Paul Vallely: Across the city, fans feel a shift in the balance of power

"Where were you when you were shit," Man Utd fans chanted the last time Man City played at Old Trafford.

The grammar was ambiguous but the accusation was clear enough: the ranks of the City supporters have been augmented by fair-weather fans since the gushings of Arab oil wealth allowed Manchester's second team to buy a raft of players of the calibre to mount a serious challenge to their cross-town rivals.

The chant was, for all its chippiness, a tacit acknowledgement that something significant has shifted in the psycho-geography of Mancunian culture in recent times. City, once regarded by United supporters as a comic irritant, have become a potent threat to the primacy of Old Trafford.

Today's derby is widely regarded in the city as the most important for some considerable time – or "ever" to succumb to the hyperbole of the local fanzine chatrooms. Victory for City could secure their grip on a place in Europe's top-flight next season; for United a win would keep their slipping Premier League hopes alive. But there is more to it than that.

Nor is this merely a case of the added needle brought to the fixture by the defection of that whirlwind of a striker, Carols Tevez, and the provocative "Welcome to Manchester" poster which City erected to mark the Argentine's arrival from United. A firm of bookies this week have tried to rekindle the outrage/glee that provoked with a new double-headed knocking-copy poster, in the teeth of attempts by the police to lower the temperature with a 2-mile alcohol exclusion zone around the ground and pre-match warnings to Tevez and his hot-tempered opponent Gary Neville after their previous on-pitch spats.

No, what has seized fans on both sides is a sense that the old balance is shifting. The patronage of City's new Abu Dhabi owners – said to own some nine per cent of the world's remaining oil reserves – is finally bearing fruit with the club clearly determined to spend whatever it takes to turn City into serious title challengers next season.

By contrast, United, who can look pretty ordinary when Wayne Rooney is injured, have four key players aged 35 or over, a mountain of debt inhibiting big spending to replace them, Ronaldo and Tevez, and a manager whose retirement looms. City are in the ascendancy and United feel on the cusp before decline, in a sport where momentum counts a lot.

Old stereotypes are being inverted. City, whose boast has always been that they were the real Mancs by contrast with a United fan-base which stretches from Highgate to Hong Kong, are now looking like a bunch of cosmopolitan galacticos with a management bent on challenging the international marketing machine which has been built round Old Trafford. By contrast United, ironically, have a larger number of home-grown players. And while Tevez is the finest of mercenaries, Rooney exudes the dedication of a fan in his commitment and naive enthusiasm. For many the world in Manchester feels on the brink of turning upside down.