John Rentoul: Cameron has a winning personality. And this is why he is ahead of Brown in the polls

If we fell for it once, we can fall for it again. A Prime Minister who is a good man, interested in the practicalities of good government, who has done little that is overtly objectionable but whose public persona is embarrassing. And a Leader of the Opposition who is fluent, witty, emotionally open, socially conservative and very good at it.

In fact, the comparison is unfair on Gordon Brown, who has, at least, not pursued a foreign policy of the appeasement of ethnic cleansing, as John Major did in the Balkans. And it is flattering to David Cameron, who does not have a foreign policy at all. It is notable that he has hardly ventured abroad since he gave a speech last October seeking to replace Tony Blair's liberal interventionism with "liberal conservatism", which sounded like code for "not getting involved".

But Cameron is energetic and increasingly effective on domestic matters. He and Brown ended the political season with a score draw on substance, but a Cameron walkover on style. With political difference reduced to rival parliamentary motions on the reform of MPs' expenses, Prime Minister's Questions ended in a familiar pattern.

Brown, head down, so serious he almost seemed close to tears, persisted in making the Government's reasonable case, hoping that he would get the credit eventually for sheer resilience. Cameron, "dancing around the ring" as Blair predicted he would, lightly humiliated the Prime Minister with a sure touch verging on arrogance. Consistency? he asked, although I wasn't paying attention when Brown accused him of lacking it. "I said he was useless a year ago and I have not changed my mind since." It was cruel, but far too many Labour MPs found it funny. That is the trouble with humour: it catches people with their psychological guard down.

The best politicians have a public sense of humour. It is what stops our eyes glazing over. Without it, we lose interest and are prepared to believe the most outrageous things about our leaders, such as that Gordon Brown is a wicked man who steals from the poor to give to the rich and who is personally responsible for rising price of food and the falling price of houses. Although, without humour, this becomes the price of food "rocketing" and house prices "collapsing", which is what Nick Clegg overclaimed for the Liberal Democrats, and the whole House thought again of Vince Cable.

It may be difficult for the Bush-haters to recall it, but George W Bush is both a highly successful politician and a funny one. He once said he worried that even his White House staff thought he was stupid because they gave him an intelligence briefing every morning. And he was fond of quoting Robert Strauss, his fellow Texan: "You can fool some of the people all of the time, and those are the ones you need to concentrate on."

Well, Cameron has concentrated on them with a professionalism, and wit, that Brown cannot match. The Prime Minister's Cabinet colleagues know it. As they sit next to him in the Commons, they contemplate the 20-point Conservative lead in the polls that has become the settled fact of the summer intermission. They know that some of it is the simple weariness that afflicts a party that has been in office for more than a decade. But they know that a lot of what separates the two parties is Cameron's wit and personality.

It is certainly not policy. That was the point of two big economic speeches this week from Cameron and Osborne. There can be no difference between the parties on interest rates, because they accept, in the form of Bank of England independence, that you cannot take shortcuts to growth at the expense of controlling inflation. And Cameron and Osborne are clever enough to learn the other lesson that Blair and Brown taught them: that they have to stick to the Government's spending plans. That means that the levels of taxes, public spending and borrowing are fixed, regardless of which party forms the government after the next election.

So the message of the two speeches was the next stage of the argument that Brown failed to fix the roof when the sun was shining – that taxes might have to go up, rather than down, under a Conservative government. Which is equally true of a Labour government, because the economy is in such a state.

Nor does social policy offer any clear difference between the parties. Cameron attracted admiring notices for his reworking of the "broken society" theme last week. "It is time to talk a new language of social justice, of what is ... right and wrong," said the Leader of the Opposition, as he attacked "our country today" as a society "unfit for any decent person to raise a family and live in". Except that this was Tony Blair, on his election as Labour leader in 1994. Where Blair talked of smacking his children but regretting it, Cameron talked this week of the "naughty step" he uses to keep his four-year-old daughter in line. There is no difference of substance there.

George Osborne thought he was being clever in his speech to point out that youth unemployment is higher now than in 1997. Yet the real significance of that is how persistent the linked problems of worklessness, crime, addiction and chaotic parenting are, and how far both main parties are from solving them.

That the opinion polls are so tilted against the Government on the basis of Cameron's offer of fresh faces and an easy manner rather than policy substance ought to give courage to Brown's enemies – those are the ones on the bench next to him. I suspect that Brown will be forced out by a self-interested Cabinet rebellion by this time next year. David Miliband, the Foreign Secretary, would probably beat Ed Balls, the Children's Secretary, in a leadership election. (Harriet Harman withdrew from the contest with her curious comment, as she stood in for Brown last week, about men leaving the country should she become prime minister.)

But the longer the Cabinet putsch is delayed, the better the prospects of James Purnell, the relaxed (and witty) Work and Pensions Secretary. Purnell is working on those precisely those difficult issues of social exclusion that are central to the Cameron-Osborne critique of Labour's shortcomings. And Purnell, I am told, impresses his selectorate by being the only Cabinet minister to hold a regular surgery for MPs at the Commons to discuss welfare issues from their constituencies.

The danger for the Tories is not that the economic downturn robs them of room for policy manoeuvre, but that their reliance on the personality of their leader leaves them vulnerable to a Labour counter-strike.

John Rentoul is chief political commentator of 'The Independent on Sunday'

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