The Unified Socialist Party of Mexico (Spanish: Partido Socialista Unificado de México, PSUM) was a far-left political party in Mexico. It later became the Mexican Socialist Party (Partido Mexicano Socialista) in 1988. The PSUM was founded in November 1981 by the merger of four socialist parties: The term far left refers to the relative position a group or person occupies within the political spectrum. ... A political party is an organization that seeks to attain political power within a government, usually by participating in electoral campaigns. ... 1981 (MCMLXXXI) was a common year starting on Thursday of the Gregorian calendar. ... The color red and particularly the red flag are traditional symbols of Socialism. ...

• The Mexican Communist Party (Partido Comunista Mexicano, PCM) - the Mexican affiliate of the Communist International, formed in 1919;
• The Movement of Socialist Action and Unity (Movimiento de Acción y Unidad Socialista, MAUS) - a split from the PCM that was active in the Mexican labor movement;
• The Party of the Mexican People (Partido del Pueblo Mexicano, PPM) - a split from the Popular Socialist Party (PPS);
• The Movement of Popular Action (Movimiento de Acción Popular, MAP) - a party involved in campaigns for trade-union democracy and reform in the 1970s.

Before merging to form the PSUM, these four parties had formed an electoral alliance called the Coalition of the Left (Coalición de Izquierda) in 1977. The Mexican Communist Party (Spanish: Partido Comunista Mexicano, PCM) was a communist party in Mexico. ... The first edition of Communist International, journal of the Comintern published in Moscow and Petrograd (now Saint Petersburg) in May 1919. ... The labor movement (or labour movement) is a broad term for the development of a collective organization of working people, to campaign in their own interest for better treatment from their employers and political governments. ... Popular Socialist Party (Spanish: Partido Popular Socialista, PPS) is a left-wing political party in Mexico. ... A union (labor union in American English; trade union, sometimes trades union, in British English; either labour union or trade union in Canadian English) is a legal entity consisting of employees or workers having a common interest, such as all the assembly workers for one employer, or all the workers... An electoral alliance is an association of political parties or individuals which exists solely to stand in elections. ...

Though the PSUM was a multi-tendency organization, it generally followed the ideology of Eurocommunism. In 1988, the PSUM changed its name to the Mexican Socialist Party (Partido Mexicano Socialista, PMS). In 1989, following the presidential campaign of Cuauhtémoc Cárdenas, the PMS joined Cárdenas and other dissidents from the Institutional Revolutionary Party to form the Party of the Democratic Revolution (PRD). Pluralism is, in the general sense, the affirmation and acceptance of diversity. ... Eurocommunism was an attempt in the 1970s by various European communist parties to widen their appeal by embracing public sector middle-class workers, new social movements such as feminism and gay liberation, rejecting support of the Soviet Union, and expressing more clearly their fidelity to democratic institutions. ... Cuauhtémoc Cárdenas Solórzano (born May 1, 1934) is a prominent Mexican politician. ... The Institutional Revolutionary Party (Spanish: Partido Revolucionario Institucional or PRI) is a Mexican political party that wielded hegemonic power in the country – under a succession of names – for more than 70 years. ... The Party of the Democratic Revolution (in Spanish: Partido de la Revolución Democrática, PRD) is one of the three main political parties in Mexico. ...

Further reading

• Barry Carr, "Mexican Communism 1968-1981: Eurocommunism in the Americas?" Journal of Latin American Studies, Vol. 17, No. 1 (May 1985), 201-228.
• Dan La Botz, "Mexico’s Labor Movement in Transition," Monthly Review, Vol. 52, No. 2 (June 2005).