For years the Simpsons was my favorite show. It was often really funny and also clever and I, like many on this thread noticed how they skewered liberals too. In the last few years I just somehow quit watching without really knowing why. I think mainly because it quit being funny.
There is a lot of subtle leftist bias tho. The really stupid or evil people are usually portrayed as conservative, as are their supporters on the show.
The liberal characters, are supported by the smart ones such as Lisa. Men are much more often portrayed as idiots. In one show they even went so far as to say the Simpson men are genetically programmed to be idiots while the women brilliant.
In the few times where they really go over the line it is attacking Christianity. A good example is where the Simpsons rescue their children from (or the horror) from being baptised by Ned.