A joke is doing the rounds in the French Socialist party about Bertrand Delanoë, the mayor of Paris and new star of the left. One day, he tells Ken Livingstone, his former counterpart in London, about all the bus lanes he has been building in Paris. Livingstone replies: “Well, that will be useful for your Olympic bid in 2060.”

Delanoë insisted last week that he had recovered from his disappointment at losing the 2012 Olympics to London. At the time, he accused the British of bending the rules but now, perhaps through clenched teeth, he says, “I am serene,” and gallantly wishes London a “marvellous success”.

These days, he has his own game to focus on. Delanoë, 57, has emerged as a leftist hero, boosted by hugely popular innovations in Paris, a hatred of Ségolène Royal, the failed Socialist presidential candidate, and the growing impression that President Nicolas Sarkozy could be vulnerable to a credible challenger from the left.

“I am one of many,” he said with a modesty that he can easily afford. Yet a growing legion of friends and allies is looking to him to rescue the party from Royal in time for the next presidential contest in 2012. He is expected to stand against her in the race for the position of party secretary-general in October.

“I’ve never been egotistical,” he said in an interview on a train carrying him to London. “I’m attached to a management of the Socialist family that is collective and has a team spirit. The head of the team is the last question.”

Nevertheless, his opinion of Royal is well documented. Last year he accused her failed campaign of being a triumph of marketing over “substance and values” and described his party’s recent history as a soap opera, a reference to Royal’s separation from François Hollande, the current Socialist secretary-general who had been her lover for 25 years.

Some claimed the couple’s personal problems were at the root of the party’s pitiful performance at the polls. Whatever the case, Delanoë, the first senior French politician to have declared his homosexuality, seems determined to put the Socialists back on track. “I hope it will be the left that incarnates our future,” he said, adding that he was “more disapproving than ever of the politics of Sarkozy”.

Sarkozy’s slide in the polls has put a spring in Delanoë’s step. So has his recent re-election as mayor with a significant margin. He was buoyed by the huge success of a self-service bicycle system called Vélib (from the French word vélo for bike), which is being copied by London. It will be followed by Autolib, a similar service for cars.

The Paris Plage, a beach built on the banks of the Seine in summer that attracts millions of visitors, is another much-acclaimed project. This month he will inaugurate the first branch of a “river Métro”.

Delanoë is also credited with making the city greener and likes to set a personal example when it comes to combating pollution and waste: he drives an electric car and has auctioned off wine from the town hall cellar that was too valuable to serve at any meal.

All this has gone down extremely well in cosmopolitan Paris. There are doubts, nevertheless, about how his homosexuality might play among voters beyond the “City of Light”. He does not encourage questions about his private life but is said by aides to complain of lingering antihomosexual feelings among the French.

The French media prefer to avoid this issue, dedicating increasing space to Delanoë’s rising prospects without assessing how his sexuality might alienate a conservative rural electorate.

Delanoë travelled to London last Saturday in a show of support for Livingstone’s re-election campaign and was greeted like a long-lost relative by his counterpart as he stepped off the train at St Pancras.

Livingstone seemed in no doubt about Delanoë’s ability to capture the hearts and minds of the French: he had invited him over to boost his appeal to London’s substantial French vote and introduced him to passers-by as “the next president of France”, to which the chain-smoking Delanoë responded: “Keep cool, Ken, keep cool.”

Delanoë had a close brush with death in 2002 when an unemployed man of Algerian descent stabbed him in the stomach, telling police: “I don’t like politicians and I especially don’t like homosexuals.”

“It’s given me the sense of having a little more knowledge in relationship to life,” he said. These days a bodyguard follows him everywhere and looked nervous in Islington when political activists gathered around Delanoë and Livingstone brandishing placards in support of the Tory candidate, Boris Johnson.

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