If I disagree with Obama, am I racist by default?

No, but I suspect a large number of his loyalists will think you are. I think the biggest problem with racism is most people don't think they are, but its something we all suffer from to a larger or lesser degree. Sure, there are some who are major bigots, know it and don't care. We all are influenced by our upbringing, and try as we might to rid ourselves of it, some of it sticks. When we're honest with ourselves we realize that ones ethnicity has nothing to do with their ability to do a job.