Two-party system in the context of "Third party (U.S. politics)"

A dominant-party system, or one-party dominant system, is a political occurrence in which a single political party continuously dominates election results over running opposition groups or parties. Any ruling party staying in power for more than one consecutive term may be considered a dominant party (also referred to as a predominant or hegemonic party). Some dominant parties were called the natural governing party, given their length of time in power.

Dominant parties, and their domination of a state, develop out of one-sided electoral and party constellations within a multi-party system (particularly under presidential systems of governance), and as such differ from states under a one-party system, which are intricately organized around a specific party. Sometimes the term "de facto one-party state" is used to describe dominant-party systems which, unlike a one-party system, allows (at least nominally) democratic multiparty elections, but the existing practices or balance of political power effectively prevent the opposition from winning power, thus resembling a one-party state. Dominant-party systems differ from the political dynamics of other dominant multi-party constellations such as consociationalism, grand coalitions and two-party systems, which are characterized and sustained by narrow or balanced competition and cooperation.

First-past-the-post (FPTP)—also called choose-one, first-preference plurality (FPP), or simply plurality—is a single-winner voting rule. Voters mark one candidate as their favorite, or first-preference, and the candidate with more first-preference votes than any other candidate (a plurality) is elected, even if they do not have more than half of votes (a majority).

FPP has been used to elect part of the British House of Commons since the Middle Ages before spreading throughout the British Empire. Throughout the 20th century, many countries that previously used FPP have abandoned it in favor of other electoral systems, including the former British colonies of Australia and New Zealand. FPP is still officially used in the majority of US states for most elections. However, the combination of partisan primaries and a two-party system in these jurisdictions means that most American elections behave effectively like two-round systems, in which the first round chooses two main contenders (of which one of them goes on to receive a majority of votes).

Maurice Duverger (/ˈduːvərʒeɪ/ DOO-vər-zhay; French: [mɔʁis dyvɛʁʒe]; 5 June 1917 – 16 December 2014) was a French jurist, sociologist, political scientist and politician born in Angoulême, Charente. Starting his career as a jurist at the University of Bordeaux, Duverger became more and more involved in political science and in 1948 founded one of the first faculties for political science in Bordeaux, France. An emeritus professor of the Sorbonne and member of the FNSP, he has published many books and articles in international newspapers, such as Corriere della Sera and la Repubblica in Italy, El País in Spain, and especially Le Monde in France.

Duverger studied the evolution of political systems and the institutions that operate in diverse countries, showing a preference for empirical methods of investigation rather than philosophical reasoning. He devised a theory which became known as Duverger's law, which identifies a correlation between a first-past-the-post election system and the formation of a two-party system. While analysing the political system of France, he coined the term semi-presidential system.

The Panhellenic Socialist Movement (Greek: Πανελλήνιο Σοσιαλιστικό Κίνημα, romanized: Panellínio Sosialistikó Kínima, pronounced [paneˈlini.o sosi.alistiˈko ˈcinima]), known mostly by its acronym PASOK (/pəˈsɒk/; ΠΑΣΟΚ, pronounced [paˈsok]), is a social-democratic political party in Greece. Until 2012 it was one of the two major parties in the country, along with New Democracy, its main political rival. After a decade of poor electoral outcomes, PASOK has retained its position as one of the main Greek political parties and is currently the second largest party in the Greek Parliament.

Following the collapse of the Greek military dictatorship of 1967–1974, PASOK was founded on 3 September 1974 as a socialist party.

The National Liberal Party (Romanian: Partidul Național Liberal, PNL) was the first organised political party in Romania, a major force in the country's politics from its foundation in 1875 to World War II. Established in order to represent the interests of the nascent local bourgeoisie, until World War I it contested power with the Conservative Party, supported primarily by wealthy landowners, effectively creating a two-party system in a political system which severely limited the representation of the peasant majority through census suffrage. Unlike its major opponent, the PNL managed to preserve its prominence after the implementation of universal male suffrage, playing an important role in shaping the institutional framework of Greater Romania during the 1920s.

American electoral politics have been dominated by successive pairs of major political parties since shortly after the founding of the republic of the United States. Since the 1850s, the two largest political parties have been the Democratic Party and the Republican Party—which together have won every United States presidential election since 1852 and controlled the United States Congress since at least 1856. Despite keeping the same names, the two parties have evolved in terms of ideologies, positions, and support bases over their long lifespans, in response to social, cultural, and economic developments—the Democratic Party being the left-of-center party since the time of the New Deal, and the Republican Party now being the right-of-center party.

Political parties are not mentioned in the U.S. Constitution, which predates the party system. The two-party system is based on laws, party rules, and custom. Several third parties also operate in the U.S. and occasionally have a member elected to local office; some of the larger ones include the Constitution, Green, Alliance, and Libertarian parties, with the Libertarian being the largest third party since the 1980s. A small number of members of the U.S. Congress, a larger number of political candidates, and a good many voters (35–45%) have no party affiliation. However, most self-described independents consistently support one of the two major parties when it comes time to vote, and members of Congress with no political party affiliation caucus to pursue common legislative objectives with either the Democrats or Republicans.