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.
Third party (United States)
In the United States, if an interest group is at odds with its traditional party, it has the option of running sympathetic candidates in primaries. If the candidate fails in the primary and believes he or she has a chance to win in the general election he or she may form or join a third party. Because of the difficulties third parties face in gaining any representation, third parties tend to exist to promote a specific issue or personality. Often, the intent is to force national public attention on such an issue. Then, one or both of the major parties may rise to commit for or against the matter at hand, or at least weigh in. H. Ross Perot eventually founded a third party, the Reform Party, to support his 1996 campaign. In 1912, Theodore Roosevelt made a spirited run for the presidency on the Progressive Party ticket, but he never made any efforts to help Progressive congressional candidates in 1914, and in the 1916 election, he supported the Republicans.