The traditionally pro-European Spaniards - who joined what was then the European Community in 1986 - are the first to vote on the proposed constitution on Sunday after its approval by government leaders last year.
The 34.6 million eligible voters are being asked the simple question: "Do you approve the treaty instituting a constitution for Europe?"
Opinion polls predicted a large yes vote. But the abstention rate could attain new records, due, say analysts, to a widespread lack of knowledge of the contents of the proposed constitution despite a broad popularisation campaign and also, paradoxically, because of a strong pro-European consensus that reduces the suspense of the vote.
The media have bombarded voters with a steady feed of advertisements, some featuring celebrities such as former FC Barcelona coach Johan Cruyff reading passages from the proposed text.
But around three-quarters of the electorate still said before the vote they did not feel informed. Both the Socialist party of Prime Minister Jose Luis Rodriguez Zapatero and the conservative opposition Popular Party have called for a yes vote, as have the moderate regional nationalist parties ruling in the Basque country and Catalonia.
The European Union grew last
year to 25 member states
The vote is purely consultative, and parliament will ultimately have to ratify the decision. Spain thus kicks off the popular ratification process in a dozen of the EU's 25 member states.
With the EU bloc having grown last year to 25 member states, the constitution, which in theory requires the approval of each, is designed to simplify the body's decision-making process.