A two-party system is a political system having a system of electoral rules which happens to ensure that all, or nearly all, elected offices are held only by the candidates of the two highest vote-getting parties. Two-party systems are generally not designed to be two-party systems, but tend to favor two major political parties as an unintended consequence of Duverger's law. Important rules are:

  • First-past-the-post election system: Only the highest vote getter in each district gains a seat.
  • Candidates chosen by district, rather than by at large elections.
  • No proportional representation.

Such systems have evolved in the United States and the United Kingdom, and Canada, as well as in many small or newly independent countries such as Jamaica. While Americans and citizens of the UK often see the two-party system as natural, based on their long experiences with it, it is in fact a product of the particular rules in place. The two parties that dominate thus have an incentive to keep the rules as they are, so as to prevent electoral losses to smaller parties.

Defenders of the two-party system claim that it produces more stable governance than multiparty systems. Because uncommon ideas are non-influential, policies and governments do not change rapidly. The system allows major parties to co-opt uncommon ideas as they become more common. While smaller parties find this exceptionally frustrating, it enhances stability while allowing for ideas that gain favor to become politically influential.

Critics of the two-party system point out that stability may not be an advantage, since there are many stable democratic republics, such as Germany, that have some form of proportional representation. They argue that it leads to the following flaws:

  • Limited representation for uncommon ideas.
  • Debate thus often revolves around narrowly perceived policy ideas, rather than larger political issues.
  • Reform is exceptionally difficult.
  • The system is more easily corrupted by campaign contributions since there are fewer players to donate to.
  • In an effort to attract voters, each party will adopt planks of the other party's platform, leading to the appearance in some skeptics' minds of a one-party system. Examples include the American notion of a "Republicrat."
  • Most electors are forced to engage in tactical voting, voting for candidates that may not be their first choice.