John Rentoul: Why is Tony still here?

He gave a clear incentive to keep up the guerrilla war against him

I remember watching Tony Blair from the Press Gallery of the House of Commons 11 years ago. He stood where David Cameron now stands and taunted John Major for letting his Euro-rebel MPs back into the Conservative Party without getting anything in return. Major foolishly pointed out that Labour was also divided on the question of Europe. "There is one very big difference," said Blair, and paused for a well-judged moment. "I lead my party. He follows his." I remember the roar from the Labour side of the House. It was the full-throated roar of a party delighted to be led, knowing that history was on its side.

Last week, the wheel turned. The swash and buckle were all on the Speaker's left again; the defensiveness and the telling silence on the right. Blair has performed poorly at Prime Minister's Questions before. He has died on his feet (at the Women's Institute in 2000). He has lost a studio audience (when he asked on Question Time in the election if people found it hard to book GP appointments more than 48 hours in advance). But usually, when it matters, he rises to the occasion. Last Wednesday it did matter and he failed to do so.

The challenge Blair faces has changed and he has not risen to meet it. The Conservatives are leading in the opinion polls. They are led by David Romance (clue: it's an anagram), who has the wind of historical inevitability in his hair. His joke about Khartoum was a warmed-up one that journalists made two years ago when Blair visited the spot where General Gordon died, but he delivered it well and the point was deadly. "Why does he not trust the Chancellor of the Exchequer to take over the Government now?" There is no answer to that, as Eric Morecambe might have said; at least, not one that Blair can give in the Commons, with Gordon Brown by his side.

From now on, it will become harder and harder for Blair to answer the question that Cameron and journalists will ask: "Why are you still here?"

Perhaps I am overreacting to one parliamentary moment. There was nothing disastrous about Blair's performance. His situation is nothing like as humiliating as Major's. The Conservatives are in nothing like such a strong position now as Labour was then. The country is basically soundly governed and enjoying its longest recorded period of sustained economic growth. The same opinion polls that show the Tories on average six points ahead also suggest that Labour would be no better off under Brown. They suggest a fascinating and finely balanced contest at the next election between Brown, regarded as "tough" and "competent", and Cameron, regarded as "likeable" and "in touch".

Yet Wednesday's low point came after Monday's Downing Street news conference, with its message of appeasement for Brown and his backbench supporters. George Pascoe-Watson, the political editor of The Sun, asked: "Is Gordon Brown still the successor you would choose for yourself, or are there other members of your Cabinet right now who could do the job just as well?"

Blair replied: "Of course he is. When have I ever said anything different?" To which the accurate answer is, "Every time you have ever been asked about it." It was, as far as I am aware, the first time that Blair has endorsed Brown as his successor. The closest he came before was in an interview in January when he said, "I'm absolutely happy that Gordon will be my successor", which is quite different.

In his next answer at Monday's news conference, Blair again said the opposite of what he meant. Asked why he hadn't repeated his intention to serve a "full" third term, he said: "I don't resile from anything that I have said about this" - meaning "I do resile from what I've said in the past because I am trying to manage the next-door neighbour". But, by talking of giving his successor time to "settle down", he suddenly and unnecessarily brought forward the limit to his time in office. With the last general election now more than a year away, the argument that he has a democratic mandate to carry on has weakened another notch - he now voluntarily weakened it further, by declining to assert his right to serve a "full" term as he was elected to do. This was, presumably, supposed to reassure those Labour MPs who were demanding a timetable - a clever demand that makes it sound as if they want to know whether they have double physics on Wednesday afternoons, rather than that they are contemplating regicide. Instead, it merely fuelled the speculation about how long a time was "ample" - the word Blair used at the meeting of the parliamentary Labour Party on Monday evening.

Worse, by appearing to retreat under pressure from Brown and his supporters among Labour MPs, he gave them a clear incentive to keep up the guerrilla war against him.

It may be that the fever will abate for a day or even a week. But the combination of a demoralised party - which, apart from one blip, has been ahead in the opinion polls since 1992 - and a media obsession with Blair's departure means that the fever will be worse every time it is triggered.

Almost all of Blair's time as Prime Minister has been an unsteady decline from the glad confident morning of 1997, but last week felt like another lurch downwards. At some point the feedback loop between party, media and public opinion will become so strong that it will be impossible to escape to one of those happier interludes that now seem a distant memory. More than before, therefore, Blair is at the mercy of events. Almost anything that happens will be reported through the prism of "when is he going to go?".

There will be another Labour rebellion on the Education Bill, which will change nothing, but which will be reported as if it did. So Blair is left waiting for events that cannot be turned against him to provide a break in the clouds, which is not a strong position in which to be. Next year's local elections will further traumatise the party. It is harder than ever to see how he can survive to the end of next year.

But I hope he tries. Eleven years ago, he accused John Major of caving in to his own party's irreconcilables. After delivering his "I lead my party, he follows his" jibe - which Major later admitted was "the best one-liner he ever used against me" - Blair said "the white flag now flies over Downing Street". He must not make the same mistake.