I love this kind of historical generalization, and I've been meaning to do my own fact checking. This morning I heard about the attempted assassination of Senator Gifford (D, AZ), and I decided to finally work this out.
I obtained the following list of presidential assassins from Wikipedia. What I found even more interesting than the assassins political motivations was the fact that so many of them were insane. I was also interested to learn that many of them were not executed for their crimes.