From Peter Orszag on Bloomberg. Could not have said it better myself:

It is striking that both income inequality and political polarization began to rise sharply in the U.S. in the mid- to late 1970s. Yet many pundits airily dismiss this connection, arguing that because blue states are, on average, higher-income than red states, the link between income and partisanship must be weak. Instead, they attribute increasing political polarization to the gerrymandering of legislative districts. Both of these assertions are empirically false.