Grateful for 19 years of European largesse, Spain is on Sunday expected to vote yes to the proposed EU constitution as it kicks off the popular ratification process in a dozen of the bloc's 25 member states.

Since Spain joined the European Union in 1986 along with neighbouring Portugal, around 100 billion dollars (75 billion euros) of development aid has transformed an economy which had lagged its northern neighbours by decades, to the extent that the country has proved almost a model pupil.

"Spain arrived late for many things, the European Union included. Now it is possible to be first and to set the pace," Prime Minister Jose Luis Rodriguez Zapatero told Spanish radio on Thursday, while urging a "massive" turnout at a rally in eastern Spain.

Opinion polls suggest a clear majority of those expressing an opinion will vote in favour -- but analysts suggest the turnout will struggle to hit 50 percent of the electorate.

A low turnout would be politically embarrassing for Madrid, but there is no required minimum turnout for the referendum to be valid. The vote is purely consultative, and parliament will ultimately have to ratify the decision.

Mainstream politicians, led by the Socialist Zapatero, have been drumming home the message at public meeting after public meeting that Spain has done well as part of Europe.

"We Spaniards move forward with Europe. Back a united and diverse Europe," Zapatero has told voters, while the main conservative opposition leader Mariano Rajoy has stressed that "Europe represents the future. Say yes to Europe."

Sunday's vote will be the fourth Spanish referendum since the end of the four-decade military dictatorship of General Francisco Franco, who ruled with a rod of iron after a devastating civil war until his death in 1975.

Those 36 years of military rule, which saw Spain sidelined from Europe, gave way to a December 1976 referendum on reintroducing democracy, followed by another two years later on a constitution and a third in 1986 on whether to stay in NATO, all of which produced yes votes.

Zapatero told a meeting of flag-waving party workers in the northern city of Zaragoza on Wednesday that Sunday's vote is as significant as the post-Franco referendum which ushered in a democratic constitution.

Some regionalist parties oppose the EU text, but they are a small minority overall.

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.

Roughly half the member states, including the largest, Germany, have chosen to submit the text to parliament for ratification, rather than in a referendum.

The parliaments of Slovenia, Lithuania and Hungary have already ratified the text, as has the lower house of the Italian parliament, though the Italian upper chamber still has to ratify it.

But Zapatero has been pushing hard for Spain to set an example for those who are to hold a popular vote.

Spanish voters are called upon to answer the question: "Do you approve the treaty instituting a constitution for Europe?"

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 say they do not feel informed.

Joining several nationalist firing groups in the "no" camp is conservative former prime minister Jose Maria Aznar, who believes the treaty relegates Spain to a "small" player with little influence on the EU stage.

But unions are set to vote in favour in the belief -- and hope -- that it will uphold workers' professional and social rights.

"Spanish workers are convinced that the EU is a constant source of social progress," the secretary general of the UGT union, Candido Mendez, recently opined.

Spain has made giant strides economically and culturally during its 19 years of EU membership -- its fashion designers, chefs and film producers are in global demand -- and Zapatero has urged voters to show that they back "the process of European construction to which we owe so much."