When the Prime Minister failed to make Milburn Secretary of State for Education, the one post for which he might have stayed, the Chancellor's ascendancy was sealed. The ruling assumption of politics became the inevitability of the transition from Blair to Brown. No sooner had that conviction settled on the media-political complex, however, than it started to decay, like weapons-grade plutonium.
It would be foolish to suggest that the consensual wisdom is already wrong. But nothing lasts in politics. The Prime Minister looks greyer on his return from the Caribbean, but there has been no lessening in the energy he brings to the job. His intimates would say that, wouldn't they? But I detect no evidence that what he is doing now has ceded primacy in his mind to the admittedly interesting question of what he does next.
The point is that he is perfectly capable of another three years at the top, and that he has to convince himself and everyone else of that in order to make it possible to exit before at a time of his choosing. Looking back over the last three years, which takes in the world-historical event of Blair's premiership, the invasion of Iraq, it is evident that anything can happen over the next three.
The one person who understands this best is Gordon Brown himself. I remember an early introduction to the subtlety of his intellect when, as John Smith's understudy, he was hailed for a dazzling performance against Nigel Lawson in 1988. Instead of high-fiving his advisers, or whatever it is that Brown does when he is happy, I was told he was sunk in gloom. He had peaked too early, he said. He would attract hostility from his colleagues and become the target of sniping from his rivals. So he did, and the rest is history.
He knows that, of the many things that do not exist, free lunches and certain promotion are two of them. That is why the period since the election has been so intriguing. What, the Prime Minister's advisers are asking, has the Chancellor done in the past three months - apart from returning, behind the scenes, to his old unhelpful ways, admittedly at a lower level of intensity?
It is a mischievous question. Brown had a part to play in the Prime Minister's progress on Africa policy at the Gleneagles G8 meeting. And he delivered the eulogy at Robin Cook's funeral. But the main news story about him was his redefining the economic cycle. He got away with it at the time, but it may be that serious long-term damage has been sustained below the waterline of the ironclad.
It is a mischievous question, too, because the period since the election has been a fruitful time for the Prime Minister. I would not go as far as Anthony Seldon, who last week described it as "the most successful three months of his entire premiership". But he is right that the defeat of the European constitution, winning the Olympics, the deal on Africa and IRA disarmament should have had Blair repeatedly dancing a little jig, which is apparently what he does when he is happy.
There was something else in Seldon's assessment that left the Prime Minister unjigged, however, and that was the suggestion that Blair found Charles Clarke, the Home Secretary, "too tame". An "insider" was quoted as saying: "Clarke is more sceptical about the value of prisons, tends to side with lawyers and is worried about criminalising antisocial behaviour." That prompted Tom Kelly, Number 10's official spokesman, to take the unusual step of naming Seldon and saying: "That article was plain wrong."
As I say, intriguing. Compare the Chancellor's low visibility since the election with the Home Secretary's ceaseless activity. From the moment the bombs went off on 7 July, he has been a substantial presence on television. Not only does answer he questions directly in a way that not all politicians do, and talk faster than any media adviser would advise, he comes across as thoughtful and honest. On the day of the London bombings he said bluntly that he doubted that identity cards would have made a difference. After the attempted bombings two weeks later, he gave interviews on the police shooting at Stockwell and postponed his family holiday.
This week, Clarke has been engaged in sound politics and productive negotiations, speaking to the European Parliament and discussing anti-terrorism law with 70 fellow EU ministers in Newcastle. Practical stuff about mobile phone records, and sensible stuff -to those who do not take absolutist positions on civil liberties - about how to interpret the European Convention on Human Rights in circumstances where it seems to obstruct action against those who wish us harm.
Last week Clarke declared: "I am neither woolly or liberal or a woolly liberal." Never mind the grammar, just feel the pulse of middle England. This is not quite the same electoral district as that cultivated by Blair. Clarke's interview, in which he spoke about a confrontation with yobs in Norwich and his neighbour's fatal stabbing, was with the Daily Mirror, not the Daily Mail. But it is Blair's insight that Labour people care about law and order too, and there is no reason why Clarke's nuanced Blairism should not appeal right across the spectrum. The trick is to be tough on crime but not to sound as if you are being illiberal for the hell of it - and Clarke is pulling it off brilliantly, in a way that David Blunkett never managed.
For those purposes, a little distance between Clarke and Blair is wholly to the Home Secretary's advantage. Clarke will probably not have been upset by Seldon's "tame" report, therefore. He benefited from similar reports of friction with Number 10 when he was Secretary of State for Education.
Suddenly, Clarke - who intends to contest the Labour leadership when Blair goes - seems a credible challenger to Brown. No one else seems likely even to gather the necessary number of nominations (45) from Labour MPs (the other signal fact about the post-election reshuffle being the fact that John Reid is not Foreign Secretary).
Brown is still a long way ahead, of course. But over the past three months the shape of the leadership contests in both main parties, neither of which has yet begun, has been transformed. It is now quite possible that the voters could face the choice at the next election of Clarke - or Clarke.
The writer is chief political commentator for 'The Independent on Sunday'Reuse content