John Rentoul: The man who Cameron really fears

Tony Blair's admission that he is no longer in control of his future has had the opposite effect to that intended by the plotters trying to force him out. "I would have preferred to do this in my own way," he said. That was an understatement. He had been desperate not to put a more specific limit on his time in office because, as he said in The Times interview that triggered the crisis, "the authority of the prime minister's position becomes difficult".

He was wrong. His authority was already "difficult" in the sense that civil servants were looking through him to his likely successor. For his remaining months, at least everyone understands that his is a pulpit premiership, where all that is left to him is the power of words and the symbolism of his office. But those are important and he knows how to wield them.

He also told The Times that he would not do what he was forced to do seven days later because "what you do is you just provide for another endless bout of speculation".

Wrong again. Suddenly, the speculation has moved on from when Blair will go to who will succeed him - and on terms wholly unfavourable to Gordon Brown.

Already, the crisis of the past week has been analysed in the manner of the origins of the First World War. The fracturing of the Labour coalition by Blair's response to 9/11; his misjudgement of tone in the early phase of the Lebanon conflict; the provocations of his Times interview. Brown's role in fomenting trouble has been judiciously weighed. Did he speak beforehand to Tom Watson, the defence minister who led the revolt?

It hardly matters. As Charles Clarke observed, the Chancellor could have stopped it with a "click of his fingers" if he had wanted to. If Blair has only the authority of the prime ministerial pulpit, it is Brown who has power at his fingertips. It may be a soap-opera simplification to see what happened last week as an attempted putsch by Brown against Blair, but it is only a simplification of the truth.

It is irrelevant whether the grievances of the Brownites are justified or the conspiracy theories of the Blairites real. What is important is that it has become the "perceived wisdom" - a phrase coined by Hilary Armstrong, the Mrs Malaprop of cabinet loyalists - that Brown tried to force Blair out.

In so doing, the Chancellor has done himself far more damage than he has done to Blair. I would not say that the Blairites in government were jubilant last week, because they realise that their champion is diminished. But one minister who has always thought Brown "unfit" to be prime minister said: "Now he's proved it." Other ministers and Labour MPs who had suppressed their doubts about Brown in an effort to reconcile themselves to his inevitable succession have had them roused again.

Clarke, in describing Brown as deluded and uncollegiate, was giving voice to what they fear may be true. The backlash against Brown among MPs was the second big story of last week. One of Blair's friends asked me hopefully: "Do you think Gordon might get booed at conference?" No, he won't, but the mood has shifted.

That is what really changed last week. Blair was almost certainly going next year anyway; but now it looks as if there will be a contest to succeed him. Before last week, Brown was going to be declared leader of the Labour Party as the only candidate validly nominated. Now there will have to be a campaign.

The rules require candidates for the leadership and deputy leadership to be nominated publicly by 45 Labour MPs. As long as Brown was seen as the certain winner, the careerist bandwagon effect made it difficult for any challenger to stand. When John McDonnell of the hard left faction says he is "challenging Gordon for the leadership", what he means is that he is seeking nominations - and won't get them. Now, however, someone else might.

If Blair and Brown have both been diminished by last week's failed coup, who, apart from David Cameron, has gained?

Step forward the man who was fortuitously at the Prime Minister's side as he made his announcement on Thursday. As he waited in Downing Street to set off in Blair's car, Alan Johnson, the Education Secretary, looked at the media helicopter hovering overhead. "It's amazing what interest the media are taking in my support for synthetic phonics," he mused.

Those cabinet ministers who have spent the best part of two years accommodating themselves to the coming of Brown are now sensing the ground shifting under their feet. Even though Johnson has hardly started to feature in opinion polls, some of his colleagues have him on their radar.

One cabinet member told me he was the John Major of the Labour Party. "He never says anything. What are his views?" It was not meant to be complimentary, but the trouble with that analogy is that Major won his party's leadership and went on to win the general election.

The psychology of Brown is a complicated subject at the sunniest of times, but what he must feel now, as he contemplates the David Davis precedent, hardly bears thinking about.

Almost the entire careerist apparatus of the Conservative Party publicly committed itself to Davis's leadership campaign before it started, simply because the "perceived wisdom" was that there was no credible alternative candidate. The Labour Cabinet will not be making the same mistake - and, to the extent that it already has, is now fumbling around for the reverse gear.

No one really knows how good Johnson is, but he comes across well, has a sense of humour, a terrific life story and, I am told on good authority, he is the opponent Cameron fears. This is not just because Johnson would show up his upper-class origins, but because Johnson would mark a complete break not just with the Blair era but with the Brown-Blair era. There is a chilling logic, from Brown's point of view, in taking the "renewal" of the party of which he has so often spoken to mean moving beyond the joint architects of New Labour. That logic suggests that when Blair goes, Brown should go with him.

It is a logic that will coalesce with the sudden clamour of the moment. After the clamour for a timetable, now the clamour is for a contest. That is democratic ground which Brown will find impossible to resist. Nor can he appear to want to. A contested leadership election ought to be in his interest. Then he could present the arguments for the changes that he wants to make.

Now he is going to get one, with all the unpredictability that implies. That is the most extraordinary outcome of the week. "We have paid the price of shortening TB's term," said one minister. "But it will be worth it if we get a non-Brown succession."

Long ago, Blair was asked how he became a Labour MP. "A catalogue of errors and mistakes!" he said, happily. Thus it began, and so it will end. But Blair's mistakes do not matter any more. It is Brown who misjudged things last week. The top job was coming to him on a plate, but by allowing the plotters to strike, to wound and not to kill, he may have just thrown it all away.