At the regional level, aside from a major breakthrough from Republican Left of Catalonia (ERC), the election saw all regional nationalist parties losing votes; the break up of Convergence and Union (CiU), support for the abertzale left EH Bildu coalition falling sharply, Canarian Coalition (CC) clinging on to a single seat and the expulsion of both Geroa Bai and the Galician Nationalist Bloc (BNG) from parliament; the latter of which had maintained an uninterrupted presence in the Congress of Deputies since 1996.

[6][7] For the Congress of Deputies, 348 seats were elected using the D'Hondt method and a closed list proportional representation, with an electoral threshold of three percent of valid votes—which included blank ballots—being applied in each constituency.

The prime minister had the prerogative to propose the monarch to dissolve both chambers at any given time—either jointly or separately—and call a snap election, provided that no motion of no confidence was in process, no state of emergency was in force and that dissolution did not occur before one year had elapsed since the previous one.

An opinion article published in Público on 8 December 2014 suggested that the probable date for the election would then be either on 25 October or on a Sunday in November 2015, not counting All Saints' Day.

[1] The Cortes Generales were officially dissolved on 27 October 2015 after the publication of the dissolution decree in the BOE, setting the election date for 20 December and scheduling for both chambers to reconvene on 13 January 2016.

[11] Mariano Rajoy won the 2011 general election in a landslide running on a platform that promised to bring a solution to the country's worsening economic situation, marked by soaring unemployment and an out-of-control public deficit.

However, shortly after taking office, Rajoy's People's Party (PP) popularity in opinion polls began to erode after its U-turn on economic policy, which included the breaching of many election pledges.

[26] In its first months in power, Rajoy's government approved a series of tax rises,[27] a harsh labour reform that allegedly cheapened dismissals[28]—and which was met with widespread protests and two general strikes in March and November 2012[29][30]—and an austere state budget for 2012.

Other measures, such as a fiscal amnesty in 2012 allowing tax evaders to regularize their situation by paying a 10% tax—later reduced to 3%—and no criminal penalty, had been previously rejected by the PP during its time in opposition.

[39][40] A new Education Law—the LOMCE—received heavy criticism from the Basque and Catalan regional governments, which dubbed it as a re-centralizer bill, as well as from social sectors which considered that it prompted segregation in primary schools.

[53] Among these were a massive expenses scandal involving former Caja Madrid senior executives and advisers—including members from the PP, PSOE and IU parties and from Spain's main trade unions, UGT and CCOO—, who were accused of using undeclared "black" credit cards for private expenditures;[54][55][56] revelations that the PP could have spent as much as €1.7 million of undeclared money on works on its national headquarters in Madrid between 2006 and 2008;[57] and the Punica case, a major scandal of public work contract kickbacks amounting at least €250 million and involving notable municipal and regional figures from both PSOE and PP, as well as a large number of politicians, councilors, officials and businessmen in the regions of Madrid, Murcia, Castile and León and Valencia.

[60][61] The Monarchy had also come under public scrutiny as a result of a corruption scandal affecting Duke of Palma Iñaki Urdangarín, the Nóos case, and his spouse Cristina de Borbón, Infanta of Spain and daughter of King Juan Carlos I, for possible crimes of tax fraud and money laundering.

[63][64][65] These corruption allegations, coupled with other scandals—such as public anger at King Juan Carlos' elephant hunting trip to Botswana at the height of the economic crisis in 2012[66]—as well as his own health problems, had severely eroded the Spanish Royal Family's popularity among Spaniards,[67] and were said to have taken its toll on the monarch, who announced his abdication on his son Felipe—to become Felipe VI of Spain—in June 2014.

Social mobilization channeled through various protest actions, such as "Surround the Congress" (Spanish for Rodea el Congreso), the so-called "Citizen Tides" (Mareas Ciudadanas) or the "Marches for Dignity" (Marchas de la Dignidad).

