The author neglects the ebb and flow of the diversity of opinion on both sides. For example, the political left can be subdivided into "liberal", and "radical", and the political right as "liberal" and "conservative". When nationally, liberalism was ascending and conservatism declining, the radicals of the Democrat party are marginalized. They are less able to control the agenda of the left, but liberals of both parties have more power. Conversely, when conservatism is ascending and Democrat liberalism is declining, it marginalizes Republican liberals as well, and yet the radicals of the Democrat party more strongly control their parties agenda.