The R Word and Culture

Could it be that a word we have, for so long, attached to a community of people with legitimate disabilities, has transcended that community and entered mainstream culture?  What about the word "idiot?"  Idiot was a word that was used in the political context to define people with a mental disability and we now use that word to describe anyone who does something stupid.  What about stupid?  That is a word that once had extreme acceptance in our society but is now no longer applicable to students with disabilities, as it once was.  My point is that we are a very temperamental society.  If I told someone that their political beliefs were "retarded," how is that any different than telling someone that their political beliefs make them an idiot?  The scenario is exactly the same, with the lone exception being the word in question.  What makes "retard" worse than "idiot?"  Ideas?