John Rentoul: David Cameron's tax promises can only profit one man: Prime Minister Brown

Gordon is like a hurricane - no one knows where it will hit
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This is the column that likes to bring you answers. The capital of the Solomon Islands is Honiara. The smallest number of given squares in a sudoku that can produce just one solution is 17. The best pop single is "But I'm Different Now" by The Jam. But there is one question that this column cannot answer. That is: how would voters react to a head-to-head contest between Gordon Brown and David Cameron?

This might seem something of a problem for a political column. On the contrary, it is an opportunity, because the reasons why it is so difficult to answer are so intriguing, especially after a week in which Cameron stumbled over tax'n'spend - the central issue of British politics since the dawn of time. Who knows whether, if Brown is installed as prime minister, people will regard his experience as trumped by Cameron's personable manner? Opinion polls tell us many things, such as that Brown's record of delivering economic stability is hugely respected, and that Cameron's appeal runs far ahead of that of his party. But they are imperfect guides to the future because, as John Curtice, the professor of polling wisdom, says, "people are rather bad at predicting their own behaviour".

Certainly, Cameron's strategy and poise have so far been exemplary. Which only makes the embarrassment of last week's Tory tax report the more puzzling. We know what happened. A junior person put it on the Tory party website a day early by mistake. A senior person at Labour HQ noticed and alerted Gordon Brown's office. Ed Balls, the Economic Secretary to the Treasury, was soon on all available media outlets condemning Tory plans to cut public spending by £21bn a year. Incidentally, that shows how fast and focused the Brown/ Labour machine is, at a time when the Blair/Labour apparatus is looking a bit ragged. But the main point is that David Cameron and his shadow chancellor, George Osborne, seemed nonplussed and defensive.

They both repeated that they would not be promising "upfront" tax cuts. Which implied that there might be some tax cuts "down back". Down the back of what: Gordon Brown's sofa? It has been speculated (by me, among others) that Cameron was looking for a fight with the tax-cutting wing of his own party in order to dramatise the change wrought by his leadership. If so, this wasn't it. The impression given was of a pair of small-state Conservatives caught in their dressing room putting on their "Party of the NHS" outfits.

We can see why the report was awkward for them: the tax commission chaired by Lord Forsyth was appointed by Osborne a year ago, when Michael Howard was still Tory leader. Since then, the political world has been turned upside down by Osborne's friend Cameron, and Osborne's flirtation with flat taxes (that is, tax cuts for the rich) looks very last year. Flat taxes are right off the script written by Cameron's other best friend, Steve Hilton, the ideologue of the kinder, gentler Conservatism that is now the core message. The most telling sentence in the Forsyth report was the one saying that environmental taxes were "beyond its remit".

But that does not explain why its accidental early publication should have wrong-footed Cameron. Surely he and Osborne had already decided how to deal with it? Or are there hints here of tension between the two; early signs of Blair-Brown syndrome?

One-nil to Brown, anyway, in the unfriendly match that is the warm-up for the main event. Although it is worth remembering that Brown has not yet formally come through the qualifying stages to guarantee his place in the final. Statistically, there is still a chance that Alan Johnson could run against the Chancellor and win. As Labour MPs returned to Westminster this month, I was told that there are "50 to 60 who are Anybody But Brown", but most of them are hiding in the maquis, waiting and seeing. I am also told that the line peddled by Nick Brown, the Chancellor's unofficial chief whip, has been mildly counter-productive. He has been canvassing MPs, telling them that "the door is closing" if they want to get into the I'm Backing Gordon government job centre.

Nevertheless, the Chancellor is close to carrying all before him. Another Labour MP summed up the mainstream view to me thus: "Gordon is like a hurricane far out to sea. No one knows where it will hit, or how many roofs will be ripped off, or if we will all get through it. But we know it's coming."

Brown as prime minister remains a great whirling unknown at the centre of politics. To take just one important example: who would be his chancellor? If the shape of the next few years is Cameron versus Brown, it will also be Osborne versus... who? The safe choice would be Alistair Darling, the Secretary of State for Trade and Industry, a long-standing ally of Brown's. The aggressive choice would be Ed Balls, who has been the economic half of Brown's brain since 1992. As Balls is not yet in the Cabinet, though, such a striking promotion might be taken as a slight by some of those who are.

The clever choice, therefore, would be David Miliband, the Secretary of State for the Environment, who has already shown an impressive grasp of the climate change issue. Privately, Miliband is modest about his maths. But then, so was Winston Churchill, when Stanley Baldwin summoned him in 1924. Churchill thought he was being made Chancellor of the Duchy of Lancaster, and protested, when told he was being offered the Treasury, that he was no good with figures. "They give you the figures," said Baldwin. Miliband, with his brother, the other Ed, as chief secretary in a Department of Milibands, it should be, then. That would help neutralise Cameron's green advantage, and it would help persuade the doubters that Brown is running a government of all the youthful talents.

But we do not know what Prime Minister Brown would do until he does it. All we do know is that Cameron has not had a comfortable week - that he is capable of making a mess of things. The Conservative message on tax is now terribly confused. Either Cameron has to stick with Labour spending plans, in which case, why would the voters think he would be more competent than Brown in managing them? Or he has to offer tax cuts, in which case he has to say how they would be paid for.

One problem or the other might be soluble. But last week he fell between the two positions and ended up with the worst of both worlds.

It has been a good week for Prime Minister Brown.