First off, I am sure that Racism is indeed a factor of not only Conservative Opposition but Opposition to him in general for SOME. Some will like him FOR race and some will dislike him FOR race. Even at a time in American Society where general acceptance and tolerance is extremely widespread, there will always be the ones who choose to let ignorance interfere.
However, that is just life and we must continue to fight the ignorance even if we can't fully defeat it.
Racism however does not apply in many cases. Just because you oppose President Obama doesn't make you a racist. Many people simply disagree with his policies. Differing opinions are bound to happen with any President, and this does not always stem from the topic of race.
But on the other hand, Obama's death threat rate has piled up 400%, usually catering from the topic of Race or other occasional ignorances. No doubt Race has a factor in that.