For some reason, people tend to trust the Democrat candidate more with regard to the economy. This may be because of FDR's legendary performance during the Great Depression. Most people have never been told that it appears his policies extended rather than resolved the great depression.
What you should wonder is why, despite the almost certainty (if you believe the major media) of an Obama Presidency, the market isn't doing better. It's because investors have to have a reason to invest, and the motivation in our market is profit. Some investors may well be afraid of Obama being elected, because he seems to want to take more of that profit away. For an incredibly humorous and also easy to follow explanation, please visit Captain Capitalism's post on the subject. I promise you don't have to be an MBA to follow or enjoy this one.