You missed the point. Instead of looking at other lefty candidates he did (or didn't, whatever) support...the issue should be on what he thinks of the direction the Rebuplican party has taken us over the last 8 years. I don't think it is race related, it think it is a referendum on the republican party. Call me crazy, but I could see him endorsing Hillary at this point too.
I do agree with your assessment that he had philosophical differences with the admin and that is why he is endorsing Obama. The way the war was sold to the public, torture, civil liberties, and foreign policy would be the notable ones. I think he subscribes to traditional conservative values on domestic, economic, and foreign policy...which, I am sorry to say, this admin failed at miserably. There is very little difference with McCain with this respect, especially the foreign policy part, and that is why Obama got his endorsement.
I think it is especially shortsighted to assert that this is somehow race related. It is policy related.