Plumer draws on the work of various researchers. Here's a striking example of their findings, as summarized by Plumer:
In politics, this all matters a great deal. Larry Bartels of Princeton has recently studied the voting record of the Senate between 1989 and 1994--a time, note, when Democrats controlled Congress. He found that senators were very responsive to the preferences of the upper third of the income spectrum, somewhat less attentive to the middle third, and completely dismissive of the policy preferences of the poorest third. In one striking example, Bartels discovered that senators were likely to vote for a minimum wage increase only when their wealthier constituents favored it--the views of those directly affected by the hike had "no discernible impact."
The Bartels study is available as a PDF (928 KB).
The rest of Plumer's article is equally interesting.