I don't know what Parker and Stone's politics are specifically, but they did say at some Hollywood event that "at least he's (Bush) is better than this guy" -- at which point one of them pulled his eyes down to imitate John Kerry. For anyone who watches Conan O'Brien just picture his impression of Kerry, and that's what the SP guy did.
I will give the South Park guys credit for that episode they did on immigration with the people from the future. It was fair, and almost brilliant at times.
Their 'Passion of the Christ' episode, though, was clearly weighted more towards the anti-Gibson forces.