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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. From 1922 to 2015, Liberal Democrats and its predecessor Liberals was the third party. Since 2015, it is used for the Scottish National Party (SNP). In Scotland, SNP has been the dominant parliamentary party beginning with the 2011 Scottish Parliament election, with the Conservative the next largest party and Labour becoming Scotland's third party ever since.
In Canada provinces such as Ontario, Prince Edward Island and Quebec, nearly 50+ active political parties exist throughout the nation with several them both provincial and federal levels either did elected, formed or joined during cross-flooring.
The term "third parties" is used mostly in countries with first-past-the-post voting systems, as those systems tend to create a two-party system, so that successful smaller parties are rare to stronger two-party systems like the United States.
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. e 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 Canada politics are similar to United Kingdom politics but it does instead have either one or occasionally two and three national and provincial third parties since last few decades in Canadian politics like the national New Democratic Party and most its provincial chapters, the national Green Party of Canada and some its provincial chapters, Quebec's the Parti Québécois since 2018, New Brunswick's the People's Alliance of New Brunswick since 2018, Ontario's the Ontario Liberal Party since 2018, Alberta's the Alberta Party since 2015, Manitoba's the Manitoba Liberal Party since 2016, and the Prince Edward Island Liberal Party since 2019.
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 third political parties rarely win elections, as proportional representation is not used in federal or state elections, but only in some municipal elections. But however since 2018, Maine as become first state for adopted Majoritarian favor ranked-choice voting system to deduced spoiler effect nor vote splitting made under First-Past-The-Post system for both federal and primary elections, but not state elections due of their state's constitution only favor plural voting systems.
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 and New Democratic Party in Canada 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.