This autumn the Labour Party has embarked upon a strategic blunder. The strategy is driven by Gordon Brown's personal instinct: an impulse, an idea, so devilishly clever that even Peter Mandelson must be impressed by the cunning. Reader, get a load of this...

Labour has decided to unmask the Conservative Party... as a conservative party; to expose the Tories as secret Tories; and in a daring strategy calculated to astonish the nation, to track the Centre Right straight to its ideological lair and tell the world its precise location. On the centre right! Crikey. The Tories had better not let that get out. You mean they think people should be encouraged to do more for themselves? That Government spends too much? You can almost hear the electorate's collective gasp. Horrors! Pass it on. The Tories don't like high taxes!

Let Bromley hear the truth, let Croydon be told: David Cameron thinks a lot of public expenditure is wasted! Let Solihull and Harrogate delude themselves no longer: Tories aren't wild about the EU! And let the scales fall from the eyes of Brighton, Pembroke and Chester: Conservatives think that in a crisis even needed state spending may have to be reined back. Take the message to Billericay, to Essex man and Worcester woman: Tories are sceptical about the public sector's efficiency. Broadcast to self-employed plumbers, minicab drivers and small businesses across the nation: Tories are worried about the operation of the Human Rights Act! Leave readers of the Daily Mail in no doubt: the Principal Opposition are Conservatives.

Quake, England! Flinch, Wales! The sage of Dunfermline has seen a vision: at the next election the party challenging him and his economic record will be Tory, share Tory instincts and promise Tory things.

In the political equivalent of a stage whisper, Downing Street's communications people are spreading what is, for them, a surprisingly focused message. It is that after two years building his image as the captain of a changed party, a nice party, David Cameron is now reverting to type: nasty leader of a nasty party. Tory criticism of public spending is seized on as evidence of the cuts they plan. The Tory promise on inheritance tax (that Labour matched it is suddenly forgotten) is denounced as proof that Tories favour the rich. Their thoughts on immigration (also aped by Labour) are proof of hidden racism.

“Don't be fooled by all this ‘compassionate Conservatism'” is the message: “The Tories haven't changed.” There's one big problem for Labour in launching such a propaganda campaign, at this of all moments. It dovetails beautifully with David Cameron's own. He's working flat out to remind voters that, for all the PR gloss and “hello birds, hello trees, hello flowers” stuff of the fat years of 2006-07, the Conservative Party has not ditched its essential beliefs, especially in the bigger v smaller State debate, and in economics and welfare. Generosity was the watchword of Mr Cameron's first big push to project himself and his party, and it was so successful that it left some voters confused about what the party stood for.

Austerity, Mr Cameron now senses, must be part of the next campaign: a second stage that cannot but sound like a change of mood and even of message. So there's a clatter as the train switches tracks, and it does jar - you could hear it on the Today programme yesterday as Mr Cameron's Shadow Chancellor, George Osborne, struggled to remind us of when precisely it was that the Opposition warned us that the fat years were over - but Mr Cameron and Mr Osborne are reconciled to that. They believe that in the economic mess we're in, the electorate will be reconciled to the change of music too. They want the change to be noticed.

And in wades Downing Street, right on cue, to tell Britain that a Conservative government would be recognisably Tory. Well, praise be!

Some voters were beginning to wonder. For the Conservative Party, Labour's demon-mask line of attack is a heaven-sent distraction from some very serious questions about the busy but still par-baked opposition manifesto that seems to be emerging. Does it add up?

How will the charities and the “third sector” shoulder a heavy share of social provision? What is all this nonsense about directly elected police chiefs? How credible are the promised savings from cutting waste? Why so timid on the NHS? Can inflation-proofed public sector pensions be afforded, and are they fair? Who's paying for the implicit promises in Liam Fox's defence drum-beating? Is this really the time to proceed with a Trident replacement programme: surely it can be postponed? How about an Austerity Olympics? And - be serious - nuclear energy has to be part of the carbon-reduced solution, doesn't it? Sooner or later these questions will have to be asked, but if Downing Street wants to distract us from serious criticism by prancing around pointing pantomime fingers at the Tory leader and shrieking “He's a Tory! He's behind you!” - then that's just fine by Mr Cameron.

In the present crisis, both presentationally and in substance, the Opposition leader has not one but two tightropes to negotiate, and Mr Cameron's speech yesterday moved him deftly but quite sharply forward on both.

He needs (as I've suggested above) to darken the backdrop from waving trees and scudding clouds, and to toughen up the Opposition's ideological profile. Yesterday's speech contained more uses of the term “Centre Right” than I've ever heard from Mr Cameron: indeed I do not remember his using it at all until recently. But he still needs to protect his energetically promoted aura of modern tolerance and personal generosity. He should succeed in keeping the balance, not least because it's true: Cameron really is a modern and un-bitter person; but he really is an instinctive Tory too.

The second line is even harder to walk. He must avoid being sucked into a knee-deep treacle of bipartisan “national” politics - yet at the same time avoid the name-calling and pie-chucking that the British public so detest. His speech yesterday gave sharp notice of his determination not to pull punches, without sounding too gleefully adversarial.

From the Brown camp, Yvette Cooper's response - to accuse Mr Cameron of “juvenile political games” - was itself juvenile. Mr Cameron needs no lessons in bipartisanship from Mr Brown or Labour, who gave John Major hell over Black Wednesday (though they had supported the policies leading to it) and (while professing pro-Europeanism) tried to wreck the Maastricht treaty, even attacking the Liberal Democrats for being bipartisan.

Today's Tories should rejoice in the differences between themselves and Labour. Their 1997 campaign depicting Tony Blair as a socialist demon in a mask of moderation hinged on a national dread of the far Left - and didn't work, anyway. Cameron Conservatism will never inspire that dread.

Credibility, not demonology, should be Labour's pitch.

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