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A two-party system is a form of government in which two dominant parties hold offices on every level of politics, from regional to national. It shouldn't be mistaken with the notion that voters only have the option of voting for one of two parties. As in the U.S. and many other countries, governments run by a two-party system often allow for third-party candidates to run for any office if they can wrest enough votes away from the two dominant parties. Two-party systems are a political phenomenon associated with plurality voting systems, in which the candidate with the most votes wins the political office. Duverger's law theorizes that plurality voting systems have a high likelihood of turning out two-party systems.
Many nations throughout the world have had two-party systems: Democrats are pitted against Republicans in the U.S., Conservatives versus the Labour Party in the United Kingdom (UK), and the Democratic Party versus the Liberal Democratic Party in Japan. Governments that act on the cooperation of more than two parties are called coalition governments. Coalition governments are often parliamentary in style. Some governments operating under a two-party system have the potential of transitioning into a coalition government in the event of a split election, such as with England after the 2010 election, which saw the Liberal Democrat and Conservative parties forming a coalition.
It is difficult for third parties to destabilize a two-party system for multiple reasons. In some countries, voter loyalty, or perhaps just as powerfully, familiarity, with two dominant parties makes it difficult for lesser known, less established third parties to garner an effective number of votes. This, however, doesn't mean third parties don't play an important role. In fact, they play one of the most important roles in two-party politics — that of the spoiler. Even if a third party doesn't win any elections, it does have the potential to pull some votes away from one of the dominant parties, possibly enough to influence an election’s outcome. In 1912, Teddy Roosevelt, running as a Progressive in the US, took nearly 28 percent of the popular vote, beating out Republican candidate Taft's 23 percent. With Taft's votes severely weakened by Roosevelt, Democrat Woodrow Wilson won the presidency.
Maurice Duverger, a French politician and sociologist, theorized that two-party systems are a natural result of nations which elect their officials by a majority vote on a single ballot. He reasoned that parties with similar views would merge in order to avoid being outmuscled by opposing parties, and that would cause other parties with similar views to, in turn, merge as well. Any third party, he thought, would rarely if ever have a significant shot at a majority rule given this phenomenon; the two-party system would only continue to reinforce itself. His views are commonly taught in political science.