The electorate might not appreciate an unnecessarily early election driven by Mr Blair's concerns either about his health or his place in history, writes Frank Millar, London Editor.
Is there any point now to a continuing Blair premiership? The Prime Minister finally, if only briefly, escaped the shadow of the British hostage, Kenneth Bigley, on Thursday night with his sensational disclosure about his recurring heart problem wrapped inside a declaration of intent to serve a full third term in office.
After a conference week in Brighton overshadowed by Mr Bigley's terrible plight here was another, if obviously very different, human story to which the first and appropriate response was concern for Mr Blair himself and sympathy for his wife and children during what will be an anxious time.
However, it was Mr Blair's chosen terms - coupled with the revelation that he and Cherie are already planning for political retirement with the purchase of a £3.6 million home in London's swish Connaught Square - which ensured that swift reassurance about the "routine" nature of Mr Blair's hospital treatment yesterday instantly gave way to a renewed focus on the question of his leadership and the Labour succession.
In fairness, Mr Blair had no choice but to try to answer the question about how long he intends to remain in office. It would otherwise follow him into the general election campaign, enabling the Conservatives to develop their "Vote Blair, Get Brown" theme. And it is possible Mr Blair thinks to have bought time, intending to use continuing health worries actually to quit quite soon after the election, expected next summer.
The terms he employed on Thursday night suggest otherwise. Asked when he would stand down, one close aide replied "Very shortly" before what would otherwise be his fourth election.
Nor was there any cause for embarrassment in pressing questions about matters of crude political calculation at a time of personal and family stress. For it was instantly clear that this announcement was indeed driven by political calculations, and the message unmistakable. Not only was Mr Blair attempting to close down speculation about his leadership, he also thought to shut the door on Gordon Brown's ambition to succeed him.
True, there were kind words for the Chancellor, who knew nothing of Mr Blair's intentions until he stepped off his plane in Washington on Thursday night. But they were not the words he wanted, or might once have expected, to hear.
Asked if his announcement had helped Mr Brown's prospects, Mr Blair conspicuously declined the opportunity to name his friend of 20 years his natural successor. "I have a huge respect for Gordon . . . I'm sure there are lots of people who want to do the job . . . and I think he is very capable of doing it, don't misunderstand me at all. But the reason I want to stay is to see the job through. I've begun it. I want to see it through."
All this leadership speculation might be what the Deputy Prime Minister, John Prescott, and others like to call "soap opera". However, it is authentic soap opera scripted by New Labour's own high command, and it now seems set to run and run.
How could it be otherwise? Supporters of the Chancellor certainly have no difficulty interpreting Mr Blair's coded message about "lots" of other likely contenders as a signal that he will be backing one of them.
And the implications for the Labour Party as a whole of the Prime Minister's announcement are indeed, as the Guardian put it, "volcanic". Mr Prescott himself fuelled speculation about the leadership some time back when he spoke of the "tectonic plates" shifting in anticipation of Mr Blair's eventual decision to go, and he was unconvincing yesterday in response to Conservative chairman Liam Fox's prediction that Labour would now find itself locked in "a permanent war of succession".
What are young and ambitious middle-ranking ministers to do as they look to their prospects in a post-Blair era? It would certainly be in the nature of politics for them to divide into armed camps around whichever candidate they think most likely to succeed.
What of the humble backbenchers - of whom there should be more - eyeing their own constituency majorities as Labour contemplates the possibility of a fourth term?
How close to the event would they actually permit Mr Blair to remain in office while increasingly not in power? Would not their natural preference be for a comparatively early departure so that the new leader can recast the cabinet, refresh and reinvent the government and establish his own appeal to the electorate long before polling day?
And what of the candidates themselves, still seated around Mr Blair's cabinet table? Every dispute over policy - already guaranteed by Alan Milburn's return to assume Brown's old job in charge of the election campaign - might inevitably now be seen through the lens of the coming leadership contest.
Whom to oblige with loyalty and courage? The man on his way out, or the brooding Scot who at this writing must still seem the likely victor? Which brings us to the increasingly combustible Blair-Brown relationship. In any rerun of the battle over a contentious issue like university fees, would the Chancellor again call off the attack dogs?
There is danger here, too, for Mr Brown. As was evident from his more restrained conference performance on Monday, he almost certainly calculates it is not in the interests of his inheritance to have this relationship implode so close to a general election.
That calculation could change if there is anything to yesterday's speculation by Trevor Kavanagh, the highly respected political editor of the Sun, that Mr Blair might cut and run for an early election with the intention of forcing Mr Brown to the Foreign Office, or out of the cabinet altogether, by Christmas.
This seems improbable. The electorate might not appreciate an unnecessarily early election driven by Mr Blair's concerns either about his health or his place in history. It is surely also the case that Mr Blair is still less able now to do anything about his troublesome Chancellor.
And if Mr Blair really is staying on because of his obsession with his place in the history books, can he count on Mr Brown to help secure it? Or might the Chancellor thwart him at the last?
Having been given the clearest signal that Mr Blair is opening the way for other possible successors, it also seems improbable that Mr Brown the prime-minister-in-waiting would want to infuriate the Euro-sceptic press by stretching himself to help Mr Blair win a referendum on the new European Constitution. And without victory in that, it seems a safe bet we can forget a subsequent poll on British membership of the euro.
Maybe Mr Blair knows best. And Labour is still set to win the election if only by default of the opposition. Yet it seems an extraordinary thing to invite the country to contemplate life in the post-Blair era, while seeking re-election on the promise of great things still to do.