I think Dr. Guy has it right. There are plenty that speak "from the left" and do so in ways that reflect very poorly on their causes and issues. Lawrence O'Donnell (who engaged in the great "liar" screamfest at the author of "Unfit for Command"), Chris Matthews (and his rant at Michelle Malkin) and his friend Keith Olberman, and a host of others like the aforementioned radio host above reflect very poorly on the side of the left. Whether they speak for everyone, or only a portion (or even just one), they are seen as representatives of the left, and they paint an image of the left with a broadbrush (in other words, they paint the image themselves, so that the right doesn't have to).
There are plenty on the right that are not any better though. Rush Limbaugh has been over the top many times lately (and before), and so have others.
My point is that both sides should try to get out of the gutter and stop letting the other side have such poor images to poke at.