John Rentoul: The trouble for Tony Blair is that he's forgotten how it looks to ordinary people

What struck me about his evidence yesterday was the curious omission of any expression of sympathy for Dr Kelly's family

It all went wrong in Basra. On the morning of 29 May this year, Tony Blair addressed 400 British troops from the verandah of a former presidential palace. It was a short speech, but long on rhetoric. "When people look back on this time and look back on this conflict, I honestly believe they will see this as one of the defining moments of our century." There was a touch of Thatcher, both in his nationalism - "You have made this whole country, our country, hold its head up high" - and in his personal responsibility for the Iraq war - "I know there were a lot of disagreements in the country about the wisdom of my decision to order the action."

Honest disagreements he could cope with, in his view of himself as a heroic national leader overcoming the doubters back home, armed only by his shining conviction in the rightness of his cause. What he could not take, he told the Hutton inquiry yesterday, was an allegation that, had it been true, "would have merited my resignation". That morning, the BBC had aired the "extraordinary allegation" that his office had included things in the Iraq weapons dossier against the wishes of the intelligence services.

So there he was, visiting "liberated" Iraq after a successful war which had deposed a ghastly dictator with fewer casualties than had been feared, and all he got from the journalists with him was carping questions following up the BBC's "absurd" claims.

There were more echoes of Margaret Thatcher in his evidence yesterday. "Parts of the BBC," he said, did not cover the war in "as objective a way as it ought".

The nerve that Andrew Gilligan's report touched that morning was, therefore, more raw than usual. Even so, Mr Blair's assertion that the BBC report was an attack on him as Prime Minister, on the intelligence services, "and on the country as a whole" sounded like his predecessor at her most shrill attacking the enemy within. And Mr Blair is usually extremely good at putting on the thick skin.

He is, as has often been observed, one of the most media-sensitive politicians, a good judge of how things will play with journalists, as well as being a fine judge of public opinion. Yet one of his strengths has been his ability to suppress his real views in order to smooth his path through the prickly egos of the media-political complex.

On that morning in Basra, however, it seems Mr Blair began to lose the sense of proportion and self-restraint that had served him so well in the past. It was presumably this experience that led him to authorise Alastair Campbell to attack the BBC in extravagant language at the Foreign Affairs Select Committee four weeks later.

That was the error of judgement that precipitated a crisis from which Mr Blair and Mr Campbell probably could not have emerged with credit - even before David Kelly's death.

It hardly mattered with what skill Mr Blair deployed the arguments in his favour yesterday. The fact that Dr Kelly apparently took his own life creates an almost irresistible assumption in the public mind that the Government must in some way be responsible.

Mr Blair was therefore constrained in what he could say yesterday. He made several important points, not least to remind people that the September dossier was not designed to make the case for war but to make the case for action against Saddam Hussein, and that the first port of call afterwards was the United Nations, which decided to send in the inspectors, not the invasion.

He also tried to recall the decision-making perspective of people who could not have known that Dr Kelly might take his life.

But he could not say what he presumably thinks, which is that he was traduced by Dr Kelly through Mr Gilligan and (to a lesser degree, such that Downing Street did not seem to notice it at the time) Susan Watts on Newsnight, in that Mr Campbell and/or Number 10 did not include the 45 minutes claim in the dossier against the wishes of the intelligence services.

Nor could he say that he wanted Dr Kelly's name to become public - although he came close, saying that he and his senior colleagues had assumed the scientist was "of a certain robustness". Instead, he had to rely on the argument that the name was bound to become public anyway and he could not withhold material information from parliamentary committees.

There was, therefore, a lack of authenticity in yesterday's evidence, however effortlessly he may have survived a day that had - once again - been billed as his greatest test. But that has always been the case with Mr Blair. His ability to mask his real feelings has been one of his strengths. That is how he was able to pose for so long as the political equivalent of JK Rowling's Mirror of Erised, in which everyone sees that which they most desire. But it is not a magic that will keep working for ever, and his real personality, with its furies, its pique and its pride, was bound to show through, so that he could no longer be all things to all people.

That process is now well advanced. In the constant internal battle between political caution and Gladstonian conviction, especially about world affairs, caution is increasingly being thrown to the winds. The war in Iraq took away a large section of Mr Blair's natural support; the death of Dr Kelly and the process of the Hutton inquiry is eroding a further section before our eyes. Some of that is inevitable, and some of it is the accumulated price of past spins.

As Richard Sambrook, BBC head of news, slyly pointed out in his evidence, the corporation was reluctant to accept Downing Street's initial rebuttals of the Gilligan report at face value because of its experience of previous denials. In the cases of Cherie Blair's dealings with the conman Peter Foster; of Martin Sixsmith's "resignation"; of Lakshmi Mittal's Britishness; and of (ancient history, this) Mr Blair's lobbying of the Italian Prime Minister on Rupert Murdoch's behalf - in all those cases early denials had to be modified in light of later information.

Even so, I cannot help thinking that Mr Blair has lost some of his genius for divining public opinion.

What struck me about his evidence yesterday was the curious omission of any expression of sympathy for Dr Kelly's family. When James Dingemans, the inquiry counsel, asked him if he had anything to add, he said no. A note of humility and empathy, such as was shown by the next witness, Gavyn Davies, would have been second nature to Mr Blair once. He has forgotten what he once knew better than anyone else in politics: how non-political people see things.

j.rentoul@independent.co.uk

The writer is the author of 'Tony Blair: Prime Minister', published by TimeWarner Books