By William Horsley
BBC European affairs correspondent
Three champions of the EU constitution - the leaders of France, Germany and Spain - are launching a joint campaign in Barcelona to persuade Europeans to endorse it.
Spain is about to become the first EU state to hold a referendum to ratify it.
Mr Zapatero believes he can lead the battle for the constitution
Can this trio successfully sell the constitution to their own nations and gain ground in a Europe-wide political battle?
The fate of Europe's first continent-wide constitution will be put to the test in 10 referendums over the next 18 months. It could be a fiery baptism.
So all eyes will be on Spain on 20 February, when it becomes the first to face the test.
Spain's youthful new Socialist leader, Jose Luis Rodriguez Zapatero, is confident of jumping safely through the hoop.
Opinion polls show the Yes vote outnumbering the No vote by an overwhelming margin.
For Mr Zapatero, that is a badge of honour. But others who follow may stumble.
Remote and bureaucratic
If just one of the 25 EU member states fails to ratify it, the constitution cannot legally take effect.
Then enemies of the constitution would try to bury the whole scheme.
The 10-year project to build a "united Europe" would be thrown in doubt.
So, on Friday, Barcelona will be the stage for some high political theatre.
Two big guns of the Yes camp - President Jacques Chirac of France and Chancellor Gerhard Schroeder of Germany - are joining Mr Zapatero at a public rally to build momentum for Yes votes across the EU.
A decade-old project for a united Europe is about to be tested
The trio want to portray the EU as the best vehicle for fulfilling a dream of European unity and greatness.
Last month, at a ceremony to launch the giant new Airbus A380, the product of a European consortium, Mr Zapatero called it a great achievement that showed Europe was "unstoppable" as a world power in science and industry.
Yet about a third of all European voters say they still know nothing about the constitution.
Many see the EU and its institutions as remote and bureaucratic.
Mr Zapatero wants to show other pro-EU leaders how to win over hearts and minds for the EU - and for a constitutional treaty which both clarifies and extends the powers of a European level of government for 450 million people.
The Zapatero government has sent out millions of copies of the constitution to citizens' homes. At a football match in Madrid in January, 70,000 fans were given pamphlets urging them to vote.
Most Spaniards feel good about the EU because membership has symbolised Spain's graduation to a club of prosperous democracies, after the dark years of Franco's fascist dictatorship.
Yet Spain's United Left party condemns the EU constitution as a capitalists' charter.
Sharp criticism has also come from the mainstream conservative opposition.
February 2002: Convention starts work
June 2003: Draft submitted to EU Thessaloniki summit
December 2003: Brussels summit fails to agree final text
May 2004: EU enlarges to 25
June 2004: Text agreed
Leaders of the Peoples Party have denounced Mr Zapatero for giving up Spain's special voting privileges during tense EU negotiations in 2003. But the party still favours a Yes vote.
One of the toughest battles could come in France, a country at the heart of the EU integration project.
Last year, when President Chirac announced his intention to hold a referendum, about seven out of 10 French voters said they would vote Yes.
Now that has fallen to barely six out of 10, and the No campaign has gathered strength.
Why? Partly because the French are dismayed at their nation's loss of influence in the enlarged EU of 25 states.
And they are overwhelmingly hostile to the prospect of Turkey's EU membership, which is on the cards within 15 years, now that the EU has agreed to let the Turks open talks on the terms of its future membership.
A sign that the public mood is cooling was an internal vote among rank and file members of France's Socialist Party late last year.
It recorded only a modest majority in favour of the constitution.
The French left sees it as a tool of Anglo-Saxon-style capitalism. France's trade unions are showing their power again through a series of strikes in protest at the dismantling of the protected 35-hour week system and other reforms ordered by the Chirac government.
Opposition to the vote has united left-wing parties across Europe
Several right-wing groupings, including the so-called "souverainistes" as well as the populist National Front of Jean-Marie Le Pen, are campaigning against the constitution because they say it will lead to an undemocratic "super state".
The French National Assembly has recognised that the EU constitution takes some sovereign powers away from France. It amended the French constitution a few days ago in preparation for ratifying the European one.
There is another "Factor X" which could boost the No side - French voters have a track record in recent years of upsetting the government in power at every opportunity.
Could the EU constitution fall victim to this trend?
Nobody knows, but uneasy times lie ahead for many European leaders.
The No side also has the potential to cause an upset in the Netherlands, where a referendum is due this summer - as well as in Denmark, Britain, Ireland, Poland and the Czech Republic, where referendums will be held later.
In just two other states, Luxembourg and Portugal, scheduled referendums look certain to bring a clear Yes.
In Germany, ratification must take place by a vote in the Bundestag, in view of the country's post-war legal ban on national referendums.
So far three EU states - Lithuania, Hungary and Slovenia - have ratified the constitution by parliament votes.
That leaves 22 to go before the autumn 2006 deadline.
This week marks the start of many set-piece battles in which the people of Europe will make their voices heard.