This list does not include political organizations that do not run candidates for office but otherwise function similarly to third parties. For non-electoral political "parties", see here.
This section includes only parties that have actually run candidates under their name in recent years.
This section includes any party that is independent, populist, or any other that either rejects right-left politics or doesn't have a party platform.
This section includes any party that has a left-liberal, progressive, social democratic, democratic socialist or Marxist platform.
This section includes parties that primarily advocate for granting special privileges or consideration to members of a certain race, ethnic group, religion etc.
Also included in this category are various parties found in and confined to Indian reservations, almost all of which are solely devoted to the furthering of the tribes to which the reservations were assigned. An example of a particularly powerful tribal nationalist party is the Seneca Party that operates on the Seneca Nation of New York's reservations.
This section includes parties that primarily advocate single-issue politics (though they may have a more detailed platform) or may seek to attract protest votes rather than to mount serious political campaigns or advocacy.
A number of third party, independent, and write-in candidates have performed well in many U.S. elections.
Greens, Libertarians and others have elected state Legislators and local officials. The Socialists had 600 mayors at one time before World War I, including Milwaukee, Wisconsin; New Haven, Connecticut; Reading, Pennsylvania; and Schenectady, New York. There have been 20th Century governors elected as independents, and from such parties as Progressive, Reform, Farmer-Labor, Populist, and Prohibition. There were others in the century before. However, the United States has had a two-party system for over a century. The winner take all system for presidential elections and the single-seat plurality voting system for Congressional elections have over time created the two-party system (see Duverger's law).
Third party candidates sometimes win elections. For example, such a candidate only won a U.S. Senate election twice (0.6%) since 1990. Sometimes a national officeholder not affiliated with and endorsed by one of the two major parties is elected. Currently, there is only one U.S. Senator, Angus King, who is neither Democratic nor Republican, while no U.S. Representative hails from outside the major parties. Although third party candidates rarely actually win elections, they can have an effect on them. If they do well, they are often accused of having a spoiler effect. Sometimes they have won votes in the electoral college, as in the 1832 Presidential election. They can draw attention to issues that may be ignored by the majority parties. If such an issue finds acceptance with the voters, one or more of the major parties may adopt the issue into its own party platform. Also, a third party may be used by the voter to cast a protest vote as a form of referendum on an important issue. Third parties may also help voter turnout by bringing more people to the polls. Third party candidates at the top of the ticket can help to draw attention to other party candidates down the ballot, helping them to win local or state office. In 2004 the U.S. electorate consisted of an estimated 43% registered Democrats and 33% registered Republicans, with independents and those belonging to other parties constituting 25%.
The only three U.S. Presidents without a major party affiliation were George Washington, John Tyler, and Andrew Johnson, and only Washington served his entire tenure as an independent. Neither of the other two were ever elected president in their own right, both being vice presidents who ascended to office upon the death of a president, and both became independents because they were unpopular with their parties. John Tyler was elected on the Whig ticket in 1840 with William Henry Harrison but was expelled by his own party. Johnson was the running mate for Abraham Lincoln, who was reelected on the National Union ticket in 1864; it was a temporary name for the Republican Party.
Currently Bill Walker of Alaska is the only independent Governor in the United States, having defeated incumbent Republican Sean Parnell in the 2014 gubernatorial election. He is also the first independent Governor since Alaska became a state (although not the first third party governor).
In winner-take-all (or plurality-take-all), the candidate with the largest number of votes wins, even if the margin of victory is extremely narrow or the proportion of votes received is not a majority. Unlike in proportional representation, runners-up do not gain representation in a first-past-the-post system. In the United States, systems of proportional representation are uncommon, especially above the local level, and are entirely absent at the national level. In Presidential elections, the majority requirement of the Electoral College, and the Constitutional provision for the House of Representatives to decide the election if no candidate receives a majority, serves as a further disincentive to third party candidacies.
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 has a chance to win in the general election he 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.
