Pardon the delay in response, had to give the issue some serious thought.

I think that the main problem is stated in your last paragraph, above. On the one hand, as killslowly states below, there have always been morons who would do things like wear a t-shirt with a Democratic donkey in the crosshairs. On the other hand, people in authority are not supposed to encourage them. When you have someone like Bill O'Reilly being a rude asshole to his faux liberal guests on national television and being rewarded with contracts renewed in perpetuity, when you have Ann Coulter writing about shooting liberals and getting invited to the GOP convention, you say from a position of authority that general codes of polite conduct are out the window. When you have behind this a Republican party that has embraced more and more of the 'total war' concept that has percolated up through the College Republicans since the 80s and when you have Rove's creatures playing the 'cry treason' game, you have a situation where some of the aforementioned morons start to think they can get away with a lot more than a few offensive t-shirts and bumper stickers.
On the other hand, I can't help suspecting that this isn't the first time in the Republic's history that we've been in such a situation. All the other times it proved to be a bad idea by the people pushing it, too.