Dynamist picks this up with some self contradicting commentary like this:
Serif types are the original typographic forms of the Roman alphabet. Sans-serif types are derivative letter forms that have been stripped of their serifs (as the name implies, if you know a little French!), a simplification that lends itself to short, simple messages.So is a bumper sticker a short simple message? Yup. So why wouldn't we want to use sans-serif fonts again? He goes on to talk about some typographic snobbery to back his case for Kerry up.
Honestly I find the social implication of the bumper stickers to be more interesting. I've lost track of the number of people with both a Bush sticker and "support the troops"/yellow ribbon/American flag on their cars as well. I've seen two people with Kerry stickers and any of those features. Not that I'm questioning their patriotism or anything.