If you could go back in time and do, or bring, one thing to mess up the timeline what would it be?

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.