Spain's snap parliamentary election on July 23 is unlikely to produce a working majority for any one party, with complex pre- and post-ballot alliances holding the key to governing Spain.

Below are the main groups vying for power:

SPANISH SOCIALIST WORKERS' PARTY (PSOE)

* Leader: Pedro Sanchez * 2019 total votes: 6.8 million * Lower house seats: 120 (out of 350) * Ideology: Centre-left, social democracy

Founded in 1879 by printing workers and intellectuals, PSOE is Spain's oldest active party and one of two that have dominated the political landscape since Franco's rule ended with his death in 1975. It has been in government the longest since then (1982-96, 2004-11, 2018-23).

In 2019, PSOE and hard-left Unidas Podemos forged Spain's first coalition government since the return to democracy.

PEOPLE'S PARTY (PP)

* Alberto Nunez Feijoo * 5 million * 89 * Centre-right, mainstream conservatism

The PSOE's main rival, the PP was formed during the transition to democracy by former Franco regime officials - including six ex-ministers - but after a relaunch in 1989, it veered to the centre and has headed the national government twice (1996-2004 and 2011-2018).

VOX

* Santiago Abascal * 3.7 million * 52 * Far right, national populism, anti-immigration

Vox split from the PP in 2013 when anti-separatists within the latter pushed to suppress Spain's semi-federal system that grants regions a certain degree of autonomy. With a mix of social ultraconservatism and economic neoliberalism, it campaigns against feminism, immigration, LGBT rights and Islam.

Regarded as too toxic by the PP national leadership, Vox and the PP have nevertheless teamed up at municipal and regional levels, and Abascal has said he is open to a post-election alliance on a national level.

PODEMOS ("WE CAN")

* Ione Belarra * 3.1 million * 35 * Hard left, progressivism

Podemos emerged in 2014 from the 15-M protest movement in the aftermath of the 2008 financial crisis that hit Spain especially hard. Founded by a small group of academics - mostly political scientists - its goal was to overtake PSOE from the left with a platform that railed against the two-party establishment and promised greater social equality. It peaked in 2015 with 5.2 million votes but has since haemorrhaged support after multiple internal squabbles and splits.

SUMAR ("UNITE")

* Yolanda Diaz * N/A * N/A * Progressivism, environmentalism

A personal initiative of charismatic Labour Minister Yolanda Diaz, Sumar combines several left-of-PSOE parties such as the United Left coalition that includes the Communist Party of Spain - of which Diaz is a member - as well as the Valencia region's Compromis and Mas Pais ("More Country"), which split from Podemos in late 2018. It remains unclear whether Sumar and Podemos will run on the same ticket in July. They have until June 9 to reach an agreement.

PRO-INDEPENDENCE PARTIES

* Oriol Junqueras, Laura Borras, Arnaldo Otegi, CarlesRiera, Ana Ponton * 2 million (combined) * 29 (combined) * Full independence for the regions of Catalonia, BasqueCountry and Galicia

Several parties in parliament fight for their regions' secession from Spain. The left-wing ERC (Republican Left of Catalonia), centre-right Junts per Catalunya (Together for Catalonia) and anti-capitalist CUP (Popular Unity Candidacy) joined forces for the region's failed independence bid in 2017.

EH Bildu (Basque Country Unite) and BNG (Galician Nationalist Bloc) are two left-wing parties advocating independence for the Basque Country and Galicia, respectively.

Most of these parties have backed the Sanchez government in parliament.

OTHERS

* The centre-right and regionalist PNV (Basque NationalistParty), which now has five deputies whose occasional support hasbeen crucial to pass some of the government's signature bills. * The centre-right and regionalist Canarian Coalition, twolawmakers. * The centrist Regionalist Party of Cantabria, one lawmaker. * Teruel Exists, a party demanding more government attentionto the depopulated northeastern province of Teruel. Its lonelawmaker, leader Tomas Guitarte, was the instrumental vote tomake Sanchez prime minister in January 2020.

DROP-OUTS:

* The centre-right Ciudadanos party won 10 seats in 2019 buthas since imploded, with most votes being picked up by the PP.It will not run this time.

(Reporting by David Latona; Editing by Andrei Khalip and Giles Elgood)