Crazy People in the Mainstream

This question stems from a conversation I was having earlier today in which I argued that, for today's Republican party, people like Ann Coulter are accepted into the mainstream and have no clear equivalent on the left. Why does it seem that a conservative pundit can - oh, I don't know - joke about poisoning a Supreme Court justice or talk about blowing up the New York Times building and still be viewed as a voice for average Republicans? Surely anyone on the left of similar vitriol would be viewed as a "fringe" pundit.