I hope they find WMD. How else are we to trust the Prime Minister?

Tony Blair's foreign tour has not been a smooth affair. From Kuwait to Warsaw the Prime Minister has been dogged by a persistent question: where are Saddam Hussein's weapons of mass destruction? How frustrating it must be for a supposedly victorious war leader. The weapons are nowhere to be seen. The same damned, wretched question pops up wherever he goes.

At his news conference in Warsaw, Blair angrily blamed opponents of the war for clinging on to this issue. What's the hurry over the weapons? Celebrate the liberation of Iraq! The problem with his anger is that it was not the opponents of war who made weapons of mass destruction the central issue. It was President Bush and, more evangelically, Blair himself who chose to do so.

Bush always appeared muddled in his public advocacy, raising erratically the prospect of regime change as much as the issue of Saddam's weapons. Blair was clearer: he was going to war to rid the world of Iraq's armoury. This was a point he made repeatedly and with mesmerising persuasion in Downing Street press conferences, speeches and interviews for several months. It was not a passing aside as part of a wider argument. From September of last year (at his press conference in Sedgefield and in his speech to the TUC conference a few days later) until the outbreak of war the Prime Minister highlighted, with a messianic zeal, the dangers to the world of Saddam's weapons.

Now Blair appears irritated when the issue is raised. From talking of little else for six months he has moved to a point where he does not want to speak about the weapons at all. His irritation is misplaced. What is at stake is not an arid academic debate about the origins of the war. It is the much broader issue of trust, the relationship between a political leader and the voters.

In a way that cannot be underestimated, trust is at the core of the New Labour project. New Labour's origins lie in the rubble of the Labour Party's fourth election defeat in 1992. Post- mortems carried out by the party's leadership at the time reached a single conclusion: the electorate still did not trust Labour. They did not trust it to run the economy. They did not trust the leadership. They did not trust a party that had still not fully reformed itself.

With a disciplined and ruthless brilliance, Blair and Gordon Brown set about regaining voters' confidence. They resolved to promise no more than they could deliver. Blair scrutinised every single proposal in advance of the 1997 election to ensure that it met the "trust" test. Most specifically, Labour was not trusted to "tax-and-spend" so Blair and Brown agreed to adopt Tory plans on tax and spending until they had changed people's perceptions. This might have slowed the pace of reforms, but their determined incrementalism transformed the way that Labour was seen. It was trusted.

So why did Blair risk severing the umbilical cord of trust between the voters and the leader over the weapons of mass destruction? Let us be clear. The cord will not be broken with one clean cut. At some point evidence of sorts will be found in the desert. The Prime Minister would not risk making this prediction so emphatically in recent days if there was any doubt about that. But what is obvious is that the threat posed by Saddam was never on the scale that he suggested. If it had been, the Iraqi leader would have used the weapons in the war. In a desperate contrivance, Donald Rumsfeld, the US Secretary of Defence, has come up with the wackiest excuse of all time - that Saddam might have destroyed his weapons in advance of the war: "OK boys, we are about to be attacked, so get rid of your weapons immediately!"

At the very least the missing weapons and the fact that none were used during the war show that Blair exaggerated the threat posed by Iraq. I do not believe that he forced the intelligence services to fabricate evidence - that would be a scandal to make Watergate seem like a mild speeding offence. Much more likely, he presented tentative intelligence reports in the most dramatic and persuasive light. Under New Labour Downing Street has always resembled a newsroom. What's the story? What's the top line? How are we presenting this? These questions whirl around all the time. In the siege-like atmosphere before the war it is easy to imagine the scene: "Let's highlight this warning about Saddam's weapons! Let's put some life into this stale prose!" It did not matter where the warnings or the stale prose came from - a single uncorroborated intelligence source or an ancient PhD thesis.

Quite possibly, Blair genuinely misread the scale of the threat as described by the intelligence reports. The course of this government on several fronts would have been very different had Blair held other ministerial posts before leaping to the very top. In this particular instance his lack of previous ministerial experience meant that he never had the chance of observing at close quarters another prime minister handle intelligence reports, nor read such reports from the perspective of being a foreign secretary or a defence secretary. And what a different perspective: Robin Cook read the crucial reports in the Foreign Office up to 2001 and reached the conclusion that Saddam was less of a threat than he had ever been.

Here I fear that a bleak tragedy is being played out. There are many complex reasons why Blair chose to go to war, but I have no doubt that one of them was a determination not to be seen as "Old Labour". Old Labour was regarded as anti-American and weak on defence. New Labour would be pro-American and strong on defence. But, like a character in a film noir, when the Prime Minister tries too hard to purge his party of its past he lands himself in trouble. Some of his worst headlines have arisen, for example, from his attempts to woo business leaders, partly to show that Labour has changed: I must be "New". I must not be "Old".

Now we have a terrible twist. In attempting to be boldly New Labour on the international stage Tony Blair has inadvertently risked his own credibility. And that was what New Labour was meant to be all about: a governing party that could be trusted. I get no satisfaction from making this observation. Personally, I hope bucketloads of lethal weapons are discovered so that the Prime Minister can be partially vindicated and therefore trusted again on other issues, such as Europe and public services. For, unless the weapons are found and shown to have been a threat, will anyone believe him again? While trying to achieve the opposite, Blair has taken us back to 1992. The issue of trust hangs in the air once more.