Labour's ranks. Those with a taste for schadenfreude who were happy to let Livingstone hang himself as the doomed Labour candidate against Boris, will demand an alternative. They will remind us of Ken's weaknesses - the cab bills, the Scotch in the morning, the failure to expose Johnson's frailties in the last election.

If there is a sniff of a chance of victory for Labour in the London election they will fight to block Livingstone. But they'll need an excuse, and a good one at that.

Which brings me to the hunt for "a British Obama". This has not received much publicity, but it is an issue exercising many in the Labour party. They argue that Britain's minorities are not adequately represented in Westminster and that Parliamentary structures make it hard for ethnic minority politicians to rise to the top.

London, they say, would be an ideal launching pad for the first prime minister of "post-racist Britain" - the equivalent of Obama's base in Chicago.

How could Livingstone, one of the earliest and most committed campaigners for celebrating the capital's multicultural heritage, resist the call to step aside if the right candidate could be found for the May 2012 London election?

So, who might this "British Obama" be? Three London MPs fit the bill - one of whom, in my view, is way out in front...

• David Lammy, elected Labour MP for Tottenham in 2000 at the age of 27. He is reputed to be close to Obama, their friendship predating the Illinois senator's run for the presidency. However, there is little evidence that Obama's charisma has rubbed off on his friend. He is currently Minister for Higher Education and Intellectual Property, a job that gives him little chance to shine in public. To put it bluntly, Lammy is dull.

• Dawn Butler, Labour MP for Brent South since 2005. Currently a whip, pretty low in the government pecking order, and a casualty of the expenses scandal. Will her claims for a jacuzzi and for £18,000 worth of food bills be forgotten by then? Probably not.

• Sadiq Khan, Labour MP for Tooting since 2005. A true Londoner, born in his local hospital and educated at schools in his constituency. Before he entered Parliament he was a leading human rights lawyer and campaigner on Muslim issues. He takes pride in the fact that the son of a London bus driver is now Transport Minister. Because his boss is Lord Adonis, who can only appear in the Lords, Khan is the department's top man in the Commons. Transport is a high-profile issue in London and if he can make the most of his Commons appearances in the final nine months of Brown's government, he has the chance to make a real name for himself. And he is squeaky clean on the expenses front.

Move along the bus, Mr Livingstone.