The two Mancunian rivals have opened up a gap at the top despite their weaknesses, writes Richard Jolly.
Gloves are off, in the red and blue corners of Manchester
It is a tale of one city.
The Manchester of 2012 has distinct similarities with Liverpool in the mid-1980s, with the Milan, Moscow, Lisbon, Athens and Istanbul of the past.
It is a place where a country's league title is being contested by neighbours. It is a local affair with national consequences.
That scarcely seemed likely when Chelsea began November on top of the Premier League. They start December tied on points with the overachievers from West Bromwich Albion.
"It is a two-horse race already," said the former United captain Gary Neville, citing the six-point gap between the European and the English champions, who are hot on the collars of the leaders.
"We're still ahead in the league but City are on our tails and it's going to be a long season," said Sir Alex Ferguson.
Indeed, one long season has given way to another. Last year only goal difference separated the Mancunian rivals, twinned on 89 points but divided by the final-day emotions of delight and despair.
This year promised to be different, especially after Chelsea's £80 million investment in the summer. It means Roman Abramovich's employees cannot admit defeat. "Why can't we [win it]?" asked interim manager Rafa Benitez. "We have to keep going."
And, wary of their quality while ignoring the backdrop of instability at Stamford Bridge, others are reluctant to rule out the Champions League winners.
"Chelsea could be [in it]," said Roberto Mancini. Ferguson added: "I don't think you can be certain of saying it's going to be a two-horse race at this time," he said.
But history suggests it will be.
"Over the years we've always hoped that the top five or six are all capable of winning the league but eventually it ends up a two-horse race," Ferguson added.
"It's been like that for years, and last season there was big gap between ourselves and City [and the rest]."
Indeed, it was a 19-point gulf. Another is threatened now as others have fallen away while the Manchester sides, while damned by faint praise, have pulled away.
Each has an enviable element to their record. City are the sole unbeaten side in English league football. United have won an improbable 79 percent of their games.
Both have displayed the spirit of champions. City have gained 14 points from losing positions which, under any other circumstances would have garnered them rather more praise.
But United have 18, turning defeat into victory six times already. The series of comebacks both have mounted suggests next Sunday's derby could be a topsy-turvy affair. Before then, City host Everton with the chance to leapfrog their neighbours, if only for a couple of hours because United kick off later at Reading.
These are sides equipped with resident super-subs - Edin Dzeko in blue, Javier Hernandez in red - who tend to leave it late.
The accusation is that they camouflage other failings. Neither United nor City have peaked, making their early-season eminence all the more remarkable.
These are teams with winning habits, rather than squads packed with players particularly well.
Robin van Persie has proved prolific for United but Wayne Rooney has only scored in one league game.
Last season's outstanding player Antonio Valencia has been more subdued, the veterans Ryan Giggs and Paul Scholes may finally be in decline and the defence have conceded with unusual regularity.
City, too, have faced questions about their rearguard, whether their set-piece marking or Mancini's habit of altering between three and four defenders.
The imperious Vincent Kompany experienced an early-season dip in form, just as the midfield totem, Yaya Toure, has not been at his marauding best.
Their classiest players in the final third, David Silva and Sergio Aguero, have both been sidelined for a month of stop-start campaigns. They had scored 21 more goals at this stage of last season.
And yet, despite the criticisms and the concessions, despite going behind more than either anticipated, they are together in front. Again.
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