This week the Opposition seized control. Right through until the general election, and starting with the party conference season that opens with the Liberal Democrats in Bournemouth today, the most gripping of our national debates will take place by opposition demand, in opposition territory, and on opposition terms. From this weekend onwards, the Opposition have the ball.

Earlier this year, to many raised eyebrows, including some within his own Cabinet, Gordon Brown created a dividing line with the Opposition. The crucial question at the election, he said, would be “Labour investment versus Tory cuts”. He placed his personal judgment and authority firmly on the investment side of that divide.

This week the Prime Minister has shuffled over his own dividing line and joined the Conservatives, the Liberal Democrats and almost all political and economic commentary. Crossing to the other side, he has fallen in with the very consensus he had pledged his party to reject.

It would be difficult to overstate the significance of this capitulation. The debate will not, after all, be about whether to cut public spending, but about where and how. It is the debate the Opposition has for 12 months been demanding. Mr Brown starts now from the impossible position of trying to argue that he will be best fitted to do what until last week he had been adamant should not be done. But that’s his fault.

And the indignity of his eleventh-hour capitulation to reality has been made worse by Thursday’s shocking update on that reality. The deterioration in our finances is snowballing.

The Office for National Statistics reports an imbalance between spending and revenue of unprecedented and unsustainable proportions — far worse than many thought or Brown’s demented Candy Mountain rhetoric implied. His characterisation of opposition warnings as alarmist and verging on the unpatriotic now appears to verge, itself, on treason. Opposition alarm has been, if anything, understated.

To add to Labour’s difficulties it has emerged from a Treasury leak this week (one might fantasise that the much-put-upon Chancellor himself is the Tory mole) that Mr Brown had been advised all along that spending would have to be cut, on almost exactly the scale and within almost exactly the timescale that the Opposition had been insisting, even as he poured scorn and indignation on their argument. Moreover the Treasury had been making plans accordingly. So not only Mr Brown’s capability but his integrity have been called into question. And that’s his fault too.

Let’s talk about fault and credit. George Osborne’s speech to his party conference in Birmingham a year ago merits re-reading. That speech, together with David Cameron’s grim emergency opening statement warning of the severity of the crisis, stands out amid the routine noise that most political conferences are composed of. There are speeches that make an immediate splash, then sink, unremembered. Mr Osborne’s made no great splash, but seems to resurface, and with added cogency, whenever we update the economic news. It looks truer and truer as the months pass.

At the time what he said was arguable but unproven. The Shadow Chancellor stuck his neck out early in the domestic and international financial crisis by placing the Conservative Party firmly on the side of fiscal caution at a time when many, in the business and financial community, were saying that refloating the economy was the overriding priority, and paying the money back a problem for the fairly distant future.

Not so, said Mr Osborne (to some pinstriped sneers from the rentier Right as well as the public sector Left): unsustainable debt is a problem now. For the Lib Dems, Vince Cable was sounding some similar warnings with equal clarity and courage.

For both men the courage was political as well as intellectual. Even from those who knew that Mr Osborne was correct about debt, he and his party leader risked (and heard) rebuke.

“The Opposition’s job is to oppose,” ran the complaint. “Why volunteer to be lightning conductors for public anxiety about cuts in public spending? The problem may be of Labour’s making, but the solution will be unpopular, whoever recommends it. Let Labour propose such things when, in the end, they’re forced to — and face the music. This argument will win without your help. Keep your heads down until it has. You aren’t the Government: why take the flak?”

It is among the more bizarre truths of recent history that the most slavish practitioner of this broken-backed doctrine of opposition has been the Prime Minister. It remains an open question, both among human psychiatrists and tropical zoologists, whether the creature that inserts its head determinedly into the sand does so because at some deep and awful level it knows the truth, or because it is blithely oblivious to the truth.

Whatever the answer, it’s weirdly clear that Gordon Brown has been waiting for the truth, but seen no need to anticipate its arrival.

Well, there have always been those who advise that it’s no part of political leadership to go out among the populace with a chalk and blackboard and instruct, but that events will teach all necessary lessons; and the wise politician seeks out a hole and hides there until they do. And in opposition, if not in government, that’s not always wrong. Certainly there’s little to be achieved by railing against the spirit of your times years before your idea’s time has come. Now that it has, the Conservatives deserve credit for recommending the obvious before it was obvious. It means — or ought to — that we will listen to them next time too.

And it means — or ought to — that the City and banking voices who whisper to Conservative journalists that Mr Osborne is some kind of a political pipsqueak who has failed to treat their opinions and expertise with the deference their grotesque remunerations deserve should be sent packing. They and their like got us into this mess. Where were they when the conventional wisdom was that the bubble would never burst? Helping to puff it further. Why should we listen to them now? Maybe Mr Osborne should try harder to conceal his impatience with their special pleading; but heaven forbid that he should revise it.

Like my sketchwriting Times colleague Ann Treneman, if I hear anyone repeat again that Labour didn’t fix the roof when the sun was shining, I shall scream. But, if not repeated, the cliché can be updated. Gordon Brown failed to call in the roofers even when the rain came and the water was pouring through. This week we learnt that Mr Brown too is thumbing through the Yellow Pages.

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