Today Mr Prudence gambles it all on red

It is a gamble because no one can know whether it will work, and while the costs are obvious, the benefits have to be taken on trust. The costs are a further weakening of the Government's already shaky hold on financial reality - a further increase in the gap between what the Government spends and what it manages to raise in taxes - and at the very least an increase in the public-sector debt to be borne by future generations.This year government will borrow £78 billion, next year £118 billion and it will be 2015 before the budget comes back into balance.

Critics led by David Cameron argue that this verges on the reckless when set against Britain's current debt position. It leaves far behind the levels of prudence laid down by the European stability pact for government borrowing. Indeed, as the deficit soars, it will exceed the seven per cent of gross national product last seen in the depths of John Major's 1990s recession, or when the IMF was called in back in 1976. Who in the past 15 years dreamed for a moment that we would ever see such days again?

To work, the measures will have to mitigate the economic downturn so that more people stay in work, fewer businesses go bust, people successfully hold on to their homes and the fabric of society holds together. Whether it works depends on if it boosts confidence enough so that banks, businesses and consumers all start behaving more normally and start believing once again that the future will be more prosperous than the past.

This is a tall order - so tall, in fact, that to ask whether it will work is to ask the wrong question. Clearly, all those pessimists are not going to change into optimists overnight. As the economy slows down there will still be insolvencies, foreclosures, bankruptcy, unemployment and probably the odd riot. But the hope is these will be far fewer than they would have been without the measures.

The case for government action is that the problems in international trade and banking are sucking billions out of the economy and forcing it to contract. Confidence is so low that cutting interest rates will not work in time and there is no one else in a position to step in to keep the economic wheels turning. The risk of doing nothing and watching the engine seize up is far greater than the risk of playing fast and loose with the public finances and running the risk that it might overheat. This is why it is so difficult for the Tories to oppose.

So the question should be not whether it will work, because something is surely better than nothing. The more damaging question for Labour, in fact, is whether it is enough. A stimulus of £20 billion over this year and next amounts to one per cent of GDP per year - but how many people will change their behaviour because £1 in their pockets becomes 101p? And if people save rather than spend, then the measures will fail. There are many economists, including plenty in the City who are no fans of big government, who say the stimulus needs to be four per cent of GDP - or in excess of £60 billion - to be properly effective.

This is where the Government could be embarrassed. A bigger stimulus is out of the question because under Gordon Brown the public finances have become so weak that such a move would blow the remaining faith in the credit-worthiness of the country out of the window and cause the pound to collapse. That could be a prelude to our following Iceland into bankruptcy or another IMF rescue. His past profligacy has imposed a very real constraint on the Government's freedom of action now, however loath Gordon Brown is to admit it.

Rather than nail him on this, though, the Tories have been sucked into a debate about whether even £20 billion is affordable. This is irrelevant: no voter believes the country will go bankrupt over 1p in the pound - and even an extra £78 billion of total new borrowing is not an impossible sum when set against existing levels of government debt and a total GDP of about £1,500 billion.

There may be much huffing and puffing about the debt burden on future generations but the fact is future generations won't notice - any more than we notice that we are still paying for the debts run up by government to pay for the First and Second World Wars.

Gordon Brown is betting that they will notice even less if spending this money now pulls the economy out of its nosedive. It will still have to be paid for, which is why the Tories are warning about a tax bombshell in the future, and why Darling announced a 0.5 per cent increase in National Insurance from 2011 and a new top rate of income tax of 45 per cent for those on more than £150,000 - reminding the voters that all this seeming largesse comes at a price.

However, the bigger Tory problem is more likely to be that the bulk of taxpayers will benefit from the immediate measures, most of which will also be politically popular because they strike a chord of fairness. Such targeted measures are economically as well as politically smart because the quickest and easiest way to boost an economy is to put money in the hands of poor people, who spend it quickly, whereas richer people may put it in the bank or go on holiday.

Similarly, making permanent and increasing the rebate to compensate for the ending of the 10p tax band puts money in the pockets of those most likely to spend immediately. The 13-month long VAT cut of 2.5 per cent complicates life for retailers but should also work and if together they kick-start Christmas spending, they may save some of the high street from going bust in January.

The root of the Tories' problem is that the economic crisis has turned everything on its head and many of the traditional emotional touchstones simply don't work any more. The Prime Minister is clearly relishing this. Thus, in a speech to the CBI this morning, he said that "extraordinary times require extraordinary actions" and that this "was not the time to become prisoners of the old dogmas of the past".

That may be true but one of those old dogmas, which he used to preach, was the virtue of fiscal prudence. The Tory challenge is to get him to admit that he has nothing to put in its place.