Copyright on Political Speeches?

There is no such requirement. It's a speech given by someone to public at large. So unless the speech was written by someone, who had chosen to take out a copyright on the speech before it was delivered to the public, there are no legal restrictions at all. You are free to use the RFK speech. You can always check with library of congress for this famous speech and obtain the name of the Original author of the speech. Perhaps during this search you may even discover if there are any copyright restrictions. To the best of my knowledge, there aren't any.