Boris Johnson is one of a kind: a Conservative who has turned upper-class English eccentricity into a political asset in a country where being posh is an electoral liability.

Instantly recognisable thanks to his messy mop of platinum blond hair, the mayor of London has perfected a personal brand based on a comic talent and a likeable, shambolic style that conceal a hard core of self-belief and ambition.

The power of his brand is such that his name has entered the national vocabulary. The self-service bicycles for hire that he launched in London are now firmly established as Boris bikes, while his support for building a new airport in the Thames estuary led to the idea immediately being dubbed Boris Island.

As he campaigned for re-election in affluent Putney in west London, working-class Tower Hamlets in the east or suburban Harrow in the northwest, his celebrity pulling power was obvious as voters crowded around to have their picture taken with him.

Many people like him, although his image as the archetypal toff, or man of privilege, doesn't charm everyone. One passing motorist shouted an expletive at him through his car window, to which the mayor said thank you with a regal wave.

In 2008 he was elected mayor with the biggest personal mandate in British history. If he wins a second term on May 3, the London Olympics this summer will provide a global stage for this natural showman, whose humour has helped him to shrug off blunders and scandals that would have ended many careers.

With his comic credentials well-established, Johnson, 47, is now seeking to project himself as a serious figure. On the campaign trail, he is toning down the jokey persona.

The truth is the mayoralty is an incredibly serious job, and people don't feel that you're taking them seriously unless to some extent you take yourself seriously, he said in an interview after a walkabout among voters in Putney.

It's res ipsa loquitur (the thing speaks for itself), he added in a trademark burst of Latin, an incongruous touch in an interview conducted next to some rubbish bins down a side street as a car waited to take him to the next campaign stop.

ELVIS ON MARS

Alexander Boris de Pfeffel Johnson, to give his full name, was a contemporary of Prime Minister David Cameron at Eton, one of Britain's most exclusive private schools, and at Oxford, where they both joined the Bullingdon Club, notorious for its members donning tail coats, getting drunk and trashing restaurants. Johnson and Cameron are very distantly related via remote royal ancestor King George II.

The connection, and supposed rivalry, between the two has been the focus of years of scrutiny in political circles, where there is perennial speculation that Johnson would like to succeed Cameron in Number 10 Downing Street.

I don't think he has a clear plan to reach Downing Street but I think it's a little bit more than a dream in his case, said Matthew Parris, a prominent columnist who worked for The Spectator magazine during Johnson's years as editor.

He thinks it's an outside possibility and he keeps an eye on his chances, said Parris, who like Johnson is a former Conservative member of parliament.

Cameron's attempts to broaden his appeal by playing down his privileged background usually backfire, but Johnson embraces his image as a toff and makes it funny. The irony is that Johnson is not part of the traditional establishment embodied by Cameron.

Johnson's complex family tree spreads to Turkey, England, France, Switzerland and Germany, with Muslim, Christian and Jewish branches and an unusual cast of ancestors ranging from a minister in the dying days of the Ottoman Empire to a German prince and a leading English suffragette, Millicent Fawcett.

The current crop of Johnsons also treads the path of fame. Boris's sister Rachel, a high-profile journalist, is now editor of the magazine The Lady, while his brother Jo, who used to edit the influential Lex column in the Financial Times, is a Conservative member of parliament.

According to an unauthorised biography by Sonia Purnell, sibling rivalry played a big part in Boris Johnson's rise to fame as a journalist, politician and celebrity star of the BBC's hugely popular satirical quiz show Have I Got News For You. But will it propel him even higher?

Johnson once said he was as likely to become prime minister as to find Elvis on Mars, but with the mayoral election looming, he will not be drawn on the subject. Like any politician in a campaign, he says he is fully focused on the job at hand.

BEWARE THE BUMBLING BUFFOON

For a man of legendary eloquence, he is surprisingly vague when asked what makes him passionate about being mayor. He gives a rambling response lasting several minutes and the first policy he mentions is his plan to introduce driverless trains in the London Underground - hardly a rousing battle cry.

Asked what special skills he brings to the job, he says: I'm a bit of a quiet bully. Yeah. I mean no, in a good way. I'm a very good-natured sort of bully. I think that enables me to get things done.

