There's evidence to support that idea.
From Ezra Klein (via Andrew Sullivan) points to a book that makes the case:
Ezra Klein explores inequality's roots in a review of Winner-Take-All Politics, by Jacob S. Hacker and Paul Pierson:
[T]he book’s greatest strength is its easy command of political science data, which sets it apart from most of the other studies of inequality that have been released. Perhaps the most shocking study the authors cite comes from Martin Gilens, a political scientist at Princeton University. Gilens has been collecting the results of nearly 2,000 survey questions reaching back to the 1980s, looking for evidence that when opinions change, so too does policy. And he found it—but only for the rich.
“Most policy changes with majority support didn’t become law,” Hacker and Pierson write. The exception was “when they were supported by those at the top. When the opinions of the poor diverged from those of the well-off, the opinions of the poor ceased to have any apparent influence: If 90 percent of poor Americans supported a policy change, it was no more likely to happen than if 10 percent did. By contrast, when more of the well-off supported a change, it was substantially more likely to happen.”
In part, this is because politicians began to need money more than they had before, as the costs of campaigns started skyrocketing. The predictable outcome? Both parties have been relying more on wealthy donors and less on labor unions.
