It has often been said - not least by Alastair Campbell himself
allegedly - that Tony Blair can't manage without his spin doctor.
Alastair Campbell: Dominant figure
Well now, probably sooner rather than later, we are going to find out. It seems pretty certain that Campbell has decided to go and that he has formally told his boss and best friend of his decision.
Downing Street has dismissed reports of the spin doctor's departure as wishful thinking, but most commentators believe the only thing that remains unclear is precisely when he will leave.
That will depend on when he believes he can quit with his reputation intact and when it will not damage the government.
But whenever it comes, it will still be one of the most pivotal events for the prime minister since he was elected leader.
When Campbell goes the prime minister will be left more alone in terms of close support than at any time since he was elected leader
And how he reacts to it will help decide his own future. For the best part of a decade, Blair and Campbell have been inseparable.
There are any number of stories about how the prime minister, when put on the spot by the media, would look around him like a hunted man and
plead: "Where's Alastair?"
Now he is facing the prospect that when he is under that sort of pressure in the future, he will have no one left to turn to.
He is going to have to fly solo. He has already lost his gatekeeper, Anji Hunter, with whom he had been friends since college days.
His closest political friend Peter Mandelson, while still a close confidante, is no longer constantly at his side.
And his chief of staff Jonathan Powell is said to be looking for employment elsewhere.
So when Campbell goes - and Cherie aside - the prime minister will be left more alone in terms of close support than at any time since he was elected leader.
However, there have been growing signs that he and Campbell had been preparing for this day - although they clearly didn't expect it to come under these circumstances or at this time.
It has long been believed that Alastair was ready to leave Downing Street.
And it appears likely the two men had discussed it even before their crunch meeting on Thursday night.
On recent foreign trips, Campbell took a visibly less hands-on, some would say uninterested, approach to the press pack.
The iron grip of old had given way to a far more relaxed even disinterested demeanour.
And, it has to be said, the prime minister appeared to manage perfectly well.
A number of those close to him have long argued that he needed to abandon the Campbell security blanket and present "the real, unspun Blair" to the world.
They have been arguing that the days of spin should have been ended some years ago before they started doing more harm than good.
Campbell's decision to retreat from the daily press briefings into the Downing Street backrooms was supposed to be a symbol that the government had moved onto fresh ground.
It spoke volumes that many Westminster watchers just saw that as another piece of spin.
And, unless there are some real changes in approach from Downing Street after Campbell's departure, it may be hard for the prime minister to persuade a sceptical public that he really has turned over a new leaf this time.