According to Wikipedia, Carson appears to have been a sort of squishy RINO politically, which I guess would make him sort of middle-of-the-road. In the article, he is quoted as saying he "didn't want the show to become a political forum". From the article:
In his book, Carson's former lawyer Henry Bushkin stated, he "was by instinct and upbringing definitely Republican, but of an Eisenhower sort that we don't see much of anymore.... Overall, you'd have to say he was anti-big: anti-big government, anti-big money, anti-big bullies, anti-big blowhards." Carson served as MC for Ronald Reagan's inauguration in 1981 at the request of Frank Sinatra.
My wife and I watched an episode of Carson this week and found it to be really refreshing. Classy host, classy guests, nothing like today's late night crap.
Dick Cavett was very clearly a liberal Democrat and really pushed his point of view on his show. Mike Douglas was more of an old fashioned Democrat and old fashioned entertainer.
I'd distinguish a bit between Jon Stewart, who's obviously a liberal, but sometimes said surprising things, and Stephen Colbert, who's sticks much more to the party-line. Stewart wasn't always as predictable as Colbert and was never as vicious.