The Parliament of La Rioja was the devolved, unicameral legislature of the autonomous community of La Rioja, having legislative power in regional matters as defined by the Spanish Constitution and the Riojan Statute of Autonomy, as well as the ability to vote confidence in or withdraw it from a President of the Autonomous Community. Voting for the Parliament was on the basis of universal suffrage, which comprised all nationals over eighteen, registered in La Rioja and in full enjoyment of their political rights. Additionally, Riojans abroad were required to apply for voting before being permitted to vote, a system known as "begged" or expat vote. The 33 members of the Parliament of La Rioja were elected using the D'Hondt method and a closed list proportional representation, with a threshold of 5 percent of valid votes—which included blank ballots—being applied regionally. Parties not reaching the threshold were not taken into consideration for seat distribution. The electoral law provided that parties, federations, coalitions and groupings of electors were allowed to present lists of candidates. However, groupings of electors were required to secure the signature of at least 1 percent of the electors registered in La Rioja. Electors were barred from signing for more than one list of candidates. Concurrently, parties and federations intending to enter in coalition to take part jointly at an election were required to inform the relevant Electoral Commission within ten days of the election being called.
Election date
The term of the Parliament of La Rioja expired four years after the date of its previous election. Elections to the Parliament were fixed for the fourth Sunday of May every four years. The previous election was held on 22 May 2011, setting the election date for the Parliament on Sunday, 24 May 2015. The President of the Autonomous Community had the prerogative to dissolve the Parliament of La Rioja and call a snap election, provided that no motion of no confidence was in process, no nationwide election was due and some time requirements were met: namely, that dissolution did not occur either during the first legislative session or within the legislature's last year ahead of its scheduled expiry, nor before one year had elapsed since a previous dissolution. In the event of an investiture process failing to elect a regional President within a two-month period from the first ballot, the Parliament was to be automatically dissolved and a fresh election called. Any snap election held as a result of these circumstances would not alter the period to the next ordinary election, with elected deputies merely serving out what remained of their four-year terms.
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. 17 seats were required for an absolute majority in the Parliament of La Rioja.
