I actually sort of disagree with Gilens and Drum that “the most obvious source of influence over policy that distinguishes high-income Americans is money.” 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.