A senior figure at Downing Street put it concisely: "One day, Tony will come in and say, 'the bad news is that I am going. And the bad news is that you are going too'."
Even Blair aides, few of whom would survive the transition to Gordon Brown, know that the moment is getting nearer. They also know that Mr Blair's desire to surprise us by announcing he is quitting when we are not expecting it is going to be much harder to achieve.
Some Blair acolytes claim the opinion polls showing Labour six or eight points behind the Tories will bring rebellious Labour MPs to their senses. I suspect they will draw a different conclusion: that Mr Blair should be on his bike faster than David Cameron.
To be fair, most people around the Prime Minister know the ground has shifted beneath his feet. He expected the Brown-ites to shake the tree after the local elections to see if Mr Blair would fall off his branch. He didn't expect Mr Brown to do it himself by calling for "renewal".
Nor did Mr Blair expect mainstream Labour MPs to call on him to reach agreement with Mr Brown on a timetable for the "stable and orderly transition" he promised a year ago. When they joined forces with the Brownites at a meeting of the Parliamentary Labour Party (PLP) on Monday, the Prime Minister realised he was in deep trouble.
He tried to get out of it by promising his successor "ample time" to settle in before the next general election. But that wasn't enough. The nadir of a terrible week came the following morning when Mr Brown said on GMTV: "Remember when Margaret Thatcher left, it was unstable, it was disorderly and it was undignified." One Blair ally replied: "That was a declaration of war, given what Tony had promised the PLP. Gordon was publicly threatening Tony with the same fate as Thatcher."
A day earlier, at his monthly press conference, Mr Blair had dismissed parallels between his plight and the fall of Baroness Thatcher in 1990 in her third term. Now it didn't feel so different after all.
The fury in No 10 was palpable. "Brown has got his finger on the nuclear button and -this time - he keeps pressing it," one Blair adviser said. The metaphor was apt: in previous bouts of the "TB- GBs," the Chancellor and Prime Minister pulled back from the brink because they faced mutually assured destruction-"Mad" for short.
The shifting sands were visible at Prime Minister's Questions on Wednesday. …