Maybe the reason calling Obama a socialist, a communist, and a Marxist is failing as a political strategy is that people are no longer afraid, or unwarrantedly skeptical, about those terms. (Or maybe because enough people know he isn't those things, but go with me on this for a moment...)
Perhaps people have concluded that the vision of "free markets" touted by the right doesn't fulfill its promises, and rests on a moral foundation that has been worn away by its own hypocrisy. Perhaps people have concluded that there's really no such thing as the free market. Perhaps they have, at least, concluded that regulations are inevitable and acceptable and that it's ridiculous to try to scare us with visions of market intervention when we have just committed the largest act of intervention into the market since the Bolshivek Revolution.
Perhaps some people have even concluded that most people would be better off even under an antiquated socialist system than they are now, and they no longer buy the line that such a system would be inevitably despotic and totalitarian. Having seen the last eight years in America, can we really disagree?