I am not saying someone is actually planning it in their basement, but move back a few years and remember the Oklahoma City bombing...
This was perpetrated by wackos who really believed what they were doing was "good", although their rationalisation of it was rather thin for a sane person...
We will also never know why JFK got shot by Oswald... Whatever were the reasons... Or to proove my point that some really unstable people exist out there, let's look back at Reagan's assassin-wannabe... He said it was to impress Jodie Foster! Way to go! I'm sure she was all moved by this gesture...
To go back to current events, I watched people shout, live on national TV, at McCain for him to get tougher. For him to take on the "Osama, an... arab". People who in their minds are genuinly frightened at what their country would become if Obama was to get elected. And by not much of a stretch, might actually think that being "friend with a domestic terrorist" is actually conspiring to take control of the country on their behalf.
Some message boards are filled with hate towards Obama, and the reactions from the crowds at recent town hall meetings I have seen, with McCain getting begged at to get "tougher" in his rhetoric, having to comfort an old woman that Obama is NOT an arab, is NOT a terrorist, despite their latest ads calling into question the "true" nature of his relationship to Ayers, indicates a very unstable situation from my perspective.
Out of 300 000 000 US citizens, what are the odds that some crackpot who fell in 2000 for the dirty campaign against McCain in the primaries then, and in 2008 really believed that this 72 year old man is a liberal in disguise, out to set back and go against what this unstable person believes, won't plan and carry out such an evil and despicable plan as to take revenge on John McCain.
I really hope this is not the case. I really do not wish any harm to come on either Obama, Biden, Palin or McCain. Democracy must be understood by all to be preferable to any form of violent process in the selection of national leaders. But, presidents, candidates and other public figures have fallen in the line of fire on too many occasions.
I have no doubt the Secret service agents responsible for the well-being of both party's tickets are considering such scenarios, as they always do, but the sad thing is that whoever looses won't benefit from this protection after the elections.
So to conclude: Although I do not have information that someone IS planning something against one of the candidates, either a loosing McCain or winning Obama, it is NOT farfetched to think this might be possible in the current context...