How Boris ditched the jokes and became a serious threat to Ken

The Tory candidate has refused to play the fool - and the strategy appears to be paying off, says Andrew Grice

Boris Johnson shoots hoops during a visit to Stockwell Park High School (Getty Images)

"It is discipline," barked Boris Johnson. "You have got to concentrate very, very hard." It sounded as though he was describing the self-imposed straitjacket that has prevented him making any catastrophic gaffes during his campaign to become Mayor of London.

In fact, the Tory MP and journalist was admiring the discipline of a dance class at Stockwell Park School but he could have easily have been speaking about himself. When he became the Tories' standard-bearer, many in his party feared he would self-destruct.

As he toured the school, the press pack pursued him with the same question: whatever happened to Joker Boris, the witty television game show host? He has cracked the best jokes at the mayoral debates, but Serious Boris has been very much in the ascendancy.

"There is absolutely no distinction, theological or otherwise, between the Old Boris and the New Boris," Mr Johnson insisted. "They are of the same nature, the same substance." He then said it again in Latin, as if to prove it. "I will continue to speak up, in the way I always have done. This is a very serious job."

He admitted he had "cut down greatly" on alcohol during the campaign – to "lose weight" rather than to prevent Labour's gaffometer springing into life.

I asked him how Londoners could be sure they would get Serious Boris if he moves into City Hall on Friday. "Because I am 100 per cent determined to do the best possible job for London," he replied. "With every day that has passed I have been more and more excited by the potential of the job."

Mr Johnson had some rare words of praise for Ken Livingstone. "Some of the things he has done have been good. I like the way he champions London's diversity. I have come to understand in the course of the campaign that you have got to speak up for people who have faced discrimination and prejudice. You cannot be neutral or have a laissez-faire approach."

Another example of his discipline is that, while the man everyone knows as Ken calls him Boris, he will not reciprocate He always refers to his main opponent as "the Labour Mayor," trying to cash in on the Government's unpopularity.

Yet he insisted he would not be relying on the 14-point Tory lead in this week's ComRes poll for The Independent to carry him to victory. The same goes for the abolition of the 10p lower rate of income tax. "I think Londoners are looking very much at the issues that concern them in London, particularly on what I have had to say about crime and disorder."

His campaign guru Lynton Crosby, a hard-nosed Australian political strategist who also ran Michael Howard's campaign at the 2005 election, has no doubt that crime is by far the "number one issue" on the doorsteps of the capital.

To Labour's frustration, the media has focused more on Mr Johnson's personality than his grasp of policy detail. For the most part, he has kept the show on the road. But he did crashover his priority plan to replace "bendy buses" and bring back the Routemaster. He variously put the cost at between £8m and £100m.

Then Mr Johnson had to swiftly remove from his website a statement by Brian Cooke, chairman of the TravelWatch watchdog. Although he endorsed the Tory candidate, he dismissed Mr Livingstone's plans to give pensioners free 24-hour Tube travel as a "mad idea," putting Mr Johnson's strong support among over-55s at risk.

Yesterday, Mr Johnson announced triumphantly that Kate Hoey, the Labour MP for Vauxhall, would be the first member of his administration as his sports adviser. After an intervention by furious Labour whips, she issued a statement insisting she would be a non-partisan adviser and was backing Labour in tomorrow's election. However, her readiness to work for Mr Johnson added to the impression he is the man with momentum.

The man and his manifesto

Age: 43

Experience: Former editor of The Spectator; MP for Henley since 2001

USP: Has He Got News For You

Key policies: Would head anti-crime drive by chairing Metropolitan Police Authority. Would scrap proposed £25 congestion charge for gas-guzzling vehicles and consult local people on the future of the western extension to the charge zone. Free bike hire scheme. Would scrap Ken Livingstone's target to make 50 per cent of new homes affordable but build 50,000 more affordable homes by 2011. Would cut wasteful spending at City Hall.

Campaign: Australian strategist Lynton Crosby has made crime the number one issue

Strengths: He has populist appeal and self-deprecating humour

Weaknesses: There are doubts that he could maintain the self-discipline of his campaign during a four-year term as Mayor