What's with the liberal infatuation with bumper stickers?

I’m pretty darned liberal OK, real liberal, but I don’t quite get why whenever you see a car plastered all over with bumper stickers (e.i. >3), they’re always lefties. Some pretty much cover the whole back of the car.

Now, I am in a very blue state, which may explain why someone in a nice car might not want to call attention to his conservative leanings. At most, you’ll see a “Jesus fish” or a little “Bush” or “McCain” campaign sticker, but no slogan-slingin’ stickers. Are any of you seeing it differently elsewhere?

What do you think is the appeal to liberals of this particular form of expression?