Both Mr Blair's party and his cabinet remain anxious about a war on Iraq

On one level, Mr Blair yesterday made his meeting with a deeply restive Parliamentary Labour Party tomorrow no less difficult than it was always going to be. It was impossible to leave his news conference thinking that the prospects of war have suddenly diminished. He did not rule out supporting a US invasion of Iraq in the absence of a second UN resolution. He did not resile one jot from his conviction that Saddam is seeking to rebuild his arsenal of weapons of mass destruction. He did not in any way retreat from his determination that the Iraqi dictator should be divested of it one way or the other – a determination which, he maintained persuasively yesterday, he had harboured since well before 11 September.

And that will make a lot of people anxious, not just in the Labour Party or his own cabinet. Clare Short, who voiced her passionate and "deeply worried" commitment to a UN-backed solution on LWT's Dimbleby on Sunday, is in tune with the Labour mainstream.

Mr Blair, of course, was speaking yesterday to the country as much as to his own party. Some senior ministers have even wondered privately if Mr Blair's famous antennae have deserted him, whether he no longer has his legendary feel for British middle opinion. Haunted by Labour's anti-American past, Mr Blair made known his determination almost from the moment of George Bush's inauguration that he would not be outdone by a Tory party – not then seen to be as ineffective as it has come to be – in familiarity and sympathy with the new Washington administration.

In fulfilling that ambition, the most anxious of the doubters ask, did he go too far the other way, becoming too pro-American for some of those very voters who had been so appalled by Labour when it was anti-nuclear and anti-Nato 20 years ago? Mr Blair was no doubt partly seeking yesterday to dispel this idea, never more so than when he said he would be urging the President to act on Iraq if he wasn't already doing so.

But while there are still many politically radioactive variables in this hypothesis-strewn no-man's-land between peace and war, there was a kind of new and emerging clarity, too. For a start, Mr Blair, unlike the Rumsfeld faction in Washington, doesn't see 27 January as the inflexible deadline for starting the countdown to war. Given a working assumption that the UN inspectors have not by then found the clear evidence of a material breach within the terms of Resolution 1441 – ie either mass destruction materiel or a proveable effort to conceal it – then they should have more time to find it.

First, he appears to think that's a precondition of going to war. Even though he (and, by her own interesting admission on Sunday, Ms Short) is convinced by intelligence reports that Saddam has such weaponry, it's necessary for a democratic war that potential coalition partners – and their peoples – to believe too. Secondly, he is confident enough in Dr Hans Blix's abilities and intentions to believe he will find just such evidence.

Against this background, you can understand Mr Blair's occasional frustration that he is being attacked for things he hasn't yet done and may not do. He did play a decisive part, in league with Colin Powell, in focusing Washington on the UN route. The new and fear-inspiring doctrines of regime change and pre-emption were narrowed and refined, at least in public, to one where the UN might be conducting a fight for the nobler purpose of maintaining its own credibility.

It's possible that the material breach will be like the elephant. The UN Security Council will know it when it sees it, and will act accordingly even, perhaps, up to the point of a second UN resolution. And that is worth waiting for. Even supposing that Saddam does not have a last-minute change of heart –and we can only pray he will – that could mean that a UN-sanctioned war would follow.

Which is what majority of people, according to the polls, would be prepared to back. In one sense this is quite surprising. For the worries that will surface among Labour MPs – in many cases reflecting their constituents – go wider than the UN formalities. Indeed, it's understandable that they want the debate before it is too late. They know that if the impulse of regime change for its own sake has been partly suppressed, it was how this affair started.

Many decent MPs fear that an invasion of Iraq, while psychologically validated by 11 September, will actually make the war on terrorism more rather than less difficult. They applaud wholeheartedly Mr Blair's concerted efforts to breathe new life into the Middle East peace process but doubt, with good evidence, whether this resonates in Washington – where it really counts – as it did resonate with the President's father during the build-up to the first Gulf War. And they note that for all the power of its brand, the Security Council's permanent membership contains no direct representation of Muslim opinion.

And yet despite all this, and however mysteriously, the UN role does remain central to the British debate. Two conclusions flow from this. The first is that Mr Blair will be in deep domestic trouble if there is a dispute within the UN Security Council over whether the material breach has occurred, and there is no second resolution sanctioning war. The trouble might just be containable, if what follows Blix's endeavours is in shades of grey rather than black and white: if, in other words, the Russians, say, and the Chinese are unprepared to sanction a fresh resolution, but are equally reluctant to take a stand against war.

It would still be very difficult; the Kosovo parallel, in which the Russians refused to back a resolution, is highly imperfect since Milosevic perpetrated a heinous act of ethnic cleansing, visible nightly on television screens. But even the Liberal Democrats have argued for a UN mandate rather than (necessarily) a specific second resolution. Without such a mandate Mr Blair would have a great deal more to worry about than the Liberal Democrats.

But the second, inescapable, conclusion is that, if Mr Blair is vindicated, as he appears optimistic he will be, and Dr Blix produces the kind of evidence that makes UN support for action to enforce its own resolution irresistible, then much of the current opposition to the war, trapped by its own logic, will have nowhere else to go. In that sense Mr Blair was right to say yesterday that the US decision – so far – to follow the UN route was a "two-way street." Such a war wouldn't be one that many people on this side of the Atlantic want, from the Parliamentary Labour Party to most the Arab world. But it would then (and only then) be possible for Mr Blair to champion it as a war to enforce the credibility of the UN as well as to disarm Saddam Hussein.

Tomorrow's meeting of the PLP will certainly be difficult; but the one that really counts will be when hypotheses no longer govern the debate. And that moment of truth may be several weeks away, at least.

d.macintyre@independent.co.uk