What struck me most about the Leno monologue - people weren't laughing at the Bush jokes. Everytime one bombed he'd get testy at the audience "oh. I guess you can't handle the serious jokes" or something like that. No. They don't laugh at the unfunny jokes.
Leno's charicature of republicans has always been that they are cold hearted hatemongers. But the jokes aren't working because they're off the mark.
On the other hand, it's apparent that several of his writers listen to Rush Limbaugh religiously, and not just to do "opposition research".
The talk show host who does the best job of not letting ideology get in the way of a funny joke is John Stewart. Politically, he leans way left, but if there's a joke there he'll tell it. More often than not, a joke on the Daily Show will make fun of self-important pomposity in the media or by a politician. Though if it's a toss-up, he'll definitely go for the joke that slams Republicans.
Leno needs to be funny no matter who is in the White House. He made his grandkids rich off of Clinton. The Daily Show is a different matter in that they need to be funny but their writers are getting more political as we get closer to the election. They are still generally funny but its just getting more and more lopsided. Libs LOVE the daily show since they think this is the only place where the REAL news gets heard. Sad, but true. Check their chat rooms.