This is the day Tony Blair has probably been dreading ever since he entered Downing Street.
Campbell "became the story"
His right hand man, the other half of his brain, Alastair "the king of spin" Campbell, is quitting - and pretty soon.
Of course, the prime minister has known for many months, at least, of his communication chief's intentions.
But facing the reality of losing his most trusted friend, ally and protector will still give him a few sleepless nights.
There are already too few such allies left in Downing Street.
It has also been an open secret among political journalists that Mr Campbell and his partner Fiona Millar, Cherie Blair's aide, were planning their exits.
The "family life" reason is not anywhere near the full story, but it is certainly a part of it.
The couple have young children and have been increasingly aware that their working lives was demanding too much of the time they would otherwise want to spend with them.
So it has always been a matter of when rather than if. And that was Mr Campbell's quandary.
It is a quandary that has been made even more difficult by the circumstances surrounding the Iraq war and the subsequent row.
He wanted to go with his reputation intact and when it would do no damage to his boss.
There was a danger he would either leave while there was still a major question mark over his behaviour over the Iraq affair or when it could be described as the rats leaving the sinking New Labour ship.
And, inevitably, the choice of timing raises a number of questions about his actions.
Question of timing
The surprise announcement - and even though his intentions were well known, the timing still came as a surprise - followed his appearance before the Hutton inquiry during which he robustly denied any attempt to "sex up" the Iraq dossier.
It also came just the day after the prime minister gave an equally robust defence of his position before the inquiry and on the day Lord Hutton published a raft of new evidence to his inquiry.
The only thing that is absolutely certain is that Alastair had broken his own golden rule of spin doctoring - he had become the story.
The reaction to his announcement proves that beyond any doubt.
It received the sort of coverage normally reserved for senior ministers, even prime ministers.
Attempts to suggest that Downing Street had abandoned spin were never likely to be believed so long as Mr Campbell was there.
And the furore which has accompanied the David Kelly affair has reignited all the concerns over spin.
Mr Campbell has been at the very heart of that row and the way it has been handled in Downing Street.
It has been rumoured that the prime minister attempted to calm his friend down as the row appeared to be spinning out of control.
Tony Blair will probably want to use this opportunity to be seen to try to finally put the days of spin behind him, if it is not already too late.
New that former Labour aide David Hill will become Mr Campbell's successor are a major step down that path. Mr Hill is a well-respected and liked character who is universally viewed as straightforward and "unspun".
Political journalists like and trust him - although whether that is a recommendation or not is an open question.
Like Mr Campbell, his partner, Hillary Coffman, also works in Downing Street as a trusted, hugely professional and experienced aide.
Then there is the question of what Mr Campbell does next.
There will be no shortage of lucrative job offers. But he has insisted he does not want another big job but to "write, broadcast and make speeches."
So, what about the book based on his Downing Street diary? That would command a huge sum and become an instant bestseller.
But without revealing the sort of detail Tony Blair and others will never want to see the light of day, it would lose much of its selling point.
Leave it until after the Blair premiership has passed into history and no one will want to know. This could be Ali's new quandary.