Mr Cameron may not yet be a national figure, but he was undoubtedly the star of a remarkable show in Blackpool this week, the most exciting - and positive - Tory conference of the 24 I have attended.
The 38-year-old shadow Education Secretary, whose allies did not believe he could win a few weeks ago, is now a serious threat to Mr Davis, struggling to keep the label "front-runner" after a bad week, and may eclipse Mr Clarke as the standard-bearer of Tory moderates.
My abiding memory of the week is not from the conference hall, but from BBC2's Newsnight programme. Frank Luntz, a former pollster for the US Republicans, conducted a focus group among swing voters who watched a film of the five contenders. Mr Cameron triumphed; the only flicker of support for any rival was for Liam Fox. One woman participant said: "David Cameron has reinvented politics for me." Mr Luntz declared that he had never seen such a dramatic turnaround in opinion.
So Mr Cameron is what his party has been looking for - "Tory Blair". When Tony Blair joined the Shadow Cabinet in 1988, Philip Gould, Labour's pollster, carried out a similar "people metering" exercise to gauge the public's response to the party's frontbenchers by measuring reactions to a video of them speaking.
Like Mr Cameron, Mr Blair outscored his more senior rivals. As Mr Gould recalled: "Even when he was faltering or not at his best, the line [on the graph] would shoot up when he started to speak. He seemed to be able to connect with the public in a way that transcended rational explanation. It was a response qualitatively different to that of any other politician."
Mr Cameron may have the "X factor", but it does not guarantee that his party will elect him. I suspect that many Tory MPs will be reluctant to vote for a younger man; young cardinals usually vote for old popes and there are genuine worries about his lack of experience. They may prefer the heavyweight old pretender, Mr Clarke, to the young one. Mr Fox, a dark horse who is coming up on the rails, could benefit most from any slide in Mr Davis's support.
This week, the Tories finally regained their hunger for power. The hard core of activists who travelled to Blackpool suggested they might swallow their doubts about Mr Clarke's Europhile views. But I am not sure that the armchair members in the country will feel so charitable - or hungry. They are the bulk of the 300,000 people who will decide the leadership election and their average age is over 65. In 2001, they elected Iain Duncan Smith rather than Mr Clarke. I left Blackpool feeling that only half of the party, if that, really acknowledges the scale of the task facing the Tories. In the warm bubble of the conference, it felt as if the Tories were back in business. But the party still needs to give itself some electric shock therapy to show the voters it has got the message after three election defeats.
True, all five leadership contenders acknowledge the party must change. The Tories are all modernisers now. The "one nation" mantra, until recently used only by a minority sect on the party's left, is universally accepted.
Yet there is precious little agreement about how to change. Mr Cameron's conference speech was light on policy, although he does have a Blair-like plan to transform the party in his first six months as leader. Mr Clarke's message, in contrast, is: "Le change, c'est moi." To head off the threat from Mr Cameron, Mr Clarke needs to say more about his policies.
Mr Davis and Mr Fox made clear that their appetite for change is limited. They do not think the Tories need to trash their own brand and took a sideswipe at Francis Maude, the party chairman, whose "change or die" speech was one of the best of the week, but was inevitably overshadowed by the leadership beauty parade.
Mr Maude, a sort of licensed torturer, administered what he privately called "masochism" to his party's members. It needed to be done. He cited research showing that people liked some of the policies adopted by the Tories, but liked them a lot less when they were told they were "Tory policies". If that's not a tarnished brand, I don't know what is.
Although Mr Maude wasn't lynched, many people in his audience didn't like it and there was much criticism in the margins of the conference. So it is too early to say whether the Tories really "get it". Their choice of leader will provide the answer.
Labour certainly fears Mr Clarke and, I am sure, is starting to fear Mr Cameron. True, Gordon Brown wiped the Commons floor with his friend George Osborne when he was installed as shadow Chancellor. Mr Brown is a master tactician who will adjust his game to meet whatever Tory threat faces him. When he takes over from Mr Blair, he is not going to revert to Old Labour, a hope to which far too many Tories still cling.
But it would be ironic if, after having been kept waiting so long for the premiership by Mr Blair, Mr Brown's long-deserved tenure at No 10 was quickly interrupted by Tory Blair.Reuse content