Democracy and Inequality in the USA Bonica et al. 2013 doi:10.1257/jep.27.3.103

The title of the paper asks:
"Why Hasn't Democracy Slowed Rising Inequality?"

The paper shows the the beginnings of an answer: that the parties have been taken over by the very rich the top ten-thousandth section of the economic pyramid.

The second image is for the Democratic party alone, with a more detailed breakup:

Another paper from 2014 by Gilens and Page doi:10.1017/S1537592714001595 shows how "economic elites and organized groups representing business interests have substantial independent impacts on U.S. government policy, while average citizens and mass-based interest groups have little or no independent influence."