Here's a post by Matthew Yglesias on the relevant details:
Government By The Rich: Via Kevin Drum... Martin Gilens... the most important fact about inequality and American politics—when rich people and average folks disagree, rich people always get their way....The autocracies that are falling in the Middle East are an example of this phenomenon. The idiocy of the Republican govenor in Wisconsin is an example of this. The rigid politics of federal Republican politics in the US is an example of this. The Middle East is solving their problem by toppling regimes. How will this problem be fixed in America?
I would say the most obvious mechanism here is socialization. The president, the senior White House staff, the cabinet secretaries, the senators, the House members, the senior congressional staff, and the lobbyists, association heads, business executives, governors, mayors, foreign officials, and media celebrities who they interact with are all personally pretty high income. You get into the top decile of the US income distribution with a household income of $138,000, so the entire congress is in the top ten percent. What’s more, political elites tend to have college roommates, siblings, in-laws, etc. who are also prosperous.
Obviously the fact that rich people have money to spend on politics doesn’t hurt either. But I would never underestimate the human desire to believe that one is doing the right thing, and thus the importance of socialization to determining bias. Nobody in Washington seems to know that the public is clamoring for higher Social Security benefits and more federal spending on health and education largely, I think, because this isn’t what the people they know personally are clamoring for.