But now it has happened, just when his behaviour was under the glaring spotlight of the Hutton inquiry, wrong-footing many political commentators.
The question is echoing around the corridors of power: Why now?
Tony Blair’s right hand man, or puppet-master as some would have it, has gradually emerged from the shadows since Labour swept to power in 1997, regularly appearing in the top two in surveys of Britain’s most powerful.
His media-savvy approach to political presentation made Labour seem almost too good to be true, and once the political honeymoon had passed, suspicions that style was winning over substance began to be felt from the party grassroots to the broader electorate.
He says he is leaving to spend more time with his young family. By coincidence, his partner Fiona Millar also plans to quit her job – as Cherie Blair’s spokeswoman.
In his parting statement, Campbell has claimed that the reason he stayed was out of loyalty to Blair, who needed his closest aide to shield him through the march to war in Iraq, a public relations disaster waiting to happen.
As in all his work for Blair, Campbell dedicated himself to the call to arms with ferocious determination, and the combination of his hubris with the sad fact of Blair’s indefensible position at George Bush’s shoulder have proved his undoing.
It turned out his new general and he got on famously. A fellow keen jogger, Bush chatted amicably with Campbell about his training for the London marathon during the notorious war summit in the Azores, when Portugal played host to a coalition of the US, UK and Spanish leaders, who attempted to justify their pre-emptive strike on Iraq in the face of world opinion.
The war in Iraq that followed swiftly on the heels of this jogging summit has hastened Campbell’s fall from grace, as throughout the build-up, the invasion, and the occupation, accusations of media manipulation and outright lying have plagued him, his boss, and his boss’s boss.
Thrust further into the spotlight by the Kelly affair, Campbell was named in a BBC report that claimed he had personally overseen the “sexing-up” of a dossier produced to justify the war in Iraq. Dr David Kelly, a top government scientist, committed suicide having been outed by the Ministry of Defence as the source for the claims.
It may take a miracle to rescue Blair from the morass of his Iraq adventure, but the two at the top must have realised that the British electorate is no longer in thrall to the miracle of spin.
Campbell, the one-time pornographer, may feel that the inquiry has proved his sexing-up days are long past. The prime minister had stood by his man, giving a bravura performance in the dock just the day before Campbell’s announcement, standing by the truth of the intentions behind the dodgy dossier on Saddam’s weapons, if not the letter. What is more, intelligence chiefs also appeared to back up Campbell’s story when they testified before Hutton.
Clearly both Blair and Campbell believe that there is not enough evidence to prove conclusively that they lied about - rather than misjudged - Iraq’s belligerence towards distant countries.
Perhaps Campbell thinks that his name has been cleared and that he can begin a blissful early retirement, but perhaps it is his all-consuming loyalty to his master that has again proved the motivating force in his choices. He may have sacrificed himself to help save his boss's reputation.
It might take a miracle to rescue Blair from the morass of his Iraq adventure, but the two at the top must have realised that the British electorate is no longer in thrall to the miracle of spin.
An end to spin?
Blair has come to be seen as a leader dominated by his media manager. His recovery from the very damaging Kelly affair will be conditional on his distancing himself from spin, and presenting his government in a far more accountable and transparent way.
Tony Blair’s arch-rival within the Labour government, the dour and meticulously straight talking chancellor Gordon Brown, has been careful to keep apart from the entire war effort, leaving himself untarnished for an assault on the PM’s job when he judges the time to be right.
The absence of Campbell in itself will not be enough. Blair needs a lot of luck for things to begin going his way - in Iraq, as well as on the neglected home fronts of Britain’s schools, trains, and hospitals - if he is to bounce back from the worst crisis of his premiership.
But for Blair to ward off Brown’s threat, which may come sooner than he expects, he first needs to expel the smokescreens thrown up between him and the public by Alistair Campbell and his Machiavellian ilk.