El Derby: Barcelona v Real Madrid

There are many great club rivalries around the world but there is only one El Derby

Consider this: "It's amazing how one man alone has managed to lift the morale of a whole nation." These are the words of a doctor I know in Barcelona who has only a passing interest in football yet has the measure of the impact Ronaldinho has had on the Catalan people. He is right to be amazed. If you stand back for a minute from the hallucinations we football fans share, but fans of Barcelona FC possess to an acute degree, it really is amazing that: a) a multi-national, multi-racial football team should have become the chief repository of the pride and identity of a nation rich in history, architecture and art; and b) a Brazilian who until a year and a half ago probably never even knew Catalonia existed should have infused Catalans with a confidence and sense of self-worth they had been distinctly lacking in recent years.

They have had a hangdog 21st century. Barcelona last won a Spanish title in 1999. For the past five years they have been eclipsed by the team against whose successes and failures they define themselves, the team that has come to symbolise the indignities and injustices Castille has heaped on Catalonia down the centuries. Real Madrid have been winning things, but even when they haven't their "galaxia" has shone more brightly on the global stage. This season, though, the Real stars have been dimming and Ronaldinho's genius - in his case the word is no exaggeration - has been embellished by the radiant new talent Barcelona acquired in the summer.

Thanks to the toothy Brazilian and an accompanying ensemble made up of a Frenchman (Giuly), a Portuguese (Deco), a Swede (Larssen) and a young man from Cameroon (Eto'o), the Catalan people are walking taller than at any point since the millennium began. For the first time in a dozen or so fixtures they go into "el derby" - also known as "el superclasico" - believing that their team will win, that their strike force has the firepower to avenge ancient wrongs, annihilate the old enemy.

That faith has been diluted, though, afterlast weekend. Barcelona lost for the first time in the league this season, 2-1 away to Betis, and Real Madrid beat Albacete 6-1, with Ronaldo and Zidane producing the kind of display you would expect of players who between them have won six of the past eight Fifa World Player of the Year awards.

All of which has generated higher expectations than usual for tonight's game at the Nou Camp, which is saying quite a lot of what has become the biggest domestic league encounter in the world. One may argue that Celtic-Rangers or River Plate-Boca Juniors stir just as much, or more, intensity of feeling within their own countries. But Barcelona-Real Madrid is a fixture that has acquired ever more global prominence as television spreads the big European matches to every corner of the planet. Especially because of Real's policy of hoovering up the world's most famous footballers.

What is the significance of tonight's game? First, if Real win they will be just one point behind Barcelona in the league, when a month ago the gap between the two sides seemed so immense it was almost impossible to imagine them ever catching Barcelona up. A Barcelona victory would extend their lead at the top of the table to seven points, which could give the Catalans a rush of confidence so bracing it makes them unstoppable. Because the fact is that, apart from that blip last weekend against Betis, Barcelona have been playing the best football in Spain this season. Ronaldinho's virtuoso performances, the electric pace of Eto'o and Giuly, the creativity and composure in midfield of Deco and the Catalan Xavi (brilliant against England in midweek) have all come together in a formidably compact unity, marshalled from centre back by Catalonia's very own "crazy horse", the inexhaustible Carles Puyol.

The second reason why tonight's "derby" is so special is that the outcome will go a long way towards answering the question whether Real are history or not. There is a view among Barca fans that this Real Madrid has reached its sell-by date. That Zidane, while the world's greatest player this past decade, has not got the stomach for battle that he once had. That Ronaldo, while still capable of those runs that have been terrorising defences all over the world, has become a mood player, up for it one day; resting the next. That Roberto Carlos has lost some of that explosive force that made him the world's most extraordinary left back. That Figo lacks the bite of yesteryear. That after a fairy-tale start to his Real career last season, Beckham the footballer has been swallowed up by the handsome prince.

Whether one agrees with all this or not, there is an objective fact. Half the galacticos are reaching an age where it is biologically impossible to sustain top form week in, week out. The question really is whether this season will see them produce one last hurrah, whether they still have the energy and the appetite to end the campaign with a big trophy.

There are at least a couple of reasons for the Real camp to keep the faith. The first is a quite remarkable statistic. Despite the fact that Real failed in their bid to capture Patrick Vieira from Arsenal, and despite them continuing to play with a cavalier formation (midfield "defence" is anchored by Guti and Beckham), they have an excellent defensive record in the league this season. Of the 78 teams that make up the four big European leagues (England, Italy, Germany, Spain), only two have conceded fewer goals, Chelsea and Juventus.

Another reason why it is not entirely fanciful to imagine that Real might win today is Michael Owen. The evidence so far is that when Owen does badly, the team does badly. When he plays well, so does the team. It seemed during the first six weeks of the season that the England striker had made an catastrophic error in moving to Madrid. Not only was he spending more time than he ever had in his life on the bench, but when he did he get a run he fluffed his chances. He looked like a little boy lost. But then he started scoring, and Real started winning. Real have a better record overall in the past month than Barcelona.

It is unlikely - barring serious injury to Ronaldo or Raul -- that Owen will ever become the first-choice striker. The best he can hope for, except when Real opt for a strike force of three, is to be first choice on the bench. But were he to come on in the second half tonight and win the game for Real, in enemy territory, he will secure the devotion of his team-mates and the Madrid fans forever, clinching himself a memory that - whatever the future may hold for him - he will cherish till his dying day.

Yes. That's right. Tonight's game is that big.

John Carlin is the author of 'White Angels', a book about Real Madrid, published by Bloomsbury, £16.99