This is a fun chart, not a definitive word of God statement of anything. (No wagering!)
From a work called "Polarized America: The Dance of Ideology and Unequal Riches" that correlates income inequality and political polarization. (Not just party totals, but looking at the extremity and difference of views. For instance, they say the current congress is the most polarized since reconstruction since there are almost no moderate Republicans left in office.)
Notice that (according to this, at least) when Reagan took office income inequality was at it's lowest and the political spectrum fairly compressed. No wonder we needed a Reagan revolution! Won't someone think of the rich?
Also striking: the low level of polarization during the very intense 1960s and 1970s. This paper could have been called "WINGNUTS ARE A LUXURY" because it looks like when times are tough people have less patience in extremism.
And also striking: polarization leads inequality. Correlation is not causation, but one would expect that sort of lag if equality shifts were largely attributable to legislation.
Theory: I suspect a line labeled "religion in public life" would track these other two lines pretty darn well.
http://polarizedamerica.com/#POLITICALPOLARIZATION