Mr Blair doesn't seem to realise that being sincere is not enough

In the years just after the war, during the fag end of the music-hall, there was an act called Peter Brough and Archie Andrews. Brough was the ventriloquist and Archie his dummy. It was later turned into a programme on the wireless, as we said in those days, entitled Educating Archie. It never diverted me greatly - my taste was more for Take It From Here and The Goon Show - but inevitably I saw photographs of the two participants. They both looked uncommonly like Mr Tony Blair today.

This was - or is - surely remarkable, for Mr Blair to resemble not only the ventriloquist but his dummy as well. Perhaps it is his ancestry in show business that is coming out; or perhaps he would have been like that anyway. Whatever the explanation, one of the Prime Minister's principal characteristics is that he constantly impersonates himself, or what he thinks of as being himself.

On some occasions the act is more convincing than it is on others. That is only to be expected. To switch the analogy from broadcasting to the fine arts, as Ms Tessa Jowell would no doubt do: at some times it is a genuine Tony Blair, but at others you have to be content with Studio or even School of Tony Blair.

This refers to the quality, the accomplishment, of the performance. It has nothing to do with the intrinsic truth of what he is describing. Indeed, it is the other way about. The greater the falsehood, the greater the speech, the interview or whatever it is required to be. Grasp that, as the late Malcolm Muggeridge used to be fond of remarking, and you grasp all.

Mr Blair has had his moments with Mr John Humphrys ("I'm a pretty straight sort of guy") and with Sir David Frost as well, though fewer, if any at all, with Mr Jeremy Paxman or Mr Jon Snow. Undoubtedly he has reserved his finest performances for the House of Commons.

When, two years ago, he was chosen as The Spectator's Parliamentarian of the Year, there were puzzled grumbles in some of the papers. He had never, they said, shown much interest in the House, which was true enough. In fact he never went near the place if he could help it, which was also correct. The gossip-writers came up with all kinds of fanciful explanations of why such an apparently perverse award had been made by a Tory publication.

What happened was simple enough. Originally the judges wanted to give the gong to Sir (as he then wasn't) Gerald Kaufman. One of them piped up and said that, great though his admiration for Sir Gerald was, and admirable parliamentarian as he might be, the sponsors, then Zurich Financial Services, deserved, so to speak, a bigger bang for their bucks. They deserved a major political figure, or someone who could plausibly be presented as such. The other judges agreed. After some discussion, they came down in favour of Mr Blair.

In the subsequent period, he has not let them down. He has several times performed that most difficult feat for a Commons speaker: he has changed people's minds. To what extent should the observer take into account that he has changed them in what the observer thinks is the wrong direction; or that the politician has based his case on false premises, fallacious reasoning or both; or that he does not really believe what he is saying at all? They are among the most difficult questions of political writing.

Clearly, you cannot treat a Nuremberg Rally on exactly the same terms as the Gettysberg Address. Adolf Hitler and Abraham Lincoln cannot be judged by their eloquence or by their effect on their followers. Still less can they be judged by their sincerity, by their belief at the time that what they were doing was right. No more can a British politician be judged in this way. We do not choose our leaders for their sincerity. Yet this appears to be the sole (or, if not the sole, the predominant) criterion by which Mr Blair wishes to be judged today.

I believe this is the way he wants to be judged, but not his account of what has gone before. Does anyone think that, if Saddam Hussein really had possessed the fearsome arsenal which Mr Blair claimed he had at his disposal, the UK and the US would have invaded Iraq with quite such abandon? In similar circumstances, they would not have touched North Korea; nor, presumably, would they today. Mr Blair clearly made a political decision to support the US invasion of Iraq when it was taken in September 2001 or whenever it was (for some authorities put it earlier). What has followed since then has been ex post facto justification for what was a prearranged course of events. To be fair to Mr Blair, he was always in favour of removing Saddam because he was a bloody tyrant. The weapons of mass destruction provided a convenient pretext which an alarmed public would swallow.

Last week a clear party line emerged, additional to Mr Blair's self-justificatory and unapologetic speech in the House on Wednesday. This was to the effect that his sincerity, honesty or good faith - the precise wording varied - had now been confirmed by no fewer than four independent inquiries. It was obediently trotted out by the assorted ministers who were appearing on the numerous by-election programmes on Thursday night and Friday morning. Most members of the public would, I suspect, be hard put to it to tell you what these investigations were: they were, I assume, those by the Commons security committee, the Commons foreign affairs committee, Lord Hutton's inquiry and Lord Butler's inquiry.

Well, the first two investigations are now largely discredited. While the Hutton report is not quite in that situation, it nevertheless suffers generally from the additional detail in the Butler report and, in particular, from Lord Butler's revelation that the intelligence services had withdrawn from the claims subsequently made on their behalf by Mr Blair and other members of his government - something which the truthful and honest Mr Blair somehow neglected to volunteer to Lord Hutton.

Mr Michael Howard is perfectly entitled to ask whether the country would now willingly go into a war which had the Prime Minister's seal of approval. There is no spectacle so ridiculous as the Labour backbenchers in one of their now periodical fits of patriotism. They sound much like Tories. And yet Mr Howard neither asks nor answers the question whether he would have supported the war on Iraq if he had known then what he does now. Presumably he would, as he still contrives to support it.

Judged by the by-elections, the citizens of Birmingham and Leicester are not much impressed by him. But then, they are not greatly enamoured of Mr Blair either. They can vote and, in May 2005, may still be voting for Mr Charles Kennedy with a clearer conscience.