[2] For two days following the attacks, the government kept blaming the terrorist organization ETA for the bombings, even in spite of mounting evidence suggesting the involvement of Islamist groups.

The government was accused of misinformation, as an Islamist attack would have been perceived as the direct result of Spain's involvement in the Iraq War, which had been highly unpopular among the public.

The perceived abuse of the PP's absolute majority throughout the legislature, with a focus on Spain's involvement in Iraq, was said to have helped fuel a wave of discontent against the incumbent ruling party, with the government's mismanagement on the bombings serving as the final catalyst for change to happen.

[7][8] The day after the election, Zapatero announced his will to form a minority PSOE government, supported by other parties in a confidence and supply basis.

[10][11] Voting for the Cortes Generales was on the basis of universal suffrage, which comprised all nationals over 18 years of age and in full enjoyment of their political rights.

[12][13] 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.

[26] The Cortes Generales were officially dissolved on 20 January 2004 after the publication of the dissolution decree in the BOE, setting the election date for 14 March and scheduling for both chambers to reconvene on 2 April.

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.

[33][34] 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.