He may be implicated, but Coulson is too important to lose

Denis MacShane, the sharp-eyed Labour MP, was one of the few people outside the Conservative Party to spot the embattled spin doctor Andy Coulson yesterday – in an inner courtyard of the House of Commons beneath Big Ben.

"At seven o'clock this morning I saw a hunched figure with a suit bag and a mobile phone walking across Speaker's Yard," Mr MacShane told the Commons. "I thought he was on the way out, having been fired."

He was wrong. Rather than fire him, David Cameron and his supporters closed ranks around the former tabloid newspaper editor, determined not to lose his highly valued services. The Conservative leader claimed to be "relaxed" about allegations of phone tapping by the News of the World, which Mr Coulson edited until his resignation two years ago when his royal correspondent, Clive Goodman, was jailed for phone hacking.

"I believe in giving people a second chance," Mr Cameron said. "As director of communications for the Conservatives, he does an excellent job in a proper, upright way at all times."

In contrast, when another of Mr Cameron's inner circle, the MP Andrew Mackay, was caught in the expenses row two months ago, he was instantly thrown to the wolves; his parliamentary career will be over at the next election. Mr Coulson's job is safe for as long as Mr Cameron is able to protect him, and there will be an office for him in 10 Downing Street if Mr Cameron becomes Prime Minister.

Yesterday, Mr Coulson was in his office, and was said to be trying to work normally, despite the political storm raging outside. This reaction appears to break a well-known rule – perhaps cliché – of politics, ruefully stated by Gordon Brown's former spin doctor Charlie Whelan when he was forced to resign: "The job of press secretary becomes extremely difficult if the press secretary, and not the department he serves, becomes the story and the subject of excessive attention."

But Mr Coulson is more than a "press secretary". He is the man in the office next to David Cameron's, and one of the brains behind the whole Cameron operation, who has guarded his boss's reputation as fiercely as he is now being guarded.

Mr Coulson is the one who reminds Mr Cameron not to forget the staples of Tory politics – crime and tax, and good relations with the mass-circulation Conservative newspapers.

Mr Coulson is credited with a coup at the start of the expenses row. On day one of the revelations published in The Daily Telegraph, the newspaper generously praised the Tory leader for his "straightforward" expenses claims. They did not mention that Mr Cameron had taken a £350,000 mortgage on his second home, on which he could reclaim the interest, at the same time that he had paid off a £75,000 loan owing on his London home, which he could not claim. If that information had been public from the start, it would have made Mr Cameron's handling of the row very difficult. Andy Coulson is credited with persuading the Telegraph to go easy on his boss.

But that same expenses scandal means that there are people in the Conservative Party who would not mind seeing Mr Coulson humiliated. They resent the way it was used to rid the party of some of the old guard, with their duck islands, moats, and tree-lined estates, while party modernisers with questionable expenses claims were protected.

The Tory chairman of the Commons Culture and Media Select Committee, John Whittingdale, made it plain yesterday that he is not going to defend Mr Coulson if evidence emerges that he was personally implicated. Mr Whittingdale was Margaret Thatcher's political adviser 20 years ago, and his devotion to her legacy makes him less than enthusiastic about the Cameron operation.

The danger for Mr Coulson now is that revelations of phone-tapping could become so embarrassing that they outweigh his usefulness to his boss. But that has not happened yet. For the time being, it is business as usual in the office next door to Dave's.