The US political system is designed to swing back and forth between two right-wing parties. It would be less successful at dividing people and keeping out opposing ideas if only one party won all the time, and they know it.

The smart way to keep people passive and obedient is to strictly limit the spectrum of acceptable opinion, but allow very lively debate within that spectrum - even encourage the more critical and dissident views. That gives people the sense that there's free thinking going on, while all the time the presuppositions of the system are being reinforced by the limits put on the range of the debate."