I don't think it's likely in any event that a US president will be assassinated. It would take a major plot involving some of those who were closest to him (not that that couldn't happen - but it would be very difficult to arrange, and hence not very likely). Security around US presidents has been much tighter since JFK. The president is simply not put in positions where anyone who was not secure could have access to him (an open limo in a motorcade, for example).
Nonetheless, even as I write this, the words of Michael Corleone occur to me: "If there's anything we've learned, if history has taught us anything, it's that you can kill anybody." I've always wondered whether Puzo and Coppola might have meant those words in the script of Godfather II as an allusion to possible Mafia involvement in the JFK assassination.