What "leftist side?". I think Im misunderstanding you. Cuz my answer would be, the one to the left.
I didnt say mainstream, I said Leftist. Not all leftists are mainstream, and not everyone on the Right are either. But you need to attend some "Peace Rallies" in DC if you think none of the leftist groups are advocating Violence.
I've personally witnessed large groups of Socialists and left wing anarchists trying to incite violence at these rallies... or should I suppose those guys in their homemade riot gear with their riot shields cut from 50 gallon steel drums egging on the cops back in 08 weren't trying to incite anything, just there to pass out flowers and peace and love?
There are always gonna be idiots to the far side of both the left and right. Now name a left personality that is like that. Name a left person that uses fear and hate to drum up support for thier theories. I can name left talking heads that I think are a bit over the top, such as Obermann, but I've not seen the incendarary stuff in the left like that coming from the right. Perhaps that is because the left is in power at the moment. I don't know.