Two-party system
A two-party system is a party system where two major political parties dominate the government. One of the two parties typically holds a majority in the legislature and is usually referred to as the "majority" or "governing party" while the other is the "minority" or "opposition party". Around the world, the term has different senses. For example, in the United States, Jamaica, and Malta, the sense of "two-party system" describes an arrangement in which all or nearly all elected officials belong to one of the only two major parties, and third parties rarely win any seats in the legislature. In such arrangements, two-party systems are thought to result from various factors like winner-takes-all election rules. In such systems, while chances for third-party candidates winning election to major national office are remote, it is possible for groups within the larger parties, or in opposition to one or both of them, to exert influence on the two major parties. In contrast, in the United Kingdom and Australia and in other parliamentary systems and elsewhere, the term "two-party system" is sometimes used to indicate an arrangement in which two major parties dominate elections but in which there are viable third parties which do win seats in the legislature, and in which the two major parties exert proportionately greater influence than their percentage of votes would suggest.
Two-party system
Duverger concluded that "plurality election single-ballot procedures are likely to produce two-party systems, whereas proportional representation and runoff designs encourage multipartyism." He suggested there were two reasons why "winner-takes-all" systems leads to a two-party system. First, the weaker parties are pressured to form an alliance, sometimes called a "fusion", to try to become big enough to challenge a large dominant party and, in so doing, gain political clout in the legislature. Second, voters learn, over time, not to vote for candidates outside of one of the two large parties since their votes for third party candidates are usually ineffectual. As a result, weaker parties are eliminated by voters over time. Duverger pointed to statistics and tactics to suggest that voters tended to gravitate towards one of the two main parties, a phenomenon which he called "polarization", and tend to shun third parties. For example, some analysts suggest that the Electoral College system in the United States, by favoring a system of winner-takes-all in presidential elections, is a structural choice favoring only two major parties.