Reddit reviews The Unheavenly Chorus: Unequal Political Voice and the Broken Promise of American Democracy

Almost all activism tends to be dominated by those that are at least relatively well off. This is because participating in political activism is partially a function of being able to have the time and resources to actually do it. Now, this does not mean that poor people or others do not care about these issues, but that they just might not have time to participate in it.

If you want some scholarly literature:

• Process and Protest: Accounting for Individual Protest Participation by Alan Schussman and Sarah A. Soule
  
• The Unheavenly Chorus by Schlozman et al

This one looks at the many (and often hidden) political advantages enjoyed by the wealthy class: The Unheavenly Chorus.