[a] Voting was on the basis of universal suffrage, which comprised all nationals and resident non-national European citizens over 18 years of age and in full enjoyment of their political rights.

In order to be entitled to run, parties, federations, coalitions and groupings of electors needed to secure the signature of at least 15,000 registered electors; this requirement could be lifted and replaced through the signature of at least 50 elected officials—deputies, senators, MEPs or members from the legislative assemblies of autonomous communities or from local city councils.

[11][12] However, the Spanish Supreme Court annulled HZ lists and banned them from running on 22 May 2004, as it considered that the candidacy's promoters and half of its candidates had links with the outlawed Batasuna and with the ETA environment.

The table below lists opinion polling on the victory preferences for each party in the event of a European Parliament election taking place.

The table below lists opinion polling on the perceived likelihood of victory for each party in the event of a European Parliament election taking place.