IT'S passing strange.

All of a sudden, David Cameron is opposed to fiscally-responsible government. George Osborne no longer wants to crack down on those living on hand-outs. Nick Clegg is disowning a policy Liberals have pursued for a century. And Ed Miliband disdains one of progressive Labour's oldest dreams.

Stranger still, they are all, truly, in this together, "100%", as Mr Cameron says. Not even a flimsy sheet separates these bedfellows. Not one of these professional politicians, veterans of focus groups and polling analysis, can be found to embrace a demonstrably popular idea liable to solve an otherwise intractable problem. Instead, they mean to fight it with every means at their disposal. This is beyond mysterious.

There will be only one mention of "devo max" in this article. That was it. I'm with Canon Kenyon Wright. I refuse to use a phrase better associated with a soft drink. Besides, even in its literate form the term is misleading. In the context of Scotland, a referendum, and the SNP's pursuit of a form of independence, it's better to talk of federalism, or of maximum autonomy within the UK. Better still, let's just call it the Other Idea.

No-one has yet attempted to define it fully, after all. Nor shall I. This isn't the place, mercifully, to talk much about corporation tax regimes, or the reorganisation of Her Majesty's Revenue & Customs. Whether maximum autonomy would force an answer to the West Lothian question is also an issue for another day. The other idea is still a rough idea.

But we all know roughly what it means: the money (most of it) raised in Scotland would stay in Scotland. Westminster would retain control of defence, foreign policy and a couple of other things yet to be decided. It's the logical refinement to home rule and an alternative to "full" – unless you're a picky republican – independence.

We know something else about the other idea. The Westminster Government and Opposition are united – implacably, it seems – against it. Their entire efforts are now directed towards keeping it off the referendum menu.

You could say that stands to reason. The other idea has been defined, these past few days, as Alex Salmond's "consolation prize". If he can't win independence – and the polls still say he can't – the First Minister will settle for the alternative without skipping a beat. In most things that matter, he would have the benefits of independent government without the difficulties.

Fiscal autonomy without monetary responsibility would allow him, if the mood struck, to run up deficits safe in the knowledge that Scotland was backed by the pound and underwritten by the Bank of England. Fiscal autonomy would allow him to compete over corporation tax. And does anyone really believe that Mr Salmond would shut up about foreign policy in the event of another British war?

The other idea would also demonstrate to Mr Cameron what his celebrated "neverendum" joke really means. Oh, how they laughed, in Westminster. They thought it had something to do with the SNP not really wanting a proper referendum. Wrong.

The origins are Canadian, a reference to the old habit of the Bloc Quebecois of never accepting a plebiscite result as definitive. Lose one referendum, demand another: neverendum. Mr Salmond would use the other idea for the same purpose, it is said, as just one more staging post.

But why should that matter? A threat would only arise were Mr Salmond to succeed through fiscal autonomy. Isn't it central to Unionist doctrine that such a thing is utterly impossible? As Mr Cameron's back-benchers never tire of saying, we are mere subsidy junkies, whingeing spongers. The core thesis is that Scotland cannot survive, far less succeed economically, without England.

So why not just let Mr Salmond discredit himself once and for all? Let him have the purse strings for a while, and let the world see how he fares. If the Unionist thesis is correct, it would cost England nothing. Indeed, given "the well-known fact" that the Scots take more from the Treasury than they give, money would be saved for the poor old English taxpayer.

Better still, from a Tory perspective, Holyrood would be taught short, sharp lessons in economics and responsible politics. Deluded Scottish voters would meanwhile be given a salutary shock. This, Messrs Cameron, Miliband and Clegg could say, is what independence really means.

Speaking as one voice, they are saying exactly the opposite. They demand clarity in a referendum, but in this they are anything but clear. Mr Clegg's is the proudly federalist party. Mr Miliband's is the party that gave Scotland devolution, and the right to manage its own affairs. Mr Cameron leads people who abhor dependency and extol self-reliance.

The Prime Minister, moreover, seems prepared to gamble on the opinion polls, and to do so for – the latest cliché – "the next 1000 days". Those continue to say that Mr Salmond cannot win a straight vote on independence. The idea, then, is to cut off the First Minister's escape route, and leave him with an impossible task.

If that's the case, Mr Cameron hasn't been paying attention. It was supposed to be "impossible" for the SNP to achieve majority government by a landslide last May. Yet here's Dave, with a straightforward opportunity to save the Union, at no apparent cost to the English taxpayer, preparing to take that risk. And the other London leaders are right behind him.

What's their problem? Bluntly, what exactly are they afraid of? Kenyon Wright made a typically eloquent case yesterday against the abuse of democratic principle and the disenfranchising of a large number of Scots. That's the consequence, but not the mysterious motive. Who, in this sort of fight, turns their backs on numerous voters capable of being recruited to halt the independence movement?

The usual economic numbers are hotly contested. In a favourite SNP example, the UK spent £54 billion of "identifiable" money in Scotland in 2008-2009 while the Treasury received only £43.5 billion. A deficit, to be sure. But throw in North Sea revenues of £11.7 billion and Scotland was running a nice little surplus.

You can't depend on the volatile price of a wasting asset, cry Unionists. Tell that to other oil-rich countries, say Nationalists, and then show us when the price of a diminishing commodity is liable to fall. Tell it to Mr Osborne, too, after his raid on North Sea taxes. Then add the seabed revenues of the Crown Estate, actual and potential, while Scotland remains Britain's biggest, and growing, producer of renewable energy.

My own view, for what it's worth, is that neither bankruptcy nor bonanza would follow from independence, or from the other idea. We'd be OK, if justice were done. That's an opinion, not a slogan.

But remember: Mr Osborne is alleged to be the tactical mastermind behind the Coalition's crusade. One presumes the Chancellor has a grasp of the numbers, and of economic potential. The only rational conclusion, therefore, and the only explanation for the determination to thwart the other idea, is that he knows what Scotland could become. This is about control of energy, from whatever source, and perhaps, ultimately, about energy security.

The other idea is not my idea, or my preference. Watching the London leaders and their proxies unite against a simple democratic measure tells me all I need to know about the UK in any form. Their interests are not Scotland's interests.