A helpful rule of thumb for assessing the fanfare that habitually precedes a Gordon Brown initiative, is that there will be less to it than meets the eye. Under scrutiny, these things tend to come apart in our hands, shot through with failures of nerve and riddled with second thoughts.

So there is a chance that Monday's Pre-Budget Statement will prove a mouse, and that George Osborne and David Cameron will skid from attacking a monstrous irresponsibility to grumbling that it won't make much difference.

But the heralding - with however much exaggeration - of a big, unfunded tax giveaway has had the effect of flushing out the Conservative front bench. We now know that Mr Cameron and his Shadow Chancellor don't believe that big new borrowing will lift Britain from recession faster. They fear that it may even dig us deeper in.

In switching to this attack, Mr Cameron gambled three times this week. First that a substantial new fiscal stimulus, in the form of tax cuts to be paid for later, really is about to be unveiled. Identifying the Conservative Party firmly with scepticism about the idea, he gambled secondly that the plan will visibly fail. But the third and biggest risk he takes is that “good housekeeping” Tory scepticism will resonate with the country. He did not, after all, have to pin his colours to the mast.

We will know soon whether his first gamble was right. If No11's statement does row back from No10's pre-puffing, Mr Cameron's dire warnings may look off-target; but he can ride that.

On his second gamble - that the plan will fail - I, though no economist, do raise an untutored eyebrow at my colleague Anatole Kaletsky's confidence that Mr Brown really has “become a leader of global stature” with a plan so luminously effective that he can now trace a clear forward path to recovery. There is a Brown swagger that the Opposition is entitled to question.

Is Anatole right to assert so unhesitatingly that “a government that spends and borrows in a recession can usually repay much of this borrowing without raising tax rates, because recovery automatically yields higher revenues and reduces spending on the unemployed”? Doesn't that presuppose that recovery actually happens: fast and big enough to yield these happy outcomes? Can no borrowing be too high in a recession - or why wouldn't chancellors just think of a number and double it? Surely there is a point when the cost of debt undoes the benefits? And isn't that all that Mr Osborne and Mr Cameron are saying?

And even if Anatole is right (and is he?) that it is simply our low interest rates that have caused sterling's fall, does it end debate to remark that by making exports more competitive “the pound's decline is not a problem but a solution”? Then rejoice, Zimbabweans; prepare for boom-time, Iceland. Surely Anatole's argument is too strong. In a steep devaluation there must be a critical gradient beyond which problems grow. Again, isn't that what Mr Osborne is saying?

A Tory can accept Anatole's core Keynesian argument - that when a frightened population locks up its wallets and stashes its savings under the mattress, the State may have to borrow and spend to break the paralysis - yet still insist that there must be limits to this, and that timing matters, or the thing may go off half-cock and then have to be paid for. Isn't this, too, what Mr Osborne is saying?

The Tory leadership thinks that Monday's unfunded package may undermine confidence in our solvency and raise the spectre of higher taxes later. The Government (and Anatole) think that Monday could give the push we need, and that, in due course, the economy should be bowling along so merrily that we can pay for it then. There is a third possibility: that the stimulus will fail, but not because it is unaffordable but because it is too timid. The Brown-Kaletsky logic admits no restraint.

The logic of Tory hostility to an “unfunded” stimulus is confusing too. A “funded” stimulus is undermined as a stimulus - by robbing Peter to pay Paul. Mr Cameron's talk of future cuts in projected extra spending plays nicely to Conservative instincts about state profligacy but sits oddly in a debate about how to get out of recession now.

I suspect that the Tories are not being frank about two key planks beneath their position. The first is that they don't love state spending anyway, recession or not. Secondly, I doubt that Mr Cameron and Mr Osborne really think that, beyond stretcher-bearing for the worst casualties, there is much that a British government can do to reshape a global economic cycle. During Commons exchanges on the G20 meeting this week, John Maples, a senior Conservative, voiced this candidly: “The recession has to take its course... [or] there will not be a solid base for recovery. [Mr Brown] risks endangering that... [and] will leave a legacy of debt and taxes into the future.” To put it crudely, Monday's annoucement may amount to pissing expensively into the wind.

If that's what Mr Osborne thinks, he should say so. Which brings us to the third gamble. Could the new Tory position resonate with voters? Yes. It anticipates a change of weather already sensed: a coming chill of resignation if not despair. People are hunkering down for a period of austerity that they begin to doubt politicians can unwind.

On Monday, expect excited squeaks at giveaways that some still hope will do the trick. The language of “jump start”, “pump prime”, “injection”, “stimulus” and “seed corn” (like Mr Brown's favourite word “invest”) - implants a subliminal suggestion of prudence and good value: not really spending at all; something you can't afford not to do; a little money to (dare we say?) leverage more. In politics nobody seeking a bung calls it subsidy; it is always a one-off, kickstart or tide over.

But if summer comes and still the recession bites, Mr Brown's sorcerer's reputation may dim. With the stimulus spent and still not stimulating; the seed corn eaten, not sprouting; the grind of the pistons as the engine refuses to spark, a Prime Minister hunched over the ignition, still bragging that he knows how to start this thing, could annoy mightily.

Remember, Mr Brown's claims to cosmic leadership rest on what he says his measures will achieve, not on what they have achieved. The boasts will finally grate, and a Conservative message that if he can't whistle up a recovery, at least he should stop running up bills, should feel timely.

That is what the Opposition gamble on. For the moment Labour is incredulous at what it sees as the Tories' strategic blunder. Less spending? They've actually been stupid enough to call for less? Mr Brown, high on sleeplessness, pumped up on global status and intoxicated by one of his intermittent rushes of furious, precarious invincibility, thinks destiny is moving his way.

It is a mirage.

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