Alistair Darling's first Budget had become, long before he got to his feet, his credit-crunch Budget. In what he called difficult and uncertain times, with exceptional turbulence still whipping, like winter gales, through global financial markets, this was no time for fiscal aerobatics.

His predecessor frequently preached prudence. For now this Chancellor, like a lot of homeowners with a concerned eye on their slates, craves only stability.

Darling learned a hard lesson last October, after his first pre-Budget report. Then, with election fever raging, egged on by his mentor Gordon Brown, he indulged in some shameless policy theft from the Conservatives over inheritance tax and the tax status of 115,000 non-doms in the UK.

But that attempt to spike Tory guns backfired, as ill-thought-out proposals on that and a simplification of the capital gains tax regime began to unravel.

Now a dose of cautious consolidation - arguably Alistair Darling's habitat of choice - was called for. And that's what we got. Yes there were hints of a bigger political purpose.

Higher taxes on drinkers and the Chelsea tractor brigade offset by some modest giveaways to combat child poverty and help pensioners worried by their soaring winter fuel bills.

However, in terms of overall impact, this first Darling Budget was so stable it reportedly sent some MPs to sleep.

In the tax year from April it gives away, net, a mere £140m. By year three (2010/11), when new motoring taxes really kick in, the additional take rises to nearly £1.9bn. But that is not the whole story.

Gordon Brown's last Budget contained significant changes to both personal and corporate taxes which only come into effect this April.

And Darling's own rejigged pre-Budget report reforms also kick in.

Add them to what he announced yesterday and the overall impact of this give-something-now, take-a-lot-more-back-later approach crystallises.

The overall giveaway in 2008/09 is £1.8bn, not £140m. In 2009/10 the giveaway is smaller, but still comes in at £805m.

However, by 2010/11, by which time the next Westminster election will be history, the exchequer will enjoy a whopping £5.13bn in additional yield from all these measures.

Political cynics, who see only electoral positioning in every move every Chancellor makes, will doubtless draw conclusions on what this tells us about the timing of the next General Election.

Darling has again revised his growth forecast for this year downwards, to between 1.75% and 2.25%. That's a bit more upbeat than most other observers. But it's the bottom end of that range that the Treasury has used in compiling the rest of its forecasts. It may be closer to the prevailing consensus than it appears.

So, assuming the global credit crunch shows signs of easing by this autumn, and the UK economy does indeed avoid recession and grows that little bit faster than its main competitors, the odds on a poll in spring 2009 must be shortening.

But the architecture of this fiscal package still leaves the door open for an alternative bite at the electoral cherry later on, before that £5bn of tax rises really hits the electorate where it hurts, in its collective pocket.

David Cameron suggested yesterday that this wasn't so much a credit crunch Budget as a government facing a credibility crunch.

It's the kind of soundbite politicians can't resist. But it doesn't really help the rest of us understand the underlying political dynamic.

The Chancellor spent much the early part of his speech contrasting, favourably of course, Labour's record since 1997 on debt and borrowing, inflation and public investment, with what had gone before.

In essence, he argued, the UK had gone from being one of the most unstable economies in the G7 to the most stable.

The Tory leader countered with a lengthy charge sheet - the highest taxes in history, the highest deficit in western Europe, the highest interest rates in the G7.

He's plain wrong on the first charge. Taxes (as a share of GDP) were higher in the early Thatcher years. But there's a bigger message for both sides.

It is not at all clear that either of them can squeeze much electoral advantage from trading their economic competence in this way.

What will surely matter more in the public mind is where we are now, in economic terms, and which party is better equipped to lead us to a better place. Were the Tories to win the next election, they would have to start from where Labour left off.

And that is now a particularly challenging place. How, for instance, does this Chancellor, in his credit-crunch Budget, hope to fund that £2.6bn giveaway in years one and two.

While the credit crunch has left banks much more reluctant to lend to each other, bringing some credit markets to a virtual standstill, governments can always borrow.

And this government has decided, yet again, to let borrowing take the strain.

In his speech, Alistair Darling was keen to point out that, this year (2007/08), both net borrowing and government debt are lower than even he was expecting last October.

That was thanks to very buoyant tax revenues in January. But even the Treasury now concedes that a slowing economy is bound to generate lower tax receipts ahead.

Led by income tax, VAT and stamp duty, reflecting current troubles on the High Street and in the housing market, receipts in 2008/09 are now officially expected to be £5.8bn below what was projected just six months ago.

So borrowing really does have to take the strain. Last March, Gordon Brown was projecting net debt at £30bn next year.

In October, Alistair Darling upped that to £36bn. Now he has pencilled in borrowing of £43bn in 2008/09.

On these latest forecasts UK net borrowing won't now get back below £30bn until 2011/12.

As a result Brown's sustainable investment rule, if it survives at all, will be honoured by the thinnest whisker.

Net debt, as a share of GDP, is now projected to rise to 39.8% of GDP by 2010/11 and will still stand at 39.3% by 2012/13. The fiscal rule ceiling is 40%. And these numbers exclude the impact of nationalising Northern Rock.

So without further substantial tax rises, the scope for new public spending is severely constrained.

In its first decade in power Labour increased public spending, in real terms, by an average of 3.6% a year. The current three-year settlements sees overall spending rise by 2.2% a year.

The Chancellor indicated yesterday that, beyond that, ministers are now assuming real growth in spending will be no more than 1.9% a year.

If services such as health continue to be ring-fenced for more generous treatment, other spending departments will be squeezed very hard.

And given that £5bn rise in the tax burden already scheduled for 2010/11, it's little surprise Brown and Darling are still banging the drum about radical public sector reform.

But if they are as boxed in as they now appear to be, a Cameron government would inherit these same fiscal constraints.

The Tory leader can jibe all he likes about Britain being in a league of debt with Hungary, Pakistan and Egypt. But that's where he would find himself too.

He'll only get a chance to do something about if he convinces the electorate he has the key to that better place.

Unlike his predecessor in 1997, promising more of the same may not deliver the goods.