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He's the man who made the Tories electable (so how will Dave Cameron cope without Steve Hilton?)

The marketing guru credited with transforming the image of the Conservatives is moving to America. Andy McSmith reports on a political bombshell

In the background of television shots of David Cameron at Conservative Party headquarters, you might sometimes catch a rare glimpse of a scruffy figure in open-neck shirt and jeans, who vanishes like a shy nocturnal mammal the instant he realises he is in the picture.

Scanning the press for a mention of this elusive creature is hard work, too. During May, in the avalanche of publicity about Tory victories in local elections and the Crewe and Nantwich by-election, the name of Steve Hilton cropped up in national newspapers just six times.

He is, you might think from the portrait sketched out so far, either very shy, or not very important, or both. Actually, he is a very personable, sociable individual, and is thought to be drawing the highest salary ever paid to an employee of a British political party – an eye-watering £160,000 a year, which is roughly what the Prime Minister is paid.

Steve Hilton is the man who decontaminated the Conservative Party's brand. He helped his friend, David Cameron, look and sound like no other Tory leader and masterminded the transformation of the party from the backward-looking, authoritarian, Europe-obsessed nasty party that it used to be into the eco-friendly, sophisticated, benign political machine it now seems to be.

Now, however, Mr Hilton is off to Silicon Valley in California, for a very modern and "new man" reason: he is moving for the sake of his wife's career. He is married to Rachel Whetstone, herself a significant figure in recent Tory history, who is taking up a post as vice-president of global communications and public affairs for the internet company Google.

Mr Hilton will supposedly by back in six months. In the meantime, he will make "fairly frequent" visits to London and advise Mr Cameron remotely, from a computer on the far side of the globe, where the clocks are eight hours behind London's.

In reality, it is difficult to see how Mr Hilton can ever return to the pivotal role he has played for more than two years. The world of Westminster politics is so competitive and fast changing that people who leave find it hard to come back. It is also a world where gratitude is not bankable commodity. Though the Conservative Party owes Mr Hilton a great deal, it is not certain they want his help any more.

He was always an unusual Tory. He tried to be selected as an MP, but no constituency association would have him, which may have something to do with his dress sense. He has never been seen in public wearing a tie. Jeans and an open-necked shirt are his normal work clothes. He is even rumoured to have been seen padding about the office barefoot.

In the rare photographs of him, his hair is so sparse and shaved so close that he has turned near baldness into a fashion statement. He cycles to and from work, smokes heavily, loves to chat about politics and obviously enjoys life. Little wonder then that the grey-haired, mirthless, reactionary party elders in their regulation suits and ties wondered why the Tories were spending so much money employing this young puppy.

However, he gave them their money's worth. It was Mr Hilton who, virtually single-handed, devised a strategy that took Mr Cameron into political territory where no Tory leader had been before, to connect with the young, metropolitan, liberal middle-classes who had abandoned the Conservatives as a relic of times gone by.

When Mr Cameron cycled to and from home for the benefit of the cameras, or set off for Norway in April 2006 to be photographed with huskies, or appeared on "webcameron" video blogs, or changed the party rules to increase the number of Tory candidates from ethnic minorities, or made that speech in which he was misquoted as saying "hug a hoodie", he was following the strategy devised by Steve Hilton.

It was risky, because it was always possible that Mr Cameron would alienate lifelong Conservatives and be laughed at by the people he was trying reach. William Hague's attempts to seem modern, just eight years earlier, were an embarrassment that he quickly abandoned. Mr Cameron has stuck with the strategy, and it has worked.

It helped that he and Mr Hilton were personally so close as to appear joined at the hip. Their association goes back to when Mr Hilton turned up at Conservative Central Office in Smith Square, an eager volunteer fresh from Oxford University. His family had good reason to be suspicious of the left: his parents had fled Hungary in 1956 and changed their name from Hircksac.

