I've written before about Larry Bartels' research showing that politicians basically don't care about the views of low and medium-income individuals. The non-rich simply have no impact on their voting behavior at all. But I know you want more evidence. So here it is.
The charts below come from a 2005 paper by Martin Gilens (a revised 2007 version is here). His study is based on a dataset of polling questions about public policy issues between 1981 and 2002 (raising the minimum wage, sending U.S. troops to Haiti, requiring employers to provide health insurance, allowing gays to serve in the military, etc.) in which the responses differed significantly between the rich and the poor. On the left, you can see the impact that support from low-income voters had: when 10% of them supported a position, there was about a 32% probability of that change becoming law. When 90% supported a position, there was a....33% probability. The chart on the right shows the same for median income voters. They did slightly better, but not much.
Rich voters, on the other hand, had a much better chance of getting their way, as the steep solid line in both charts shows. Why? Gilens' guess is that "the most obvious source of influence over policy that distinguishes high-income Americans is money." This sounds like a pretty good guess to me.