In the current febrile political situation, party leaders and aspiring leaders act on the basis of two assumptions. The first is that Gordon Brown will succeed Tony Blair. The second is that Mr Brown will never make an overt bid for the leadership while Mr Blair is in place.
Every now and again the first assumption is tested. An alternative candidate to Mr Brown surfaces and is deified fleetingly by ultra-Blairites. Currently, the new Education Secretary, Alan Johnson, plays the fateful role. I suspect Mr Johnson seeks a modest greatness, but is having a more overwhelming greatness thrust upon him.
For some time, he has coveted the post of deputy leader. Mr Johnson had noted that Harriet Harman had placed her hat discreetly in the public arena. He felt he should follow suit, and therefore declared an interest. In some quarters this was taken as a Machiavellian bid for the top job. I read his intervention as a Machiavellian bid for the deputy's job.
Perhaps at some stage Mr Johnson would like to lead his party. Such a heady thought must cross the minds of cabinet ministers, especially those who are in the midst of a media embrace. Mr Johnson must have read the warm words being written about his abilities in some newspapers and wondered whether his time might come. But last week he was making a less extravagant calculation. At some point, there will be a deputy leadership contest. Mr Johnson wants his name in the frame for that post.
On one level, therefore, nothing much has changed. Mr Brown will almost certainly become the next Labour leader. None of the other potential candidates have delivered a speech on the economy, let alone run one successfully. He remains by far the most qualified and formidable candidate.
But will that job be worth having in a year or 18 months, when Mr Blair concludes that he has fulfilled his prime ministerial destiny? This is a much more urgent question than the speculation about who the contenders might be.
For Labour, the current situation is ominous, not yet fatal. The Conservatives' recovery is fragile, based largely on the fact that David Cameron is a nice chap, a light-footed leader with an astute ability to raise issues that worry many voters. In contrast, his ageing party has not greatly changed, and nor have the core of the policies, which remain Eurosceptic, tax cutting, anti-regulation and in favour of the war against Iraq. The gap between the progressive tone of its leader and the substance of the party is immense, and could be exposed easily. At the moment, Labour is too dysfunctional to try.
Soon it will be too late. If the current political narrative stays in place for much longer, voters will decide it is time for a change and opt for the nice chap on the bike, rather than return a party that appears incompetent, divided and weary of power. This is what has changed. As Mr Brown waits for the prime ministerial vacancy, he is infected by the sense of drift. At the last election, polls suggested that more voters would have supported Labour if he had been the leader. Since then his ratings have slumped.
I am not surprised. With the unique authority of a leader on his political honeymoon Mr Cameron attacks Mr Brown as a "roadblock to reform" and a political dinosaur. Some of Mr Blair's close allies brief a similar message. The Chancellor responds defensively by hailing "reform", this meaningless term that is set as a test of leadership, and then awaits the next onslaught.
Mr Brown cannot survive another year or 18 months operating in such an unflattering context, attacked by soaring Conservatives and crusading Blarites, being perceived as part of the problem as the Tories take a commanding lead in the polls, continuing as Chancellor but speaking out occasionally in code about the need for renewal, pretending to support every right-wing Blairite policy while indicating vaguely he would do things differently.
Probably Mr Brown would still win a leadership contest in 18 months' time, but he will have been so damaged, and the Government's standing so harmed, that the next election would be un-winnable. Already some polls give the Conservatives a 10- point lead. Watch that gap widen in the coming months. In the past Mr Brown has refrained from challenging Mr Blair for several good reasons. He knew such an act of regicide would divide and traumatise his party. Self-interest and the interests of Labour combined to hold him back. These factors no longer apply in the same way. Labour is tearing itself apart anyway. Mr Brown is briefed against and his supporters brief back. Below him an unofficial deputy leadership contest is under way. Everyone knows there is turbulence at the top - public protestations to the contrary only add to the sense of unease.
In the early days of his leadership, Mr Blair conveyed an attractive sense that he was not consumed utterly by personal ambition. In 1994, after the sudden death of John Smith, he told Roy Hattersley - then a Blair supporter - "Gordon wants the leadership more than me." When Mr Blair told Mr Brown at their famous dinner at Granita, in 1994, that he wanted to be leader for only 10 years, I am sure he meant it at the time. Now Mr Blair clings to power with the same deluded hunger as Mrs Thatcher in her final phase.
In a lively programme on Radio 4 last Saturday, close colleagues reflected on the fall of Margaret Thatcher. Ken Baker noted that in her final days she was determined to remain in office because she wanted to deal with rising inflation and the first war against Iraq. She was convinced of her own indispensability. For prime ministers, there are always reasons to stay. Mr Blair is in the final Thatcher-like phase. He has more to do. Only he can do it. His courtiers, a few of whom would probably prefer to see Mr Cameron win next time rather than Mr Brown, urge him on.
There are big risks for the Chancellor in launching a challenge to the leadership this year. Ultra-Blairites and others within the Labour Party would scream. Parts of the mighty media would fume. Mr Cameron would rub his hands with glee. But now, as Mr Brown waits for the comically elusive smooth transition ultra-Blairites scream, parts of the media fume and Mr Cameron rubs his hand with glee. Mr Brown might as well make his move this autumn and see if he can change the political situation once he has secured the top job. An early challenge would be bleak, but less bleak than hanging around for another 18 months.
The first assumption in British politics, that Mr Brown will succeed, still applies. For it to have much relevance, the second assumption, that he will never challenge Mr Blair, requires an urgent reconsideration.