What two party system?
Acceptable political opinion ranges from Mitch McConnell to Harry Reid. Opinions falling outside of that range are dangerous and extremist (for example, the tea party movement before it was co-opted by the Republicans.) The MSM will, however, make exceptions on an ad hoc basis (see, for example, the OWS crowd.)
The differences between the two parties are mainly about how the vast sums of wealth seized in the form of income taxes and other forced exactions are to be divvied up among the various political constituencies. Voters get to select between Coke and Pepsi according to their tastes.