Because Democrat Bill Haas doesn't seem to.
He's airing an ad on local radio station KTRS. In the ad he attacks Alaska Governor Sarah Palin. Last I checked his opponent was Congressman Todd Aiken.
He hits Palin on the Bridge to Nowhere. He uses a line reminiscent of the one that has been used on Rudy Giuliani. He says that "a noun, a verb and the lie about the bridge". If you want to listen to the ad go here.
Airing an ad like this makes me think two things:
What a dumb thing to do in the deep red second congressional district of Missouri. This ad certainly won't help someone who has to be trailing by big numbers.
I would have to think that Haas knows that he is a long shot. Running an ad like this therefore seems designed to do one thing: Help Barack Obama. If that is the case, wouldn't this ad violate campaign finance laws?