John Rentoul: Young, sunny David versus old, snarling Gordon: oh, that life were so simple

Davis's fall is a reminder of the fickleness of politics

It all happened in an astonishingly short time. Two and a half weeks earlier, on 29 September, Cameron and David Davis launched their formal campaigns within minutes of each other. Then and since, Davis consistently fell short of expectations and Cameron exceeded them. Cameron did not put a foot wrong until Thursday night, when he was tripped into giving a more specific answers to questions about drugs than he had given when the issue first came up at the Conservative Party conference in Blackpool. No, he said, he had not "snorted cocaine as an MP". But it hardly matters now, and Cameron has the same quality Tony Blair has of making mistakes only once.

Already, with unforgiving speed, the caravan is moving on. Conservative Party members will hate being taken for granted. But they can be taken for granted. Last week's YouGov poll of 600 of them suggested that they divide 72 per cent to 22 per cent in favour of Cameron over Davis. That is not the sort of margin likely to be turned around now that the first impressions of the candidates have been made. This is yet another parallel with Blair. Labour Party members were resentful in 1994, when some of them felt that the choice of Blair was being foisted on them by "the media", but the reality was more complicated, then and now. The views of politicians, journalists, party members and the general public, as reflected in opinion polls, bounce off and reinforce one another in a constant and occasionally volatile feedback loop.

Cameron it is, then. This time, the party is making the right choice. This time, MPs and party members agree. If it had been up to MPs, they would have had a third ballot on Tuesday, and at least 10 of Fox's supporters would have switched to Cameron (and possibly a few of Davis's) to give Cameron a majority. (In 2001, the MPs would have elected Kenneth Clarke when the members chose Iain Duncan Smith.)

Attention moves on, pausing briefly to relish the prospect of a Cameron-Blair match-up at Prime Minister's Questions. "Can I say how much I agree with the Prime Minister, and can I ask whether the right honourable gentleman, the member for Kirkcaldy and Cowdenbeath, does too?" Thus we come to the next part of the story: Gordon Brown. The big question now is how the Chancellor responds to the new Conservative leader.

There are, however, two parts to this question. Cameron's giddy ascent is undoubtedly bad news for Brown's hopes of succeeding Blair. It requires only a little sympathy to imagine the chill that passed through the Chancellor and his friends as they watched the Tory heir presumptive evaporate within 17 days. David Davis was the leader-in-waiting for nearly two years (he decided not to fight Michael Howard for the job in November 2003). So many Conservative MPs bet their careers on his succession that he had collected endorsements from nearly one-third of his parliamentary colleagues before the contest began. Then it all vanished, leaving his supporters variously puzzled and furious: "What did we ever see in him?"

Brown's position as likely successor to his party's leadership is stronger than Davis's was. But Davis's fall is both a reminder of the fickleness of politics and an incitement to Labour's many Brown-sceptics. In the corridors of Westminster, the search for Labour's Cameron has stepped up a gear. With a Prime Minister determined to stay on in office but off the leash, increasingly impatient for results from his public services revolution, political fortunes could be made and lost with Cameronian speed in the next two years.

The second part of the question is different. If Brown does take over from Blair, still the most likely outcome, Cameron's arrival poses less of a threat than many Tories hope. Cameron may be their best choice in a limited field, but even he will struggle to overturn the tectonic forces of history. Sure, Brown may seem old, Scottish and inarticulate against the newcomer, and it is a comparison that has been made to his disadvantage before. This time, however, his young, English and fluent opponent will be the leader of a rival party with a serious image problem.

There are parallels between Blair's bland and noncommittal manifesto for the Labour leadership in 1994 and Cameron's for the Tory leadership this year. Blair's was called "Principle, Purpose, Power" and one of its authors said to me, much later: "Looking back, one out of three wasn't bad." Cameron's is called "Why put off what needs to be done?" which must be the most stirring clarion call to stack the dishwasher or fit the draught excluder in modern politics. But the difference is that Labour's basic positions were popular in 1994 and they still are today. Blair did a lot of work before 1997 to remove policy specifics that could have turned into electoral liabilities, but he was working with the grain of a party already on course for victory.

Cameron's task is harder. He has already ditched the most obvious liabilities from the last Conservative manifesto, which he wrote, notably the plan to pay half the cost of people's private health care. Yet he has no answer to the problem that has held back the Tories for the past 10 years, which is that they want tax cuts but cannot say how they would pay for them - without making voters nervous about the health service and schools. Without an answer to that question, the main reason for voting Tory can only be that they would be more competent at administering the New Labour welfare state.

That is a tough case to make, with an education White Paper this week and more evidence all the time that NHS reforms are working. Last week the British Medical Journal reported a "striking divergence" in waiting times between England, under the reforms, and other parts of the UK, which have carried on as before. Interesting, too, that Gordon Brown in his conference speech spoke of "New Labour" - a phrase he usually avoids - "renewed". At this rate, Cameron's wedge strategy of trying to play Blair off against Brown may not work. Brown does not strike me as the kind of politician, like David Davis, whose ambitions will collapse at the first whiff of cordite.