John Rentoul: Cameron wants to portray Brown as a sociopath. Blair may have helped him

Asked by Brown, Blair accepted that Alan Johnson was 'lightweight' - Ah, the paradoxes and subtexts of the end of the Blair era!

A "big clunking fist". If that was an endorsement, Gordon Brown could have done without it. Why he wanted Tony Blair's blessing so much in the first place has long been one of the smaller mysteries of British politics. It is an established convention of democratic politics that outgoing leaders should not attempt - explicitly at least - to influence the choice of their successor. However, not only has Brown seemed desperate to secure Blair's support in the Labour leadership election, but Blair has been eager to use his endorsement, handed it out in tantalising instalments, as one of his last remaining sources of power over the Chancellor.

In January this year the Prime Minister told The Sun: "I'm absolutely happy that Gordon will be my successor. He needs the confidence of knowing he will succeed me and that's fair enough." Since then, though, the tiny flame of Blairite hope that someone else might emerge to challenge Brown for the succession has flickered back into life. It was always a bit of a candle in the wind. John Reid, the Home Secretary, has the kind of standing with the general electorate that makes the idea of him as prime minister plausible, but he would never be able to muster the 45 Labour MPs required to nominate him against Brown. The same applies to John Hutton, Secretary of State for Work and Pensions, who is less well known but may be more determined to try to stop Brown. Alan Johnson, Secretary of State for Education, was the only contender likely to be able to win the backing of the left-wing irreconcilables needed to reach the total. And he folded his cards 10 days ago.

In September, on the day of the resignation of Tom Watson, the Defence minister, which prompted Blair to put a 12-month limit on his time in office, Blair and Brown had two long face-to-face meetings. I understand that, in the first of them, Brown went through the list of possible contenders one by one in order to demand which of them was more deserving of Blair's endorsement than he was. When Brown got to Johnson, Blair accepted that he was "lightweight". Whether, in the manner of his agreeing with David Frost yesterday that Iraq had been "pretty much of a disaster", Blair agreed with a description offered by the Chancellor, or whether Blair himself volunteered the word, I do not know. There were only two people in the room, and my information comes indirectly from Brown's side of the conversation.

It was, however, a verdict that turned out to be accurate. Johnson's potential, hailed in this column and it must be suspected in at least one of the many rooms in Blair's mind, failed to fill out. Barring accidents, Brown will be elected unopposed as leader of the Labour Party next year. (The hard left will not be able on its own to muster 45 MPs.)

"Lightweight" is an interesting word. It captures something nebulous but real. I remember Blair's fury in 1986 when, as the most junior member of Labour's frontbench Treasury team, John Lloyd said in the New Statesman that his "eagerness does not amount to weight".

But it is also an interesting word because it is the same boxing analogy that Blair used in extended form in his riposte to David Cameron in last week's Queen's Speech debate. Cameron had teased Blair effectively in previous exchanges by reminding him of his half-endorsement of Brown in The Sun in January. Last week we saw Blair's delayed reaction. "The next election will be a flyweight versus a heavyweight. However much the Right Hon Gentleman may dance around the ring beforehand, at some point, he will come within the reach of a big clunking fist, and you know what, he will be out on his feet, carried out of the ring."

It worked on the day, but turned to ash in Brown's fist in the morning, when the cold words do not read so well. For Cameron, trying to depict his likely opponent at the next election as a sociopathic thug, it was a gift.

Cameron's assault on Brown is not merely a campaign of personal insult, however, but central to the strategy of contrasting hope with fear. I doubt if it is done consciously, even though Cameron and his adviser Steve Hilton are both capable of it, but Cameron has taken one of Antonio Gramsci's phrases as his text: "Pessimism of the intellect, optimism of the will." He aims to marry the traditional Tory scepticism about human nature with a sunny optimism that reminds me of nothing so much as the early Blair.

Now, though, it is worldly Blair who chides his cheerful opponent for offering false hope. "Hope is not built on talking about sunshine," he said on Wednesday. This was also the subject of a fascinating exchange that took place at the meeting of 60 ministers two weeks ago, at which Blair and Brown jointly launched the policy review leading to next year's spending plans.

I am told that Yvette Cooper, the Local Government minister, raised her concern that the emphasis on security in the Queen's Speech risked being seen as negative, and that Labour needed to come across as optimistic. She was gently put down by Blair, who suggested that she had misunderstood the purpose of the review, which was to devise the right policies for the country - and it would be only at that stage that the party should work out how to present them. In other words, Blair was accusing a Brownite of being all presentation and no substance.

Ah, the paradoxes and subtexts of the end of the Blair era! Or, to put it another way, the beginning of the Brown era. For the challenge facing the heir-presumptive is becoming clearer. He has to show that he can work with colleagues beyond his inner cabal. Above all, he has to decide what to do with Cooper's husband, Ed Balls, who would like to be chancellor. (That will not happen next year, and I have that on the best authority, namely David Cameron, who, discussing the question with friends recently, exclaimed: "God, give me Balls as chancellor!") Brown has to counter his reputation for being a political operator of ruthless aggression. He has to demonstrate that Britain under his rule would not be a dour, puritanical and pessimistic place.

For some time, Cameron has been gingering up his team in private by quoting George Orwell's words to them, saying that if they want a picture of a Brown government, "imagine a boot stamping on a human face - for ever". Now, thanks to Blair, trying to be helpful, that image has been replaced by that of a big clunking fist punching a human face - for ever.

Less of an endorsement; more of a gangster's kiss.