Nationally, ballot access laws are the major challenge to third party candidacies. While the Democratic and Republican parties usually easily obtain ballot access in all fifty states in every election, third parties often fail to meet criteria for ballot access, such as registration fees. Or, in many states, they do not meet petition requirements in which a certain number of voters must sign a petition for a third party or independent candidate to gain ballot access. In recent presidential elections, Ross Perot appeared on all 50 state ballots as an independent in 1992 and the candidate of the Reform Party in 1996. (Perot, a multimillionaire, was able to provide significant funds for his campaigns.) Patrick Buchanan appeared on all 50 state ballots in the 2000 election, largely on the basis of Perot's performance as the Reform Party's candidate four years prior. The Libertarian Party has appeared on the ballot in at least 46 states in every election since 1980, except for 1984 when David Bergland gained access in only 36 states. In 1980, 1992, 1996, and 2016 the party made the ballot in all 50 states and D.C. The Green Party gained access to 44 state ballots in 2000 but only 27 in 2004. The Constitution Party appeared on 42 state ballots in 2004. Ralph Nader, running as an independent in 2004, appeared on 34 state ballots. In 2008, Nader appeared on 45 state ballots and the D.C. ballot. For more information see ballot access laws.
Presidential debates between the nominees of the two major parties first occurred in 1960, then after three cycles without debates, took place again in 1976 and have happened in every election since. Third party or independent candidates have been included in these debates in only two cycles. Ronald Reagan and John Anderson debated in 1980, but incumbent President Carter refused to appear with Anderson, and Anderson was excluded from the subsequent debate between Reagan and Carter.
Debates in other state and federal elections often exclude Independent and third party candidates, and the Supreme Court has upheld such tactics in several cases. The Commission on Presidential Debates (CPD) is a private company.  Independent Ross Perot was included in all three of the debates with Republican George H. W. Bush and Democrat Bill Clinton in 1992, largely at the behest of the Bush campaign. His participation helped Perot climb from 7% before the debates to 19% on Election Day.
Perot was excluded from the 1996 debates despite his strong showing four years prior. In 2000 revised debate access rules made it even harder for third party candidates to gain access by stipulating that, besides being on enough state ballots to win an Electoral College majority, debate participants must clear 15% in pre-debate opinion polls. This rule remained in place for 2004, when as many as 62 million people watched the debates, and has continued being in effect as of 2008. The 15% criterion, had it been in place, would have prevented Anderson and Perot from participating in the debates they appeared in.
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A third party candidate will sometimes strike a chord with a section of voters in a particular election, bringing an issue to national prominence and amount a significant proportion of the popular vote. Major parties often respond to this by adopting this issue in a subsequent election. After 1968, under President Nixon the Republican Party adopted a "Southern Strategy" to win the support of conservative Democrats opposed to the Civil Rights Movement and resulting legislation and to combat third parties with southern agendas. This can be seen as a response to the popularity of segregationist candidate George Wallace who gained 13.5% of the popular vote in the 1968 election for the American Independent Party.
In 1996, both the Democrats and the Republicans agreed to deficit reduction on the back of Ross Perot's popularity in the 1992 election. This severely undermined Perot's campaign in the 1996 election.
Today, as in 1958, ballot access for minor parties and Independents remains convoluted and discriminatory. Though certain state ballot access statutes are better, and a few Supreme Court decisions (Williams v. Rhodes, 393 U.S. 23 (1968), Anderson v. Celebrezze, 460 U.S. 780 (1983)) have been generally favorable, on the whole, the process--and the cumulative burden it places on these federal candidates--may be best described as antagonistic. The jurisprudence of the Court remains hostile to minor party and Independent candidates, and this antipathy can be seen in at least a half dozen cases decided since Nader's article, including Jenness v. Fortson, 403 U.S. 431 (1971), American Party of Tex. v. White, 415 U.S. 767 (1974), Munro v. Socialist Workers Party, 479 U.S. 189 (1986), Burdick v. Takushi, 504 U.S. 428 (1992), and Arkansas Ed. Television Comm'n v. Forbes, 523 U.S. 666 (1998). Justice Rehnquist, for example, writing for a 6-3 divided Court in Timmons v. Twin Cities Area New Party, 520 U.S. 351 (1997), spells out the Court's bias for the "two-party system," even though the word "party" is nowhere to be found in the Constitution. He wrote that "The Constitution permits the Minnesota Legislature to decide that political stability is best served through a healthy two-party system. And while an interest in securing the perceived benefits of a stable two-party system will not justify unreasonably exclusionary restrictions, States need not remove all the many hurdles third parties face in the American political arena today." 520 U.S. 351, 366-67.