John Rentoul: Once Blair goes, anything is possible. Even an Old Etonian at Number 10

What the Conservative Party will probably get, therefore, is David Davis.

The Conservative leadership campaign is being observed from Downing Street with something approaching glee. It is not simply that Michael Howard's final throw of the dice failed, when grassroots representatives voted in favour of a sensible change in the election rules - but not by the required margin - leaving the party with a daft system. In No 10 they note that, for the two days after the Prime Minister's speech, his phrases bounced across the echo chamber of British politics. Davis and Cameron, in their consecutive campaign launches on Thursday, both said that, in order to change the country, the Tory party must change. "No one who aspires to be prime minister can talk anything other than the language of New Labour or do anything other than occupy the centre ground," said one of the Prime Minister's advisers. "Blair and New Labour have changed the political landscape."

The Chancellor has a rather more direct stake in the Tory contest, and his advisers are unsure where the threat is coming from. In the abstract, Clarke remains the most formidable opponent. He has what the party really needs and has not had in any of its last four leaders, namely the swash and buckle of effortless confidence. It is what Blair has always had, even as a greenhorn 11 years ago. Cameron has it too, helped as Blair was by a fee-paying education, but he is desperately untried. He has been in the House of Commons for only four years.

Clarke, on the other hand, has been tested in the furnace. His self-confidence is no trick of the light. He is forged out of the stuff. Yet his negatives are equally unshakeable. Flogging death-sticks to the world's poor for British American Tobacco. His part in the folk memory of a Tory past, a collage of the poll tax, high interest rates, mortgage repossessions, cuts. Then there is Europe. It is true that the question of Britain adopting the euro has gone away for another parliament. It is also true that the collapse of the European constitution removes the immediate cause of friction. But it is not beyond the wit of Brown to exploit or even create another European issue to reopen old Tory wounds.

Thus the Clarke campaign, which set the tempo with his early launch before Labour conference, has done surprisingly well with grassroots Tories. If they were convinced that Clarke was the way to win, they would probably vote for him. But Tory MPs are not so sure. None of this year's intake has yet backed him, and he has lost loyal supporters such as Ian Taylor to the Davis camp.

Yet the pulling power of Davis among Conservative MPs continues to be that of the least-worst option rather than of positive attraction. He is cleverer and more flexible than his image as a hard-nosed right-winger suggests. His theft of Blairite language at his launch last week, only an hour or so before Cameron tried the same heist, is proof of that. It wasn't just a smash-and-grab raid either, but a full stripping of the New Labour furniture. He said "more of the same" was a "delusion", recalling the modernisers' critique of John Smith, and put himself on the side of "the many not the few" and "social justice". He did it with more political art than Cameron, too, with the fabulously insincere, "this doesn't mean aping New Labour", and a manifesto of some substance.

However, his negatives remain apparent. Davis is an ordinary television performer. His advisers think he is getting better. One said last week that he can "do Clinton" - meaning that he may never be great at quick-fire debate, but can do wonkish empathy, mastering policy detail grounded in people's real-life experience. Somehow, though, he manages to fall on the wrong side of the arrogant-confident divide.

More surprising, and less noticed, he has got some of the big judgements wrong. He spent the first half of this year attacking Charles Clarke and Tony Blair's control orders, on the implied grounds that the threat of domestic terrorism was exaggerated. Then came the 7 July London bombings. Last weekend he needlessly went back to the arguments over Iraq. In a lecture at Chatham House, he said that Britain's "influence over the nature of the operation in Iraq was squandered by an obsessive insistence on prolonging UN-based diplomacy that could obviously yield no result". In other words, Blair should not have bothered with international law and simply got on with it.

Cameron has not complicated his campaign with anything quite so specific. Indeed, the only substance of his candidacy has been provided by dropping policies for which he was responsible, as head of policy under Howard, and by endorsing policies of the Government. Out go patients' passports, therefore, which were - I paraphrase - a stupid idea. In come tuition fees, foundation hospitals and academy schools, which are "right for the country".

In that, though, Cameron really is modelling his campaign on Blair. All Blair's policy changes as leader of the opposition 1994-97 moved Labour towards the Conservatives. Looking back, Blair's prospectus for the Labour leadership was vague and his manifesto in 1997 was minimalist. Cameron is right at this stage to eschew specifics.

Suddenly, the Conservative leadership campaign is a three-horse race. Cameron, who seemed to be positioning himself for the contest after next, looks as though he could come through the middle, if he doesn't crash and burn. It was telling that Brown's attack on him in his conference speech was so tired. Cameron was, said the Chancellor, an Old Etonian. Cameron's plummy accent and all it says of his social milieu is his greatest weakness. What can you say of a man who uses Nicholas Soames and Bruce Anderson to lobby undecided MPs? But what makes Cameron different from Davis and Clarke is that his negatives can be turned into strengths. He is inexperienced and fresh. He is posh and confident. He is vacuous and flexible.

What makes all this more interesting than mere Westminster village gossip is that Brown is beatable at the next election. For all the Tory angst about their 33 per cent share of the vote in this year's election, Labour was only three points ahead - and some of the advantage conferred by electoral geography will be reduced by the boundaries review. A hung parliament, at least, is within reach.

As one Tory MP, a Cameron supporter, says: "The Tory party cannot do anything until Blair goes. It cannot breathe." But when he goes, anything is possible. Even David Cameron.