I'd save RFK and give him a full two-term presidency. Just because I've always wondered how much a difference it would have made in the course of American history. It definitely seems like things took a severe turn for the worse in the late 60s and the American political system has never recovered.
I personally believe there were conspiracies to assassinate both him and JFK because they were not susceptible to being controlled by their donors or political mentors, as is the case with the vast majority of politicians. They were rogue elements with a strong potential to disrupt the status quo (i.e. gravy train) for the rest of the oligarchy, so they got taken out.
I wasn't alive for these events and I am possibly missing context. But I just see an inflection point in the 60s and I don't know if another term from Carter in 1980 would have been enough to turn things around. I also feel like he was a less impactful character than RFK