Also, why the focus on rich and middle class? Is the vast majority of america not "lower"/working class? Edit: it seems like the entire conclusion of the study is based on the influence that money has in politics.

One thing that does have an influence? Money. While the opinions of the bottom 90% of income earners in America have a “statistically non-significant impact,” economic elites, business interests, and people who can afford lobbyists still carry major influence.

Of course if you focus only on people with money then you will come up with a conflicting result... so yeah. I also feel like I am missing something here.