While it's true Carson ribbed both parties, if you watch any of his shows from the Reagan years there is a much harsher edge to his 'jokes' about Reagan and the disappointment on his face when the audience showed their clear disapproval was very clear. Carson was pretty good overall but when he had a very liberal guest on his mask came off.
I was a huge Carson fan, but you're right, at times Carson would let the mask slip. He did boast about nobody being able to tell what his politics were, so at least he was making an attempt to be fair.
I remember he had that fraud Paul Ehrlich of "The Population Bomb" infamy on a number of times. The bug-eyed look on Ehrlich's face told me he was a fanatic.
Today's late night tv talk show hosts don't bother to try to fake impartiality. Liberals can do no wrong with these clowns.
I haven't watched any of them since Leno quit.