Spain's prime minister and France's president have appealed to Spanish voters to back the EU constitution in an upcoming referendum on the issue.
Chirac and Zapatero support the proposed document
Prime Minister Jose Luis Rodriguez Zapatero said the constitution would give EU members a common identity while maintaining their individuality.
President Jacques Chirac called for a massive "yes" vote, to serve as an example to the rest of the EU.
The referendum on 20 February is the first such vote in the EU.
Spaniards are being asked to answer the question: "Do you approve the treaty instituting a constitution for Europe?"
Speaking at a meeting in Barcelona, Mr Chirac said the proposed constitution would allow help Europe "to assume its responsibilities".
"It is by unifying that Europeans will be strong. Without union, they will disappear," he added.
Mr Zapatero said a "yes" vote would give the EU "prosperity and solidarity" and would banish the concepts of war and dictatorship.
The meeting was attended by up to 2,000 people, mostly members of the governing Socialist Party.
Meanwhile several hundred opponents of the treaty gathered outside.
An opinion poll showed the majority of Spaniards still had no idea what the constitution is about, despite a multi-million-dollar information campaign.
But Spaniards are among the most passionate Europeans and say they will vote "yes" anyway, the BBC's Katya Adler in Madrid reports.
German Chancellor Gerhard Schroeder and Mr Chirac are hoping that a strong "yes" vote in Spain will set an example for their own citizens.
But so far, even with millions of euros spent on television and radio referendum publicity, it is proving difficult to motivate the public, our correspondent says.
One survey shows that only 55% of Spaniards will bother to cast their ballot.
If just one of the 25 EU member states fails to ratify the constitution, it cannot legally take effect. Slovenia, Lithuania and Hungary have already ratified the text by parliamentary vote.
Mr Schroeder and Italian Prime Minister Silvio Berlusconi pulled out of the summit in Barcelona because of flu.
But both sent messages of support which were read out at the meeting.