What a crusade! What a campaign! What a pity we're none the wiser!

Tony Blair and some of his senior ministers are in a crusading mood. The Labour chairman, Dr John Reid, has been dispatched to put the Government's case on Iraq to party members around the country. Gordon Brown has popped up on GMTV, almost protesting too much in his enthusiasm for the prime ministerial strategy. As for Mr Blair, he has been more or less everywhere, speaking persuasively at his Downing Street press conference, behind the scenes to Labour MPs, then with mesmerising force at Prime Minister's Question Time, and finally at a private meeting of senior Scottish Labour Party members in Edinburgh. At the end of this week he is in Washington. In between, he will probably be starring at a theatre near you.

Mr Blair is the most formidable and persuasive campaigner of his generation. Some of his colleagues are occasionally put up in the media to mouth precisely the same lines, and they sound nowhere as convincing. But there is a great big question at the heart of his impressive campaign of persuasion: what exactly is he crusading over? Is he sounding hawkishly resolute in the hope that Saddam Hussein backs down, thereby triumphantly avoiding war? What happens if, in the next few months, the UN inspectors fail to find convincing evidence that Saddam is hiding weapons of mass destruction? What happens, too, if there is no fresh UN resolution authorising war, but the US still decides to attack Iraq?

Everywhere we are surrounded by hypotheticals. Private ministerial briefings are littered with "what if?" questions from journalists. As one bewildered journalist exclaimed during a recent tour with a minister around the endless hypothetical situations: "All our viewers and readers want to know is whether there is going to be war, and when." To which the minister being questioned more or less said: "There is no answer to that."

So the Prime Minister's persuasive powers are being applied to a fluid and ambiguous situation. He is not out and about proclaiming: "back war" or "back the US in its war", or even "back the UN under all circumstances". Because he cannot fully declare his hand, and is probably not sure what his final hand will be, he is being persuasive over a series of possibilities. Put more bluntly, he is simply being persuasive.

Mr Blair excels at campaigning over imprecise objectives. As Labour leader in the mid-1990s he took off his jacket, rolled up his shirtsleeves and spoke passionately about the need to abolish Clause Four of his party's constitution. But what did that triumphant campaign mean? Did it signal that Labour had changed, or was about to change? No one was sure, but what a persuasive campaign! Similarly as Prime Minister, he took to the road in 1998 with his welfare reform roadshows. He was spellbinding, but he did not outline precisely what the reforms would be. At the time, he was far from sure himself what those reforms would be. So far, he has shown himself to be a less impressive campaigner when he is seeking to change minds on a specific issue. To take one example, no one would accuse him of being especially persuasive on whether or not Britain should join the euro in the next 18 months: a specific question requiring a specific answer.

I make this point not to underplay the significance of his positioning and his skill at justifying it, but to highlight the fact that we have not got to the critical moment yet. The hypothetical questions whirl around with the same intensity as they did last September. With focused eyes and forceful voice, Mr Blair is making the case for possibly no war, probably a war, possibly with full UN backing, possibly without full UN backing. The Labour leader is speaking as a conviction politician while at the same time keeping his options open.

In the short term, in relation to the domestic political situation, Mr Blair has good grounds for his seemingly unjustified self-confidence. The Conservatives are neutered, as Iain Duncan Smith supports a war at least as strongly as the Prime Minister does. Much of the discontent on Labour's backbenches would disappear if there were a fresh UN resolution. I suspect that the Prime Minister would not face a near fatal revolt if he actively sought a new UN resolution, but failed because of a veto from the likes of China or Russia. Are Labour MPs going to ally themselves with Saddam and the regime in Beijing while the rest of the UN backs force?

At this pre-war stage, there is a single nightmarish scenario for Mr Blair, the one in which the US decides to attack Iraq with little evidence from the weapons inspectors, and no fresh UN backing. I believe he has genuinely not made up his mind what to do in such a situation. He has not agonised over it, partly because he is fairly confident that the scenario will not arise in such a crude form.

There is another, broader reason for Mr Blair's confidence. He has subtly shifted the focus of debate in recent months. The questions he has raised have become absolutely pivotal, to the exclusion of all others. Almost certainly, the inspectors will find some weapons if the British dossier on the subject is remotely accurate. Quite probably the UN will give its backing for war. It is as if that is all that matters. What has been removed from the pre-war debate are the wider questions: was the current policy of deterrence and containment proving effective? Even if Saddam had weapons of mass destruction, would he dare to use them?

There is a carefully choreographed route to war: a significant discovery from the weapons inspectors and a second UN resolution. What happens then is much less clear. If there is a short and successful war, presumably the US, with Britain in tow, will turn to North Korea. That is the logic of linking rogue states to the war on terrorism: those states have to be dealt with. In which case Iraq will not be the end of the matter by any means. If there are many civilian casualties in the war the Prime Minister will be in deep trouble, even if he gets the backing of the UN resolution. If there is a terrorist attack in Britain while the war is taking place, there will be a similar backlash on the basis that the Government's priorities were wrong.

We have re-entered the world of "what if?" again. But in my view, the political dangers for the Labour leader are greater once the war has started, rather than in what happens in the weeks leading up to it. Even with UN backing, this will be seen by much of Britain, and indeed much of his party, as Mr Blair's war.