Are you serious? He is a savvy entertainer who knows distribute the jokes that the writers give him in a way that builds a larger audience. Would you ever watch his show if it was 100% biased to an opposing viewpoint? Assigning any political affiliation based on his performance is a stretch, if he would come visit CO for a shoot and talk candidly that would be different.
Since there is so little professionalism in journalists these days (commentary has replaced impartial reporting) there is a lot of opportunity for an entertainer to fill the gap. In an ideal setting Stewart would skewer all politicians equally, but he'd never have gotten a bigtime show in the today's entertainment industry with that.
South Park is more politically impartial.