A two-party system becomes a dichotomous division of the political spectrum with an ostensibly right-wing and left-wing party: Republicans vs. Democrats in the United States. Duverger's law asserts that a plurality rule election system tends to favor a two-party system. A third party can only enter the arena if it can exploit the mistakes of a pre-existing major party, ultimately at that party's expense. A major party will make a mistake and allow this to occur in the presidential election. This has already occurred in gubernatorial elections.
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