The response exemplifies one of Johnson's pet tactics, which is to hide his drive and intelligence under a veneer of vagueness, leading many people to underestimate him.

He comes across as a bumbling buffoon but it's a studied act. Underneath he knows exactly what he's up to and where he's going, said Steve Bell, an influential political cartoonist at the left-leaning Guardian, who has observed Johnson for years.

Bell said Johnson has a habit of deliberately ruffling his hair to create his signature dishevelled look.

Johnson's affability conceals a temper, which came out on April 3 when he shouted at his main rival in the race for mayor, Labour's Ken Livingstone, calling him a liar in a nose-to-nose expletive-laden tirade.

Livingstone, who is under fire over tax avoidance, had said on live radio that Johnson was in the same situation with his own tax. As soon as they were off air, Johnson let rip.

The episode is unlikely to damage a man whose strong personality is his principal selling point.

He has the enormous luck that some, if not all, of his mistakes are taken as evidence of authenticity, said Tony Travers, politics expert at the London School of Economics.

Johnson's political views, while colourfully expressed, are less distinctive than his personality. Travers labelled him a mainstream modern Conservative.

In office, Johnson has certainly been more of a pragmatist than an ideologue. He claims successes in cutting spending, reducing crime, investing in infrastructure and scrapping unpopular bendy buses in favour of a safer, greener hop-on hop-off model of bus.

His critics dispute the crime figures and blame him for steep increases in public transport fares which he says are necessary to fund crucial investment. Opponents also say he has lavished taxpayer money on vanity projects such as the Boris bikes, the new bus and a planned cable-car over the Thames.

The mayoralty, which comes with a 15 billion pounds ($24 billion) annual budget and responsibility for transport and policing in one of the world's busiest cities, has been a challenge for Johnson, who is not always known for his application or his grasp of detail.

Challenged by the independent UK Statistics Authority over questionable claims, he called the authority's boss a Labour stooge. The man in question, a non-partisan civil servant, once worked for Conservative Prime Minister Margaret Thatcher.

OPPORTUNITIES FOR FRESH DISASTERS

By far the most serious setback of Johnson's four years as mayor was the outbreak of rioting in London in August 2011.

He was on holiday in Canada with his family at the time and initially failed to realise the seriousness of what was happening. It took him several days to return home and when he finally made a public appearance, talking to victims of the riots in Clapham, south London, he was heckled and booed.

This drew unfavourable comparisons with Livingstone's response, when he was mayor, to the July 2005 London bombings. He immediately flew home and adopted a statesmanlike tone.

Johnson limited the damage in Clapham when he grabbed a broom as if to lead the post-riot clean-up. The image of him brandishing the broom soon eclipsed unfavourable coverage.

The episode encapsulated a style of crisis management that helped Johnson get away with a string of extra-marital affairs in the past that earned him the tabloid title Bonking Boris.

My friends, as I have discovered myself, there are no disasters, only opportunities. And indeed, opportunities for fresh disasters, he wrote in a newspaper column in 2004 after lying about one affair cost him a senior Conservative Party job.

That particular debacle came days after Johnson was forced to travel to Liverpool to apologise for publishing an article that made light of the city's grief over the death of a hostage in Iraq, causing deep offence. He has since joked about it, referring to Manchester during a party conference there as one of the great British cities I have yet to insult.

Such self-deprecating comedy goes down well in Britain, and Johnson is wildly popular with grassroots Conservatives. But Tim Montgomerie, editor of influential website ConservativeHome, says it is far too early for Johnson to lead the party.

It would be unlikely he would go from being mayor to being prime minister. He would go from being mayor to member of parliament to cabinet minister to prime minister, so there would be a process through which people would evaluate him, he said.

If Johnson wins a second term on May 3, he may miss out on a chance to run for a seat in parliament at the next general election in 2015, so Montgomerie's suggested career plan would not kick off until a long time in the future.

Parris says there is an outside chance Johnson could get to Downing Street one day. The question is what he would do there.

I don't know what changes he would like to make to Britain, what sort of a country he would like Britain to be, and unless he knows that and he could explain it to people you might get the impression of a very capable performer without any strong political ideas. ($1 = 0.6317 British pounds)

(Editing by Giles Elgood)