Other general causes of ineligibility were imposed on members of the Spanish royal family; the president and members of the Constitutional Court, the General Council of the Judiciary, the Supreme Court, the Council of State, the Court of Auditors and the Economic and Social Council; the Ombudsman; the State's Attorney General; high-ranking members—undersecretaries, secretaries-general, directors-general and chiefs of staff—of Spanish government departments, the Prime Minister's Office, government delegations, the Social Security and other government agencies; heads of diplomatic missions in foreign states or international organizations; judges and public prosecutors in active service; Armed Forces and police corps personnel in active service; members of electoral commissions; the chair of RTVE; the director of the Electoral Register Office; the governor and deputy governor of the Bank of Spain; the chairs of the Official Credit Institute and other official credit institutions; and members of the Nuclear Safety Council; as well as a number of territorial-level officers in the aforementioned government bodies and institutions being barred from running, during their tenure of office, in constituencies within the whole or part of their respective area of jurisdiction.

[81][82] Disqualification provisions for the Cortes Generales extended to any employee of a foreign state and to members of regional governments, as well as the impossibility of running simultaneously as candidate for both the Congress and Senate.

Concurrently, parties, federations or coalitions that had not obtained a mandate in either chamber of the Cortes at the preceding election were required to secure the signature of at least 0.1 percent of electors in the aforementioned constituencies.

[94] An agreement with Navarrese People's Union (UPN) was also reached, after a period of negotiations in which the regional party had considered to contest the general election on its own in Navarre.

After the negative results of the Catalunya Sí que es Pot alliance in the September Catalan election, Podemos, Initiative for Catalonia Greens (ICV) and United and Alternative Left (EUiA) reached an agreement with Barcelona en Comú—Barcelona Mayor Ada Colau's party—to form a joint list to contest the general election in Catalonia: En Comú Podem (Catalan for In Common We Can).

[105] In Navarre, all four Podemos, Geroa Bai, EH Bildu and I-E coalesced under the Cambio-Aldaketa umbrella for the Senate, aiming at disputing first place regionally to the UPN–PP alliance.

The Canary Islands used WET (UTC+0) instead):[111] A total of four debates involving the leaders of at least two of the four parties topping opinion polls (PP, PSOE, Podemos and C's) were held throughout the pre-campaign and campaign periods.

[130] A third, televised debate was organized by Atresmedia, held on 7 December and broadcast live simultaneously on its Antena 3 and laSexta TV channels and on the Onda Cero radio station.

[132] Online polls conducted immediately after the debate by major newspapers coincided in showing Iglesias winning,[133] while political pundits and journalists pointed on his strong performance.

[140] Opinion polls heading into the campaign had shown the PP firmly in first position, with both PSOE and C's tied for second place and Podemos trailing in fourth.

[154] After the Atresmedia televised debate on 7 December—in which Iglesias was said to have outperformed all other three with his final address[155]—and following a series of gaffes by C's leaders that had affected their party's campaign,[156] Podemos experienced a surge in opinion polls.

Rajoy revealed to them that, according to PP internal opinion polls, Podemos was rising quickly and approaching the PSOE, to the point that there was the possibility of it becoming the second political force of the country.

As opinion polls had predicted, the People's Party (PP) was able to secure first place with a clear lead over its rivals, but it lost the absolute majority it had held since 2011 in the Congress of Deputies.

[169] The party's net loss of seats (64 fewer than in 2011) and vote share drop (minus 16 percentage points) was the PP's largest fall in popular support in its history, as well as the worst showing for a sitting government in Spain since 1982.

[170] Losing 20 seats and nearly 7 percentage points to its already negative 2011 result, this was the first time since the Spanish transition to democracy that one of the two largest parties fell below the 100-seat mark.

The result was regarded as a loss for bipartisanship in Spain as a whole, as the era of bipartisan politics was declared officially over by newcomers Podemos and Citizens, as well as by both national and international media.