They call it "the red effect". It has spread down Spanish streets on the torsos of hundreds of thousands of fans wearing the shirt of the national soccer team, La Roja or "The Red", and threatens to over-run even the most obdurately separatist corners of the country.
On nights the team has notched up World Cup victories it has turned into a musical chant: "I am Spanish! Spanish! Spanish!" they shout joyfully.
Spaniards cannot recall an outpouring of national pride similar to that provoked by the country's first appearance in the World Cup final today.
"Not since the Spanish civil war have there been so many flags in the streets," El Pais newspaper reported as Madrid prepared for an all-night party if La Roja beat Holland in South Africa.
Indeed, Spain's red and gold flag still reminds some people of the civil war of the 1930s, more particularly, of the 36-year dictatorship of Francisco Franco, leader of the pro-fascist Nationalists, that followed it.
Few countries in Europe, except Germany, have such an instinctive mistrust of patriotism. Such an outpouring of national pride also raises challenging questions about Spain's vision of itself. This is a "nation of nations" say some, who see Catalonia and the Basque country as unrecognised nations.
Spain oppresses other nations, say separatists, including the Basque terror group Eta - which exacts its revenge in blood. The country's constitutional court disagrees.
"Our constitution recognises no nation but Spain," it affirmed on Saturday in a stern rebuke to Catalans who hoped a new autonomy statute might formally allow them to be known as a nation within Spain.
A million Catalans yesterday marched through Barcelona's streets denouncing the court's decision to strike out parts of the statute.
The march was led by the socialist head of the regional government, Jose Montilla, and his two predecessors. A huge flag bearing the red and yellow stripes of Catalonia preceded them.
But the march could not have been worse timed, says Josep-Lluis Carod-Rovira, deputy leader of the Catalan regional government and a leader of the separatist Catalan Republican Left party. "This is ridiculous," he complained. "We will end up with more Spanish flags being waved for the Spain-Holland match than Catalan flags."
Barcelona did not experience the same wild celebrations that provoked gridlock in parts of Madrid after the semifinal win against Germany last week, but Carod-Rovira is right that growing support for La Roja overshadows attempts to assert Catalonia's "different" identity.
Viewing figures showed three-quarters of Catalan television sets were tuned into one of the channels showing the Germany game. As horn-tooting cars and motorbikes flying Spanish flags drove around the city afterwards, flag-waving, chanting fans gathered on Barcelona's Ramblas boulevard in a previously unseen, and unimaginable, celebration of Spanishness.
The red and gold Spanish flag that is so viscerally disliked by local nationalists has even begun to appear on Barcelona balconies. Something, undoubtedly, is changing.
"You will now find people out on the streets in La Roja shirts, or with the Spanish flags that are normally considered taboo here," said Marcelino Sanchez, who was among those celebrating in Las Ramblas.
El Mundo newspaper, an obsessive critic of Catalan nationalism, talked of people "coming out of the wardrobe" to admit their love of La Roja. Some analysts say clever marketing, with the team referred to by the colour of its shirt rather than the name or flag of Spain, has eased acceptance.
Others simply say everyone wants a piece of the team's success.
City hall authorities in Barcelona bowed to pressure for a huge outdoor screen to be set up so that fans in the city could watch the final.
Catalan United Left leader Joan Puigcercos and Basque Nationalist Party president Inigo Urkullu were among those refusing to support La Roja. "These people insist on taking the joy of La Roja followers as an affront," said columnist Joan Barril in Barcelona's El Periodico.
There are other reasons why Catalans now follow La Roja. Five of the players who lined up against Germany were born in Catalonia (and one, Xabi Alonso, was Basque).
Seven players belong to FC Barcelona, the club some Catalans treat as a national team.
One of Spain's 17 autonomous regions. Capital is Barcelona. Population is 7.5 million
Spain's courts recently granted sweeping new powers of self-rule to the region. The highest court in the land has ruled that Spain is the country's only nation.
Catalans have their own language and are proud of a history which, until 1714, linked them to the independent Kingdom of Aragon. During the dictatorship of General Francisco Franco (1939-1975) Catalans were forbidden to speak their language and it was illegal to publish books in Catalan.
- OBSERVER, APBy Giles Tremlett