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In electoral politics, a third party is any party contending for votes that failed to outpoll either of its two strongest rivals (or, in the context of an impending election, is considered highly unlikely to do so). The distinction is particularly significant in two-party systems. In any case "third" is often used figuratively, as in "the third parties", where the intent, literally stated, is "the third and succeeding parties".
For instance, in the United Kingdom a third party is a national political party, other than the Conservatives and Labour, which has at least one member in the House of Commons. Since 2015, it is used for the Scottish National Party. In Scotland the dominant parliamentary party since 2016 has been the Scottish National Party with the Conservative party the next largest party and the Labour party third.
Countries using proportional representation give little advantage to the largest two parties, so they tend to elect many parties. Therefore, in those countries, three, four, or more political parties are usually elected to legislatures. In such parliamentary systems, coalitions often include smaller parties; since they may participate in a coalition government, there is not a sharp distinction with a 'major' party. In two-party systems, on the other hand, only the major parties have a serious chance of forming a government. Similarly, in presidential systems, third-party candidates are rarely elected president.
In some categorizations, a party needs to have a certain level of success to be considered a third party. Smaller parties that win only a very small share of the vote and no seats in the legislature often are termed minor or fringe parties.
Third parties face an uphill battle in terms of electoral success due to incentives placed on voters by election algorithms. Even in instances where the potential supporter may align themselves most with a certain third party, in the face of overwhelming odds against impacting the election it makes more sense to just stay home or back a coalition party in compromise. These disincentives exist primarily in the United States perpetuating two-party rule and can be alleviated through adopting electoral reform measures in the form of voting system adaptations, which, predictably, are often backed by third parties and opposed by the primary parties.
In some countries like the United States, parties with low win probability also face frequent exclusion from major debates and media coverage and denial of ballot access as well as hamstrung campaign budgets.
In U.S. politics, a third party is a political party other than the Democrats or Republicans, such as the Libertarians and Greens. The term "minor party" is also used in a similar manner. Such political parties rarely win elections, as proportional representation is not used in federal or state elections, but only in some municipal elections.
A similar situation occurs with the presidential Electoral College, where Electoral College votes are often given the candidate who receives a plurality of the vote, thus bringing up accusations that certain third party presidential candidates are "spoiling" the election or splitting up segments of voters.
Third parties usually have little chance of forming a government or winning the position of head of government. Nevertheless, there are many reasons for third parties to compete. The opportunity of a national election means that attention will be paid to the positions of third parties. The larger parties might be forced to respond and adapt to their challenges, and often the larger parties copy ideas from them. Most third parties try to build their support to become one of the dominant parties, as the Labour Party in Britain did.
In the Westminster system there is also the possibility of minority governments, which can give smaller parties strength disproportional to their support. Examples include the Irish Parliamentary Party which pushed for Home Rule in Ireland in the late 19th century.