I think that, among the politically-aware minority of the population, we would find that the spectrum of the left extends further than it does in Congress. I think only a few radical-right commentators are ideological. Most use ideology as a weapon, but actually have group-loyalty or group-hate as motivations. The actual ideologues are probably the obvious ones: the late William F. Buckley, Grover Norquist, and so on. The most visible US radical-left figure I can think of is Noam Chomsky. He is a left anarchist and a moderate pacifist, widely reviled, and largely unknown outside the minority of the politically aware. Any national network that gave him a platform would come under enormous pressure to take it away again. Yet we are talking about someone whose program would be non-violent. We might also consider ecological radicals like Jerry Mander. Another pacifist and widely hated figure, and another advocate of non-violence. For a violent leftist you'd have to look for a Stalinist (are there any left?), Maoist, Trotskyite, or the like. And here, I am ashamed to say, I know no names.
Returning to the political spectrum, probably the farthest left the Senate goes is Barbara Boxer or Bernie Sanders and they're pretty moderate by comparison to Chomsky. Not sure about the House--there might be one or two authentic radical lefties in the 535. Be interesting to see if anyone actually has done the work & already knows!