On average, 60 percent of Americans tell pollsters that they are in favor of third party alternatives. [1] Whereas early polls showed high support for the two-partysystem, in the last 30 years Americans have developed more negativeopinions. [2] The Republicans and the Democrats, however, have been able to maintain their dominance. No third party has representation in the U.S. Congress or is in contention for control of any state legislature. As Joan Bryce put it in herdissertation on third party constraints, "The extent and scope of two-party dominance within the United States is unique." [3] John Bibby and Sandy Maisel explained that the two-party system has come as a surprise to observers of American society:
That the United States should have the oldest and strongest two-party system on the globe is formany, particularly for foreign observers, a bewildering phenomenon.

http://www.matthewg.org/multiparty/ch1.htm

http://www.matthewg.org/multiparty/ch1.pdf