In July 2005, Tony Blair was riding high. He was the world statesman of the Gleneagles G8 summit, the Olympic winner, the bulldog facing down home-grown terrorists. Now, in Spring 2006, he seems enmired in pay-for-peerages sleaze. He may yet leap free of that.

But the big questions are being asked, and we have a respectable slew of answers.

As to the legacy, it seems commonly accepted that Tony Blair entrenched Thatcherism. In 1997 he came in saying its fundamentals would not be undone, and he will leave having made it very hard for anyone seriously to expect to dislodge them.

It is understood, too, that he failed as all modern politicians of the right have failed - including Mrs Thatcher: he could not wean the British off the nanny state, to which they cling for fear of something worse. They are at least logical enough to accept high taxes to pay for health and education (though not enough to make either really good), as Irwin Stelzer, half mourning, half admiring, admitted to the Today Programme in March 2006.

But what was the real character of his premiership? Blairism can't be summed up as a doctrine or a programme. Rather it was a phenomenon, and Mr Blair was only half-aware of its nature.

I have dubbed it "Messiah Politics", to capture something very personal, very idealistic, very powerful and very vote-catching.

Mr Blair used to try to persuade us that he was run of the mill. He sought to be a "pretty straight sort of guy" (when he was first tainted with sleaze). Over the doomed Dome, he appealed to the Euan Factor (referring to his son and the value of the kitchen table wisdom of the young).

But he soon aimed higher. Tony Blair has always had an almost Walter Mitty sense of his own possibilities. Commentators like Matthew Parris, Charles Moore (who in the Spectator, 18 March 2006, cited Blair's 1997 "a thousand days to prepare for a thousand years" speech) and Michael Portillo have sought to identify a self-deceiving, but also a grandiose, lack of reality in the man. Mary Ann Sieghart has identified an adolescent quality to Blair's "unrealistic idealism". He see himself, she thinks, as both Everyman and Superman.

Since his Oxford days, his biographers tell us, Tony Blair has conceived of himself as a man with a personal mission - a mission he discovered as part of his religious journey. He determined that he would be a transformational person: he would change the world. Like many a con-artist, he may have been dazzled and even puzzled by the degree to which others invested in him, but he knew how to put their belief to work. He understood the immense power of personal attractiveness when it could be put in front of TV audiences.

Tony Blair has now decided that being liked is obsolete. He casts himself as the principled loner. "I only know what I believe", he told the party conference in 2004. He seems almost to delight in Commons defeats for his measures in the War on Terrorism - as when he didn't get his 90 days detention package for suspects. "Sometimes it is better to lose and do the right thing than to win and do the wrong thing," he said in November 2005, and perhaps hugged to himself the image of a leader who is luminously right and will in time be seen to be so. Like the visionary romantic in Caspar David Friedrich's famous painting, Wanderer Above a Sea of Fog, such a figure is intoxicated by dreams, exhilarated by leadership, but consoled by martyrdom.

In March 2006, he told Michael Parkinson that the decision to join Gulf War 2 was "made by God as well". It is tempting to argue that Mr Blair was only saying, uncontroversially, that God is the ultimate judge of his actions, or perhaps that the Prime Minister was speaking vaguely. He probably doesn't suppose he has much closer guidance from his Maker than any other religious person claims. But he does have a sense of mission as grand as it is theatrical.

He was never content with little tasks: having modernised the Labour party, he thought to modernise the entire nation. But that was soon exposed as a whimsical idea. He faced a real task, and real failure, when he sought to modernise the 1945 welfare state.

Welfare reform faces the profound difficulty that most voters don't want it. Besides, Tony Blair made the enormous mistake of hating and fearing Whitehall. In love with the personal and the informal, he concentrated power in Number 10, where a coterie was made into a court and the evolution of policy overturned in favour of a blizzard of announcements (often without follow-through) and eye-catching initiatives (which had to seem to flow from the PM.) Mr Blair has returned to welfare reform too late, just as he abandoned it too early. But he also marginalised the ministries which might have delivered it when he was in full flight and had a much better chance of selling us real change.

Peter Oborne has characterised Blair's administration as uniquely mendacious. Christopher Foster is only the most clear amongst several in arguing that it damaged government. Mr Blair's Messiah Politic weren't very elegant, as we see in Armando Iannucci's The Thick of It. They marked a low point in our representative democracy, as we learned from the inquiries by their Lordships Hutton and Butler. They have involved unedifying flirtations with Bob Geldof and Bono. But they put Britain at the side of the US in Iraq. An honest prime minister working with a strong Westminster and Whitehall could not have achieved this last.

So, in one vital respect, his personal mission, including his hi-jacking of the tools of administration, enabled the largest most noble - perhaps the maddest - of his goals to be fulfilled. Mr Blair's Messiah Politics eventually led him - for a while - to be windy and lofty in the trendy causes of saving the planet from climate change and solving the Africa problem. Of course, he soon realised that nothing he could do would persuade the public quickly to take these seriously. But he did get hold of the idea of putting British military force on the side of good in the world. He started to feel this was valuable in the case of the former Yugoslavia. He warmed to it in Africa. But it became an absolute imperative in Afghanistan and later in Iraq. It was work which appealed to some of us, and commentators like David Aaronovitch have probably surprised themselves in their enthusiasm for it. Here at last was something important that could flow from the sofas in the Den.

Biographers will continue to wonder how Mr Blair, an apparently insubstantial man, managed also to be so steely. Historians will marvel at how we let him get away with it. And by the time Labour celebrates its second hundred years in the Commons, people will know whether Tony Blair picked the right side in Iraq. He may yet be judged kindly on those grounds alone.

Whatever the politicial agendas of the prime ministers who succeed Mr Blair, they have a great opportunity to undo the harm he did government.

* Richard D North's "Mr Blair's Messiah Politics: Or what happened when Bambi tried to save the world" is published by the Social Affairs Unit