I think that Domhoff is saying that wealthy people have more influence on the way the world is run. I do believe that he backs up his argument and I agree with many of the examples he gives.
On the other hand, I do have to say that there are wealthy people that don't "act" it, "look" it, or "talk" it. I guess really that there are both ways to look at it but basically its a toss up in my book.
A poor person could have an attitude problem because they're poor and a rich person could have an attitude problem because they're rich.