Alistair Darling's big gamble is not simply that an injection of £20bn can make the recession shallower and shorter. It is that the economy will recover strongly and swiftly enough to allow him - or his successor - to pay back not just that £20bn but the eye wateringly large half a trillion pounds of borrowing which he revealed today.
What's more the chancellor's bet depends on other risky gambles. First, that the British economy will recover as early as the second half of next year. Second, that the government can deliver a major clampdown on spending and huge efficiency savings. Thirdly, that the electorate will support a significant rise in taxes targeted at the wealthy but which will hit those on middle incomes too.
As such this represents a break with both the economics and the politics of the last 30 years - a break with the caution that led Labour's Prime Minister Jim Callaghan to say that you could not spend your way out of a recession and Tony Blair to shy away from increasing income tax.
If Mr Darling wins his bet the economy will be boosted and Labour will dare to dream of winning a record fourth term. If he loses, we will all be paying much much more for a long long time but it will be a Tory not a Labour chancellor who's sending out the bills
The cut in the chancellor's growth forecast for next year - minus 0.75% to minus 1.25% - is the biggest ever made by the Treasury.
However, his growth forecasts for the future are very optimistic predicting a return to growth in the last two quarters of next year and significant growth in 2010. Conveniently, this significantly improves his forecast for borrowing.
UPDATE, 03:52 PM: Wow.
Alistair Darling has admitted that borrowing will increase to a higher level than it did after the recession in the early 1990s and that national debt will increase every year until 2015. And this, remember, is based on his optimistic forecasts. If he's wrong about a swift return to strong growth borrowing and debt will be much worse.
UPDATE, 04:10 PM: Sure enough. The chancellor has announced a series of other tax changes to pay to fill the budget black hole.
National Insurance will go up in 2011 for all but it's those earning over £100k who will suffer the most.
Not only will there be a new 45p tax rate for those earning over £150k, there will also be a smaller personal tax allowance for those earning between £100k and £140k and no personal allowance above that figure. That's a major tax rise for the wealthy and may make quite a few people ask not to have their pay go above £140k.
Combined with the announcement of an increase in the personal allowance for 22 million basic rate tax payers, it's now clear what Mr Darling means by fairness.
UPDATE, 04:49 PM: The Treasury red book spells out the cost of those income tax and NICs rises.
It says that in 2011
* those on incomes over £40k and below £100k will pay on average £156/year more
* those on incomes over £100k and below £140k will pay on average £1044/year more
* those on incomes over £140k and below £200k will pay on average £3168/year more
UPDATE, 05:29 PM: Apologies I got the last two figures wrong, it should be:
* those on incomes over £100k and below £140k will pay on average £1144/year more
* those on incomes over £140k and below £200k will pay on average £3172/year more
The word from the Treasury is that no-one will be able to talk about a tax bombshell after Alistair Darling sits down as he "will have told you everything". In other words, the claim is, that there'll be no nasty surprises lurking in the pre-Budget report small print or in a secret Labour manifesto to be revealed after the next election.
It's quite a claim and it means that Alistair Darling will have to spell out significant tax rises above and beyond the new top rate of tax and a major spending squeeze as well. It would be a break not just from Labour's past but that of previous Tory governments too.
John Major and Norman Lamont ran a campaign promising tax cuts in 1992 before embarking on massive tax rises in 1993 and 1994. Gordon Brown consistently denied the need for tax rises to pay for the NHS in the 2001 election and then put up National Insurance soon afterwards. Incidentally, I'm pretty sure that that's where he'll find the money this time too - in a rise in NIC for everyone but an extra rise for richer people too.
So, why you may wonder should this government be candid now?
One good reason is to try to put the Tories on the spot. Labour's message will be - in effect - we're pledged to squeeze spending as much as it is safe to do so and we've raised taxes on those who can afford it most. Then they'll demand to know - what would you do - protect the rich and hit the poor?
In other words, a tax and spending time bomb rather than a tax bombshell.
The pledge not to raise income tax rates was one of the foundations of New Labour. It was designed to reassure voters that the party had no desire to punish those who worked hard, however much they earned. It is to be ditched in the name of fairness when a new 45p tax rate is created for those earning £150 000 a year or perhaps even more.
The sum raised - £2bn or £3bn - is relatively small but the political significance of the move is vast. It will come on a day when the Chancellor delivers not one but, in effect, two budgets both equally dramatic. The first - a crisis budget - will spend billions to get customers and the government itself spending in an effort to get the economy moving again. The second - which won't be felt in full until 2010 or 2011 - is to fill the budget black hole that can simply no longer be ignored by ministers.
Alistair Darling will say that the income tax rise will only happen if Labour is re-elected protecting him from charges of breaking the party's last election manifesto and, in effect, writing the first sentence of the next one.
