John Rentoul: Blair will suffer collateral damage but could emerge stronger - if he survives

This, then, really is it. Alastair Campbell once said that, of the weeks that Tony Blair had been Prime Minister, at least two thirds had been called the most crucial he had faced. Mr Campbell would have to accept that description of the next seven days. Not that Mr Blair's position is yet directly at risk. Of course, the failure to secure a second resolution at the United Nations will convulse the Labour Party and offend wider public opinion. But once military action begins, it is still most likely that public opinion at least can be recovered.

A short war that liberates a grateful Iraqi people will provide a measure of justification after the event. And there is the stock of respect that Mr Blair is earning now for his courage in defying his critics. In contrast to some of the public performances of his first term, he has shown confidence and resilience in arguing his case against hostile audiences of undeferential citizens on Newsnight, sceptical youth on MTV and anti-war women on Tonight With Trevor McDonald.

There is more than a touch in this of what people said about Margaret Thatcher: "You don't always agree with her, but at least you know where she stands." Mr Blair learnt a lot from Mrs Thatcher – not his politics, that is a cheap jibe, but about how politics works. He knows that, having been trapped – or having trapped himself – by his support for President George Bush on Iraq, he might as well gain some credit for standing firm.

Which is not to say that he will emerge from this crisis unscathed. Even if the war is a low-casualty affair and the negative impact on other Arab and Muslim countries minimised, the damage to Mr Blair will be severe. Clare Short will not be the only minister to resign; more than half the parliamentary party could vote against the Government; there may be a leadership challenge this autumn.

Labour Party members have never liked Mr Blair much. One of the penances of being his biographer is finding myself besieged by people who use me as a surrogate for venting their grievances against him. From the start, Labour Party members have been the worst. Tories tend to be puzzled by him, irritated by his slipperiness or enraged by his theft of their clothes. But the real vitriol comes from members of his own party. First it was schools, then it was university tuition fees, now it is Iraq at the top of the list.

Until now, however, the deal has always been that they tolerate Mr Blair because he wins elections for them. So the collateral damage to Mr Blair's personal ratings and, to a lesser extent so far, to Labour's standing, is dangerous.

That is why the critical issue is the extent to which anti-war feeling, which has spread far beyond the Labour Party, recedes to its confines after the conflict. The alienation of the Labour core cannot be ignored; it is not as if they have "nowhere else to go" because there is always the Liberal Democrats. But if Middle England returns to the fold, that will simply be the continuation of the pre-war situation, with more bitterness on the left and more admiration from the centre and right.

There is still a puzzle, though, in how Mr Blair got himself into this situation, with all the risks of the war going badly, which prompted the odds to be cut sharply yesterday on his survival as Prime Minister to the end of this year.

Mr Blair is certainly sincere in his support for military action – even if sincere is a word to use with care of such a consummate actor-politician, playing the statesmanlike gravitas against cheap party political point-scoring from Iain Duncan Smith at Prime Minister's Questions.

Iraq was the main issue he discussed with President Bill Clinton in their early meetings. He has always thought Saddam Hussein was a problem and has always been prepared to use force against him – starting in December 1998 with a bombing campaign. That was an Anglo-American venture with no explicit authorisation from the UN, and it caused barely a ripple in public opinion.

At the same time, Mr Blair had clearly decided his closeness to the US presidency would be a strategic imperative of his premiership. He stood by Mr Clinton during the embarrassment over the Monica Lewinsky affair. And he showed his determination to continue it with Mr Bush. The special relationship, he said during the Iraq debate when Parliament was recalled last September, was "an article of faith with me".

But the political and personal damage can be overstated. Just as two thirds of Mr Blair's weeks in power have been the most crucial, for about two thirds of his time he has been described as tired, sleep-deprived, looking older or exhausted. The truth is he thrives on pressure. Witness his relaxed performance at PMQs this week. Give him a few days off and he will return looking tanned, fit and full of energy.

Politically, he is weakened – for now. Clare Short can say what she thinks and he cannot touch her. Gordon Brown's statements of support reveal, on close textual analysis, the unmistakable shape of a predator stalking his prey. Yet the calmness with which Mr Blair joked in the Commons on Wednesday about Ms Short's public disloyalty suggests that he has accepted that the die is cast. He knows that if the Americans make a mess of this war, he could be tomorrow's history seminar. But if they don't, he knows he will survive and it is quite possible he will emerge in a stronger position than before.

John Rentoul is author of the biography "Tony Blair: Prime Minister", published by Time Warner Books