The key question for the coming political season: is Tony Blair still on the slide?

For half of this year he has resembled a hyperactive manager trying to slow down a run on his bank

The second most patronising thing that the late Roy Jenkins said about Tony Blair was that the Prime Minister treated political capital as a "store of value to be hoarded" rather than as a "springboard for resolute action".

The most patronising, of course, was his description of Blair's as a "second-class mind" in an interview with Boris Johnson, the editor of The Spectator, four months later in the summer of 2000. But that was simply an unguarded side-product of the man's immense intellectual vanity. He couldn't help himself, any more than he could help his reflexive and even more condescending elaboration that there was "nothing wrong" with a second-class mind because of course - he didn't say this, but it was implied - brains of the second rank are very rare and not everyone can be as clever as me. Not even Gordon Brown, whom he also patronised in that same interview. When Johnson asked if the Chancellor were cleverer than the Prime Minister, Jenkins gave a wickedly indirect answer: "Blair thinks he is."

It all depends what you mean by intelligence, really, which is why the annual fuss over A-levels and university places is so off the point. Blair got to be Prime Minister and Jenkins didn't, which ought to count for something. And then Blair took the great intellectual snob for a jolly old ride, letting him think we were going to get electoral reform and the euro in one big festival of nostalgia for the unfinished business of Limehouse, 1981.

But we will have to wait for the verdict of Andrew Adonis, the Prime Minister's adviser who is writing an authorised biography of Jenkins, to be sure who outwitted whom. For those who cannot wait, Adonis is publishing a collection of essays, including one of his own, on Jenkins at the end of next month.

But Jenkins did seem to be on to something when he accused Blair of being a hoarder of popularity rather than an exploiter of it. The Prime Minister seems to treat the hopes vested in him seven years ago as a wasting asset that will inevitably diminish over time. His political mission often seems to be to spin out that decline for as long as possible, with longevity in office as one of the most important measures of success. All that stuff about how historic it was for Labour to win two full terms; while the implication of Blair's preface to the "Big Conversation" policy document last year was that simply by being in power Labour had shifted the ruling values of the country from "selfish individualism" to "community and mutual responsibility".

Even the Iraq war, the outstanding issue on which Blair went against popular opinion, can be seen as something into which he found himself locked at an early stage - and his energetic and morally high-pitched self-justification as attempts to limit the damage. For most of last year and half of this one, he has resembled a hyperactive manager trying to slow down a run on his bank.

Of course, Jenkins was frustrated by Blair's timidity in not holding a referendum on adopting the euro. He hoped to goad him into action by writing in this newspaper, in February 2000: "The great prime ministers, those who leave a mark on history, are those who make the political weather and not those who skilfully avoid its storms and shelter from its downpours."

He produced a response all right. Within hours of publication, the Prime Minister sought an audience with Jenkins at his East Hendred country home. But the resulting heart-to-heart took Britain no closer to membership of the eurozone. Second-class mind or not, Blair is not stupid, and the idea of blowing a large chunk of his political capital on a referendum he was bound to lose did not appeal to him for understandable reasons.

But Jenkins's general criticism provides the best template for judging Blair's situation as he returns to Downing Street after a summer break of reflection and journalistic silliness. Blair's prime ministership has been a long march down from the sunny uplands of his honeymoon with all those interests in the British electorate he had not offended yet - which has recently looked more like a slide down the loose scree of Iraq.

The question for the coming political season is whether the old "mid-term blues" law that governments regain popularity in the run-up to elections still holds, or whether Blair is still on the slide. Certainly, in the closing weeks of the political season before the summer holiday, he showed his unusual power of resilience. He saw off an unwise challenge from Michael Howard, who wanted retrospectively to rewrite the terms of his support for the Iraq war, and launched a series of five-year plans for the public services that did the trick of retaining the initiative.

But resilience is not the same as renewal, and the question remains as to the extent to which Blair is capable of renewing himself and his government in office. I would say he is, to the extent that is at least sufficient to get him through the next election. His public service reforms are just enough to provide a sense of purpose similar to the Next Moves Forward initiative that saw Margaret Thatcher through the 1987 election. And he remains as dominant within his government, if not as much loved by his party, as she was.

But the threat to his hold on office will never come from the party grassroots. As with Thatcher, it will be the party in parliament that will dispatch him to the US speaking circuit if it ever decides that he is a liability. For that reason, I remain puzzled by the delphic utterances of John Prescott earlier this year. It was Thatcher's humiliation of her deputy prime minister, Geoffrey Howe, that triggered her downfall. Blair is not an electoral liability to his party in the way she was, and he has been assiduous in keeping Prescott on side, but it is still not obvious what Blair's deputy was up to in speculating openly about the succession.

Reading Anthony Seldon's book, Blair, I noticed that he quotes "one Cabinet minister" as saying, in April this year, the month before the famous Times interview: "You can feel the plates moving under the surface. People are positioning themselves for the future." So the famous "plates" were not a spontaneous one-off: as I thought, Prescott is a more disciplined talker than his reputation for mangling the language suggests.

Whatever Prescott may have meant, however, it should be clear by now that any co-ordinated push with Gordon Brown to persuade Blair to step aside for the long-term good of the party has come to nothing and that the Prime Minister intends to see the next election through. Winning an election next spring should see him through to December of next year. He would then be the longest-serving prime minister since 1900 apart from Thatcher. Jenkins, one suspects, would rather have seen him make a quixotic lunge for greatness, risking his premiership in a doomed referendum on the great European cause. That might have been heroic, but would it have been clever? I suspect the second-class mind will be vindicated in the end.

The writer is chief political commentator for 'The Independent on Sunday'