nomination. I think it would require Kennedy to actually go out and campaign with/for the man, but they could build enough momentum among the anti-war movement to get him there. With that having been said, VP Hubert Humphrey would still be the "insider favorite", so to speak, and there's a good chance he still comes out on top. But either way, if Kennedy isn't killed and we assume Nixon still wins 68, conventional wisdom says he'd sit out 72 and make a play for 76 instead. Which, assuming Watergate still happens as per OTL, is probably his best bet. He'd be well positioned to win a commanding majority, compared to the uphill battle that he'd have faced just four years earlier. That having been said, I do often wonder if Watergate could've happened sooner/been even worse, given Nixon's hatred for and paranoia of the Kennedy family. In which case, an RFK victory is possible, though not guaranteed. If he were to run in 72, and lose, he may or may not get another shot in 76 (or later) depending on just how badly he lost. I doubt he'd do as terribly as McGovern, and might be able to get it close (and maybe even pull off a popular vote win, if not an ec majorty) assuming no early Watergate.