It may seem obvious that the disproportionate political influence of the top one percent of the income distribution comes from their ability to bribe politicians with campaign contributions, but it's worth noting that the economic elite are considerably more politically engaged in a fairly comprehensive way:
For example, 41 percent of the very wealthy reported attending a political meeting. Only 9 percent of Americans did so in 2008. And 68 percent of the very wealthy reported giving money to a political candidate, party, or cause in the last four years. In 2008–a year in which “small donors” were numerous–only 13 percent of Americans donated to a political candidate or party. Again, there are small differences in the wording of the questions between the two surveys, but they are not likely responsible for the 55-point gap.
Now needless to say, if I donate $50 and you donate $2,000 then your money is going to talk louder than mine. But in practical terms, the gap between $2,000 and $50 is probably much smaller than the gap between $50 and $0. The meeting statistics are even more telling in their way. And there's considerable evidence that the rich have a much higher proclivity to actually show up and vote. I always liked the old Woody Allen line about how 90 percent of life is just showing up, and part of the story of the top one percent is that they show up. The median American household is economically struggling in a variety of ways, but isn't so cash strapped as to be incapable of offering a political donation or two. Indeed, the American middle class is pretty passionate about donating to church groups and other charities. If more of that energy were directed into the political process, politicians would pay more attention. But as things stand, the wealthy are more highly engaged across the whole range of activities and so the political process is heavily tilted in favor of their preferences.