Andrew Grice: Labour MPs think Cameron can be beaten. That's why they are in revolt

It was fitting that David Miliband uttered his coded criticism of Gordon Brown through the veil of an attack on the Conservative Party, a tacit admission that the man making the running in British politics is David Cameron.

The Tory leader's success is one of the main drivers of the growing rebellion against Mr Brown inside his own party. His critics, some cabinet ministers included, buy Mr Brown's critique that the Tory leader lacks substance, and wills the same progressive ends as Labour without providing the means. But the moves to oust Mr Brown are gaining momentum not because his enemies believe Labour can't beat Mr Cameron but because they believe it CAN.

They argue that Mr Brown can no longer land punches on the Tories because the voters have stopped listening to him.

I have spoken to several key players in Project Cameron for a series about the Tories in The Independent this week. It is not true, as some suggest, that the Tories have no policies. Oliver Letwin, their policy chief, insists they have more than Tony Blair ahead of the 1997 election and at least as many as Margaret Thatcher before her landmark victory in 1979. It is true that the Tories talk about some policies more than others, but it was ever thus. It is also the case that they often take the soft option of opportunism rather than risk saying what they would actually do. The limitations were shown during the Northern Rock affair and over the abolition of the 10p tax rate. But their evasions would have harmed them in the heat of an election battle. Beneath the Tories' confident exterior, there is an intense debate about whether they should declare more of their hand in an attempt to drive a stake through Labour's heart while the party is on the floor.

Mr Cameron is at the cautious end of the spectrum. Labour allows him to get away with his strategy of allowing the Government to stew in its own juice. He is tempted not to take unnecessary risks. With a poll lead of more than 20 points, you can see why. But there is a recognition in the Tory leadership that it needs to do more to win support for its own agenda, however tempting it might be to cruise to victory on an anti-Labour protest. Some Tories think their "real" lead would shrink to 10 points if an election was imminent.

Mr Cameron knows the media searchlight will turn to his party eventually, even if it is fixed on Labour's turmoil now. More work is required to make the Tory prospectus withstand that scrutiny.

On his busman's holiday in Cornwall, where he seemed to pop up daily in the media, Mr Cameron showed the limitations of a strategy built on mood music. Questioned by a group of BBC Radio 1 listeners, he struggled to explain how the Tories' headline-grabbing "annual limit" on immigration would do nothing to stop Polish workers taking British jobs because it would apply only to migrants from outside the EU. He also found it hard to convince a single mother that his party's backward-looking pledge to "reward marriage through the tax system" would not mean less state support for lone parents.

His exchange with the single mum highlighted another chink in the Tory armour. Shadow ministers admit they have accepted large chunks of Labour's social justice agenda, just as Mr Blair forced his party to swallow much of the Thatcher economic medicine. By fighting on Labour's natural ground, and claiming to be the true progressives in politics, the Tories recognise that Labour's social policy values are in tune with the public mood. They hope to convince people they can heal what they call "the broken society" because Labour's top down, money-is-all prescription has failed. They have a point, especially in relation to the working poor, but may struggle to persuade voters that a bottom-up vision based on voluntary groups could provide a practical substitute for the state intervention needed to ensure nationwide provision.

The Tories have a lot done, but a lot to do, to borrow a Blair slogan. Some Tory backbenchers behave as if the next election is already won. That worries the leadership, which knows otherwise, and is determined that Mr Cameron's confidence does not slip into cockiness. "The polls have changed dramatically in the past year; they can change back," said one member of his inner circle.

The ever-worsening economy adds another layer of uncertainty to the election outcome – and the viability of Tories' programme for government. The ever-growing prospect that Labour will fight the election under a different leader is a bigger threat. The Tory strategy (such as offering a "post-bureaucratic age" and completing the Blair public service reforms) is an anti-Brown one. It might not work against Mr Miliband.

Despite that, the election is now Mr Cameron's to lose rather than Labour's to win. Hence his caution. "We need him to make some mistakes to have a chance," one cabinet minister told me. There is little sign he will do so.

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