Here's a table from Gilen's work referred to below. When shifts in the preferences of high income people change, policies are more likely to change than when those of low or middle incomes do.
This makes sense when we think of policy makers - especially legislators - as being self interested actors who wish to maximize their re-election chances. Back those who can help - or hurt - you.
Does this suggest we ought to consider the US to be more an oligarchy than a democracy? A good discussion question for class.