You could say that the Conservatives have never had an easier target. All the important economic news - about growth, inflation, the public finances and the rest - in this week's Budget will be somewhere on a spectrum from ominous to bad. Later if not sooner, taxes are going to have to rise.

The best Alistair Darling can probably do is grab a few diversionary headlines. You know the sort of thing: chancellor gets tough on binge drinking; Darling goes green in crackdown on gas guzzlers; energy bosses told to slash soaring fuel bills.

The chancellor, of course, will blame the present economic woes on someone else. No, I do not mean his predecessor in the Treasury, Gordon Brown. I am told there have been some tense moments between chancellor and prime minister in recent months. Not that tense.

The global credit squeeze, Mr Darling will tell us, was an act if not of God then of reckless American bankers. Economies everywhere had been caught up in the storm. This it's-not-my-fault line worked for José Luis Rodríguez Zapatero in Spain's general election at the weekend, so why not for Mr Brown's government?

A few kind souls may indeed give Mr Darling the benefit of the doubt. For all its mishaps, including the fumbling hesitation in the handling of Northern Rock and the general chaos and confusion surrounding the taxation of non-domiciles and changes to capital gains tax, the Treasury can fairly say that it was not the architect of the subprime mortgage crisis.

The weakness lies in the fact that there had been nothing put aside for a rainy day. In spite of a decade of economic growth, the public finances have steadily deteriorated. As the excellent Institute for Fiscal Studies shows, Britain has performed badly in each of the Organisation for Economic Co-operation and Development's league tables of fiscal prudence.

In 1997, Britain held seventh position in the OECD's profligacy ranking. By this year it had moved up to fourth place. Mr Brown's mantra of prudence has been belied by the facts of spending and borrowing.

So we can scarcely blame George Osborne for making hay from the government's predicament. While it is perfectly sensible for Mr Darling to allow public borrowing to take the strain of the economic slowdown during the coming year, he should never have been starting from here.

The other day I spent an hour or two flicking through the official economic forecasts since 2000. The constant in the dozens of pages of tables and charts was that borrowing had always overshot the initial predictions - cumulatively by tens of billions of pounds. Perhaps the errors were born of incompetence, perhaps worse. Either way, they give Mr Osborne plenty of ammunition.

The shadow chancellor should not fire it all at once. He tried that when Mr Darling was finally forced to bow to the inevitable and nationalise Northern Rock. The voters were unimpressed by the overkill.

The bigger problem for Mr Osborne is posed by the question: what next? It is one thing to say that government has wasted a sizeable chunk of the hundreds of billions of extra pounds poured into public services (and the evidence here is mounting). Another to offer a credible alternative. If Mr Darling is trapped in a fiscal straitjacket, so too is Mr Osborne.

As hard as it is for the government to get its tax, spending and borrowing numbers to add up, so too, as the general election approaches, will it be for the Conservatives. Making political capital from Mr Darling's misfortunes will not be enough. Voters more often look forwards than backwards.

Mr Osborne, wisely, has eschewed promises of what he calls upfront tax cuts. In the early years of a Tory government a lower tax burden here would have to be met by increases there. The shadow chancellor has also said that, for the first two years at least, he would hold to the government's spending plans.

If that is the official position, the mood music has been otherwise. Mr Osborne's shadow cabinet colleagues are forever floating the idea that there would be more money for this or that vital service. The armed forces, we hear, are under-resourced. So, too, is the health service. Many Tory MPs, meanwhile, whisper behind their hands that, whatever the position now, once in power the Conservatives would start cutting taxes.

Part of Mr Osborne's answer is that he would cut deep into welfare bills by getting more people into work. Sounds easy? I well remember a Tory social security minister faced with that challenge during the 1990s. To lop just £1bn from the benefit budget, he remarked, was to take £1,000 from each of one million people. Which million were his critics thinking of? As for wielding the axe elsewhere, consider the fuss now being generated by taking £200m from the Post Office.

So far the opposition's nods and winks have escaped much scrutiny, as have the government's very expensive mistakes. The unwelcome reality facing both parties, though, is that the condition of the public finances (albeit dismal rather than dire) looks set to preclude significant tax cuts or spending increases any time soon. Politicians who say otherwise at best are being economical with the truth.

philip.stephens@ft.com