The Basque Parliament was the devolved, unicameral legislature of the autonomous community of the Basque Country, having legislative power in regional matters as defined by the Spanish Constitution of 1978 and the regional Statute of Autonomy, as well as the ability to vote confidence in or withdraw it from a lehendakari. Voting for the Parliament was on the basis of universal suffrage, which comprised all nationals over eighteen, registered in the Basque Country and in full enjoyment of their political rights. The 75 members of the Basque Parliament were elected using the D'Hondt method and a closed list proportional representation, with a threshold of three percent of valid votes—which included blank ballots—being applied in each constituency. Parties not reaching the threshold were not taken into consideration for seat distribution. Seats were allocated to constituencies, corresponding to the provinces of Álava, Biscay and Guipúzcoa, being allocated a fixed number of 25 seats each to provide for an equal representation of the three provinces in parliament as required under the regional statute of autonomy. This meant that Álava was allocated the same number of seats as Biscay and Gipuzkoa, despite their populations being, as of 1 January 2005: 298,830, 1,132,163 and 684,269, respectively. The use of the D'Hondt method might result in a higher effective threshold, depending on the district magnitude.
Election date
The term of the Basque Parliament expired four years after the date of its previous election, unless it was dissolved earlier. The election decree was required to be issued no later than the twenty-fifth day prior to the date of expiry of parliament and published on the following day in the Official Gazette of the Basque Country, with election day taking place on the fifty-fourth day from publication. The previous election was held on 13 May 2001, which meant that the legislature's term would have expired on 13 May 2005. The election decree was required to be published in the BOPV no later than 19 April 2005, with the election taking place on the fifty-fourth day from publication, setting the latest possible election date for the Parliament on Sunday, 12 June 2005. The lehendakari had the prerogative to dissolve the Basque Parliament at any given time and call a snap election, provided that no motion of no confidence was in process. In the event of an investiture process failing to elect a lehendakari within a sixty-day period from the Parliament re-assembly, the Parliament was to be dissolved and a fresh election called.
Parties and candidates
The electoral law allowed for parties and federations registered in the interior ministry, coalitions and groupings of electors to present lists of candidates. Parties and federations intending to form a coalition ahead of an election were required to inform the relevant Electoral Commission within ten days of the election call, whereas groupings of electors needed to secure the signature of at least one percent of the electorate in the constituencies for which they sought election, disallowing electors from signing for more than one list of candidates. Below is a list of the main parties and electoral alliances which contested the election:
Opinion polls
The table below lists voting intention estimates in reverse chronological order, showing the most recent first and using the dates when the survey fieldwork was done, as opposed to the date of publication. Where the fieldwork dates are unknown, the date of publication is given instead. The highest percentage figure in each polling survey is displayed with its background shaded in the leading party's colour. If a tie ensues, this is applied to the figures with the highest percentages. The "Lead" column on the right shows the percentage-point difference between the parties with the highest percentages in a given poll. When available, seat projections are also displayed below the voting estimates in a smaller font. 38 seats were required for an absolute majority in the Basque Parliament.