Mr Cameron, who is just four years older than Mr Hilton, was already working in the Conservative research department. Rachel Whetstone was taken on three months after Mr Hilton landed a full-time job there, in 1990. The couple are godparents to the Camerons' oldest child, Ivan. Ms Whetstone was Michael Howard's chief of staff while he was party leader. The enemies she made in that role would have made any future political career difficult, even if she had not compounded the problem by having an affair with a Tory peer, who happened to be Mr Cameron's stepfather-in-law.

During the 1992 election campaign, Mr Hilton's task, at the age of 22, was to liaise with the party's advertising agency, Saatchi & Saatchi. He impressed Maurice Saatchi so much that the agency poached him and set him working for five years on election campaigns around the world, from Colombia to Russia. Back in Britain, he was the brains behind the unsuccessful "demon eyes" campaign in 1997, which tried to convince voters there was something sinister behind Tony Blair's smile.

Afterwards, Mr Hilton set up his own company, Good Business, and co-wrote a book with the same name, which includes a long chapter on social responsibility. In that spirit, he encouraged Nike to launch a campaign against school bullying, and Coca-Cola to teach Africans about the dangers of HIV.

He also became fascinated by New Labour, turning up to events organised by the Labour think-tank, Demos. His friends included two prominent Blairite spin-doctors, Tim Allan and Benjamin Wegg-Prosser. Mr Cameron lured Mr Hilton back into full-time politics in December 2005, as his wife left politics for better-paid employment in Google.

The Conservatives insisted yesterday that the couple's move to California carried no political implications. A party spokesman said Mr Hilton would be "backwards and forwards" between London and California for six months, after which they would return to Britain. He added: "He is totally committed to David and the project of the modern Conservative Party. Nothing changes. It is not impossible to do the job Steve is doing for the party from out there, despite the time difference."

But others will wonder why someone with politics in his blood would quit the stage just when the Conservative Party has the scent of power in its nostrils. A comparison would be if, in 1995, Alastair Campbell had announced he was leaving the country for several months, expecting to return to Tony Blair's right hand when he came back.

This is the second time in a week that a Cameron adviser has moved on. Last week, it emerged that Mr Cameron's speechwriter and fellow Old Etonian, Danny Kruger, who is credited with scripting the leader's acclaimed address to last October's party conference, has also had an amicable parting of the ways. He is putting social responsibility into practice by helping his wife, Emma, to run a charity called Only Connect, that helps rehabilitate convicts.

Another Old Etonian who is no longer around Conservative headquarters is George Bridges, former campaign chief, who went on honeymoon last July, and never returned to the party. Bridges's departure exactly coincided with the return ofLord Ashcroft, the man who bankrolled the party during the lean years, who has now an office at Conservative headquarters in Millbank. Though Lord Ashcroft does not directly intervene on policy issues, his political sympathies are with the two big names in the Shadow Cabinet, William Hague and David Davis, who are identified with a more traditional right-wing Toryism than Mr Cameron and his Notting Hill crowd, who operate out of the House of Commons.

Whatever the reasons for Mr Hilton's absence, its effect must be to weaken the modernising element in the Conservative Party. Though he is an authentic Conservative, believing in Conservative causes such as low taxation, he genuinely believes in the modern setting in which he helped Mr Cameron to place traditional Tory ideas. He organised two day-long seminars, on inequality and on localism, making Mr Cameron sit through both. He even revealed to his friend Tim Allan that he had become so disillusioned with the Conservatives in the days when they were led by Mr Hague and paid for by Lord Ashcroft, that he voted for the Green Party in 2001.

How deeply David Cameron believes in all this new-look Conservatism is another question. Some suspect that he adopted the strategy as an imaginative way to dig the party out of the hole into which it sank 15 years ago. Perhaps now that the Conservatives have their biggest opinion poll lead for a generation, he has decided he does not need to try the patience of traditional Tories any more. If that is so, Steve Hilton might just as well be living in Silicon Valley.