BORIS JOHNSON—the Conservative mayor of London, darling of his party's grassroots and (it is said) likely challenger for the Tory leadership should David Cameron fall under a bus—was invited to 10 Downing Street this week. His role was to explain the merits of elected mayors to assembled municipal worthies. Mr Johnson gave them a blast of cheek, show-off cultural references and partisan appeals for his re-election when Londoners go to the polls on May 3rd. The prime minister, much of the time, gazed at the ceiling as if scanning the horizon. The cross-party audience of MPs, councillors and local bigwigs roared their delight.

Mr Johnson thanked “Dave for letting me into No 10”, while insisting that City Hall was much more interesting. Pondering why the British are a bit sceptical about mayors, he cited examples of mayoral misdeeds in the works of Hardy and Ibsen, “The Simpsons” and the film “Jaws”, in which, he noted, a town boss allowed his electors to be “eaten by a gigantic fish”. He took a swipe at his Labour rival, Ken Livingstone, whose tax-efficient financial arrangements—recently revealed—sit ill with his call for “rich bastards” who dodge taxes to lose the vote.

Above all, Mr Johnson suggested, a big part of a mayor's job is to bully central government into paying for things that voters want. By way of example, he mentioned the 1,000 extra police officers who just happen to make up point four of his nine-point “Plan for a Greater London”, plastered on billboards citywide.

Most of Mr Johnson's nine points involve spending money. Nationally, the Conservatives may be the party of flinty austerity and public-sector reform. But Mr Johnson is promising new trains and reduced delays on the London Underground, new jobs, new homes, money for small businesses and the planting of 20,000 trees. True, he boasts about cutting wasteful spending and freezing his share of London council taxes, but then assures voters that billions saved at City Hall have been freed for spending on other services. Point nine of his manifesto is essentially a vow to mug Mr Cameron—or to secure “a better deal for London from No 10”, as his posters more politely phrase it.

Mr Johnson has suggested that his London administration stands for “cost-cutting, one nation Conservatism”, ie, frugal paternalism. In fact, Mr Johnson's real beliefs, often concealed behind a smokescreen of jokes, are not so much blandly centrist as nicely balanced between swashbuckling extremes. He stands firmly at the socially liberal end of his party, a philosophy reflected in his exuberant private life. Against his party's national policies, he worries about curbs on welfare benefits and favours an amnesty for some illegal migrants. Yet at the same time he is admired on the right for backing a referendum on Britain's EU membership, and as an early voice calling for the abolition of the 50% top rate of income tax—seen by Mr Johnson as a disastrous symbol of hostility to wealth-creators.

Yet Mr Johnson's campaign has been cautious to the point of turgidity. Part of the explanation lies in London politics, which leans to the left. Those close to Mr Johnson talk of “borrowing” the capital from Labour, on the strength of their man's tousled charm, celebrity as a television panellist and newspaper columnist, and image as a party maverick. He won office in 2008 with the help of Liberal Democrat and even Labour voters fed up with Mr Livingstone (then the two-term incumbent), as well as by increasing turnout among suburban Tories who had seen the mayor as a remote metropolitan, focused on inner London.

His caution is also explained by the limited powers granted to the mayoralty on its creation in 2000 (the separate, ancient post of Lord Mayor concerns only the City of London, the central financial district). Just 7% of the mayor's revenues come from direct taxation, with transport fares supplying a further chunk. Central government provides most funds. London's mayor controls city transport, some parks, planning policy and a bit of policing, with little influence over regulation or state structures. “All City Hall does is spend money it didn't raise,” sighs a source. “London's been turned into a teenager asking parents for money.”

Wanted: a mayor with a stake in economic growth

All this confines mayoral elections to a political playpen, in which symbolic public works stand for larger battles. As mayor, Mr Livingstone, a tough lefty by instinct, imposed a congestion charge on drivers in wealthy bits of central London. In opposition, he has attacked a bicycle-hire scheme of Mr Johnson's as elitist, arguing that most users are young City of London types.

On taking office, Mr Johnson banished double-length, continental-style “bendy buses” bought by Mr Livingstone, calling them traffic-snarling and a gift to fare-dodgers. Instead, he has commissioned a new double-decker bus for the capital with an open platform for hopping on and off between stops: his blow for London heritage and for individual liberty.

This infantilisation of London politics matters. Mr Cameron's coalition government sets great store by a growing cadre of elected mayors in cities across the country. Fortified by their visibility and accountability, it is hoped that mayors can drive radical public-sector reforms through fossilised bureaucracies.

London—a cosmopolis with an economy larger than Belgium's and a population the size of Switzerland's—should be a fine test-bed for such reforms. The capital is crammed with the ambitious, the restless and those dissatisfied with the circumstances of their birth. Its mayor should be a spokesman, heard around Britain, for economic growth and openness to the world. This will not happen until City Hall raises much more of its own money, perhaps via business taxes or VAT, forcing mayors to make tough trade-offs. Playing trains and buses is not enough. London's mayoralty turns 12 this year. Time to grow up.

Economist.com/blogs/bagehot