A few parts from a longer post by Dan Little:
Polarization of American politics, by Dan Little: If anything seems self-evident about recent American politics, it is the fact that our discourse and policy debates have become more polarized. Commentators and politicians seem to have moved to more extreme positions over time so that bipartisanship and compromise are all but impossible. As for rational, honest and fact-based debate about policies like taxes, healthcare, or Social Security in the Congress -- forget it. Instead we have strident conspiracy theories, ignorant birthers with funny hair, and hateful shrieking about the President -- all intended, presumably, to whip up the faithful.
That's how things look to us as participants in the American scene. Is it possible to study this question with some degree of scientific objectivity and clarity? Can we measure polarization? Somewhat surprisingly (to me anyway) it is possible, and political scientists Nolan McCarty, Keith Poole, and Howard Rosenthal present their often surprising findings in Polarized America: The Advance of Ideology and Unequal Riches. ...
Why has the ideological distance between the parties increased so dramatically since the 1970s? The most important cause they consider over time is degree of income stratification present at a point in time (national inequalities).
The results are genuinely striking. Through a series of graphs they demonstrate a very high degree of correlation between polarization and inequalities. And they argue that this is causal: politics has become more polarized as the stakes rise for those at the upper part of the distribution. ...
The book is a remarkable and complex argument in which the authors use sophisticated statistical tools to tease out connections between their core data set -- the ideological scores of virtually all national-level legislators since the 1870s -- and other factors (constituency characteristics, party, personal characteristics, etc.). They succeed in cutting through the seemingly crazed rhetoric of conservative extremists in and out of Congress and reveal what it's really all about: protecting the economic interests of the wealthy. For all its quiet restrained tone, it's an important contribution to the Occupy argument.
What is really interesting about this analysis is that it implies that the sizzling rhetoric coming from the right -- personal attacks on the President, anti-gay rants, renewed heat around abortion and contraception -- is just window dressing. By the evidence of voting records, what the right really cares about is economic issues favoring the affluent -- tax cuts, reduced social spending, reduced regulation of business activity, and estate taxes. This isn't to say that the enraged cultural commentators aren't sincere about their personal belief -- who knows? But the policies of their party are very consistent, in the analysis offered here. Maybe the best way of understanding the extremist pundits is as a class of well-paid entertainers, riffing on themes of hatred and cultural fundamentalism that have nothing to do with the real goals of their party.