John Rentoul: Alan Johnson could move into Prescott's office tomorrow. So why not the top job?

If there is a sudden vacancy, Johnson is surely the favourite

I have seen the future, and it works. The only question is whether he really is the future. His name is Alan Johnson and he would make an excellent prime minister. Last week, he delivered his first speech as Secretary of State for Education - his first speech outside the House of Commons at any rate, because the previous day he had seen the Education Bill through its final Commons stage, known as Third Rebellion.

On Thursday, though, he gave a speech at the Institute of Education in London, in the same hall in which Tony Blair had been elected Labour leader 12 years before. It was a good speech, not least because I won a bet with a colleague that Tony Crosland would feature prominently. Johnson's predecessor from the 1960s was mentioned in the second paragraph. Johnson did not quote Crosland's profanely expressed desire to destroy every grammar school in England and Wales, but he didn't need to. Just as John Reid, when he was Secretary of State for Health, claimed that foundation hospitals were the realisation of Aneurin Bevan's vision, trust schools are now the embodiment of Croslandism.

Johnson's first task is to soothe the wounded soul of the Labour Party. Education has been a grievance with Blair from the start, and despite the fact that he is the first British prime minister to choose the state system exclusively for his children.

The mostly Labour audience thrilled to Johnson's attack on the hypocrisy of the secular middle class who use church schools. As he said, most parents already exercise choice. "They can buy a house in the right area; they can navigate the complex bureaucracy," he said. "They can even pretend to be God-fearing Christians if that helps their chances - hurrying to church for a quick knees-down on a couple of Sundays in the run-up to the new school year."

Well, it's a bit more interesting than your average New-Labour-speak, isn't it? He didn't use the word stakeholder once. Yet his message was strongly New Labour. Indeed, in his next sentence he pointed out: "These options are not available for everyone - not parents with poor basic skills themselves [or] whose first language is not English." And he was blunt with those who want to roll back choice. "Of course, every parent wants the local school to be the perfect school for their child, but sometimes this will not be the case." Until a school can be turned around, which can take "three, five or more years", it is only fair - the realisation of Croslandism in modern form, in fact - to extend "informed choice" to those currently denied it.

This is important, because the political markets have moved. The succession to Blair is no longer quite such a lock-down for Gordon Brown. Two things have happened. One is that the bottom has fallen out of the market in Prescotts. Some MPs told me last week that their local Labour parties are outraged by the Deputy Prime Minister - his sexual incontinence and/or his failure to make a proper show of contrition for it. Surprisingly for a party that cares more about financial than sexual misconduct, the Prescott affair seems to have annoyed the grass roots even more than the loans-for- lordships business.

As we report today, MPs are talking of a delegation to John Prescott to urge him to step down. He is unlikely to do so, but the simple possibility that he might flounce out of the Cabinet and resign as deputy leader of the party has increased the velocity of circulation of gossip about a deputy leadership election. Harriet Harman's campaign is up and running (she repeats her line in an interview today that a woman should stand); so is Peter Hain's; Jack Straw is discreetly making his interest known; Alan Johnson is being talked about. If there were a sudden vacancy, Johnson is surely the favourite, if he wants it.

And if he could win the deputy leadership, why not the top job? Especially as the second thing that has happened is that the opinion polls have turned against Brown. In the run-up to the general election a year ago, the polls suggested that ditching Blair for Brown would increase the Labour vote. Now they suggest that the change would depress it. What has changed? David Cameron. We have seen this sort of thing before. In 1990, the Conservative Party did what it had to do to respond to public opinion. Its MPs ditched Margaret Thatcher.

The political situation was transformed and Labour was transfixed, unable to adjust quickly enough. If it had shown the same ruthlessness as the Tories, it would have ditched Neil Kinnock and drafted John Smith. Then the outcome of the 1992 election probably would have been the hung parliament that the opinion polls predicted.

The Labour Party has not learnt from that mistake. Now Cameron is a step ahead, and Labour has again failed to respond. Blair has responded all right. He has tried to put himself on the side of the people against the Government on issues of crime and immigration. But he will not be around at the next election. As he tried to tell the party's national executive at its first meeting after the last election, next time they would have to work it out for themselves. The first thing the party needs to realise is that settled assumptions are dangerous in politics.

For 12 unbroken years, it has been the settled assumption that Brown would succeed Blair as Labour leader. Eventually, the Tories would find a leader flexible and creative enough to get ahead of that curve. The Cameron strategy of hugging Blair and dissing Brown is already bearing fruit. Last week's ICM poll in the Brown-backing Guardian was a delight to behold, as the newspaper that had aligned itself with the assumption of the Brown succession found public opinion had moved on. A battery of questions compared Blair to Brown, most of them apparently in Brown's favour. But there was one striking exception. Asked who was "best able to work with other people", 45 per cent said it applied more to Blair, while 35 per cent said it applied more to Brown.

Julian Glover, The Guardian's new politics leader-writer who wrote the leading article in March calling on Blair to go this year, was left with a delicate task. He had to explain that people say they prefer Brown to Blair but that, if Brown took over, the Conservatives' lead over Labour would stretch from four percentage points to nine. If that formation persists, Labour should ditch Brown for a more creative and flexible centre-ground politician. Johnson could just be that leader, but does the party have the sheer ruthlessness and ingratitude that the Tories showed Thatcher in 1990?