I've seen arguments in 3DHS that anyone middle class voting Republican is voting against his best interests. The argument implies that Republicans deceive all manner of folks into voting for them which implies that Republican voters are some how not intelligent enough to see this deception or to think for themselves, and that Democrats were somehow smarter and better. I'm not saying this justifies the comments you're talking about, because it certainly does not. But sometimes it is hard to be motivated to condemn someone for doing something that someone else is doing to you.
And I'll say that sort of "voting against their best interests" reasoning is wrong all around. And you're right that it is subtly racist to use it about African-Americans in particular.
I wanted to address this post by Prince from a while back. It was made in a thread started by knute that was inherently racist and I thought that perhaps pulling it out and giving a new perspective would help in addressing this separately from the other issues.
First of all I agree that condemning a group of people for not condemning another person is a bit odd. It implies that by not condemning someone there is passive approval, which on a forum like this is not necessarily true.
On the other hand, I disagree with the notion that one cannot make inferences based on class. One can even make some inferences based on race, for example we know that certain diseases strike a particular ethnic group more than another (as an example, those of Scandinavian ancestry are more susceptible to develop schizophrenia than the average population from other ethnic backgrounds). Yet, making inferences based on the intelligence of a broad group of a race or religion, for example, is purely racist in sentiment.
The problem with the above example is that the middle class is ill-defined and purposely so. "Middle class" as used by politicians is basically a code phrase for "Americans" or "Average Americans" or "you and I." It can range from someone who scrapes by to pay the monthly bills and feed the family to someone who has a five car garage and a brand new Porsche SUV for junior. In other words, it is too ill-defined to be of any practical use. You'll note that "middle class" is not a Marxian term at all. It is a rather catch-all political construct.
Having said that, all mainstream sides of the political spectrum will aim towards the "middle class" at least in rhetoric. So saying "my side is best for the middle class" is just another way of cheering for the home team.