Two types of assassins
Rick: yes, as I wrote in my most recent most recent piece for PJ, political assassins break down into roughtly two main types, and Oswald appears to have been a mixture of the two:
Political assassins tend to be of two types: the first is the coldly calculating killer (or co-conspirators) motivated by a strategic move for power and/or a political vendetta, and the second is the lone crazy person. Some, such as Lee Harvey Oswald and Sarah Jane Moore (remember her?) inhabit territory somewhere between the two (they also happen to inhabit territory on the left, a fact most leftist commentators tend to conveniently forget).
The first group do have primarily political motivations, but they are highly unlikely to have been motivated or even affected at all by casually inflammatory rhetoric. Their provocations are of a deeper sort.
How often have this first sort of killer or killers been behind political assassinations in the United States? Well, it depends who you ask; Kennedy conspiracists are adamant that such plotters were behind Oswald, and if theories about the mob’s involvement in the 1933 Mayor Cermak slaying are true, that would be another example of a group effect. Lincoln’s assassination featured a number of Confederate sympathizers who worked together and planned to take over the government, as well.
Loughner appears to be one of the purist examples of the second type -- the lone crazy -- that we've ever seen. That doesn't mean he doesn't sometimes spout statements that sound political. Nor does it mean that his craziness is necessarily of a degree that would afford him a defense in a court of law.