With trademark grandiosity, Alex Salmond unveiled his white paper on independence this week as if he had retrieved it from the top of Mount Sinai. ‘This is the most comprehensive blueprint for an independent country ever published,’ proclaimed the First Minister. It was yet another reminder of an inexorable law of politics: the larger the document, the weaker the content. The American declaration of independence managed to fit on a page. The SNP’s plan for a separate Scotland is so bald that it needs to conceal its nothingness with 650 pages of flannel.

You can look in vain in its pages for any sign of any policy that will make Scotland a better place. There will be more welfare, universal childcare, milk and honey. And income tax will not rise. To read the document is to realise the extent of the nationalist fantasy. It does not explain how any of Scotland’s (many) problems can be remedied by granting more powers to its Edinburgh political establishment. Opinion polls show Scots (especially the young) are deeply unpersuaded. The average result of polls this year so far has shown 33 per cent ‘yes’ and 50 per cent ‘no’. It was similar last year. The SNP’s position looks irretrievable.

And yet the mood of many Scottish Unionists is sombre — in some cases despairing. They want to save their country, and keep Britain strong. Yet it is no longer Alex Salmond they fear but David Cameron. The skeleton haunting the impending Unionist victory feast is so-called ‘Devo Plus’, or ‘Devo Max’ — the extension by the Westminster government to Scotland of sweeping new devolved powers, possibly covering everything except defence, foreign affairs and monetary policy. Devo Max is so close to the SNP’s ambitions it was a constitutional option in its 2007 white paper.

Incredibly, some such Devo Max settlement is now the official policy of the Conservative and Unionist party — the irony of that title is not lost on Toryism’s supporters north of the border, though many must now be described as erstwhile supporters. They have been betrayed too many times to retain their allegiance to the Conservative party.

The history of Tory betrayal of Scottish Unionism goes back to 1968 when Edward Heath, without consulting his Scottish party, delivered the ‘Declaration of Perth’, committing the Conservatives to devolution. It was the first of the Tory ‘modernisers’ who sold out the Union by going high-handedly over the heads of his Scottish supporters; the similar conduct of David Cameron today has a resonance of that initial betrayal. At that time it was Labour that defended the status quo. By the time Margaret Thatcher ended the Tory commitment to devolution Labour had changed tack; the devolutionist genie was out of the bottle and it was Labour that eventually delivered a Scottish parliament via a referendum in 1997.

The conventional wisdom then was expressed by George (now Lord) Robertson when he declared that devolution would ‘kill nationalism stone dead’. Instead, Scottish Unionism is confronting the possibility of the biggest constitutional upheaval since 1707 — more accurately since 1603. Only the common sense of Scottish voters seems likely to avert that catastrophe.

The so-called Unionist parties’ response to the SNP challenge has been one of abject cowardice. When Alex Salmond led a minority government they were too craven to call his bluff and use their parliamentary majority to hold a referendum, which would have buried separatism for a generation. After the SNP won an overall majority at the Scottish election in 2011, the UK government’s response was to pass the Scotland Act conceding more new powers (over income tax, stamp duty and land tax) to Holyrood.

By March this year, the Scottish Conservatives were launching a drive for Devo Plus, or possibly Devo Max, clearly on the instructions of David Cameron and George Osborne. It is the Chancellor, moonlighting as Tory modernism’s political strategist, who is driving the lemming stampede towards extravagant devolution. That is alarming on another front: a Chancellor of the Exchequer who does not realise that two fiscal systems are incompatible with the integrity of a unitary state does not inspire confidence. The additional absurdity of this made-in-Downing-Street devolution plan is that the one victory over the SNP that David Cameron could previously claim was excluding a second question on Devo Plus from the referendum ballot paper.

The canard that tax-raising powers would make the Scottish parliament ‘accountable’ is discredited by the history of local authority taxation in Scotland. Before the 2008 economic slump, out of four million Scottish electors only 2.3 million paid income tax. That figure is unlikely to have increased in the current economic climate. The insanity that is Devo Plus would allow spendthrift socialists to bleed wealth creators in the interests of their client constituency. Now Dave and George are bringing Devo Plus back from the dead and, in doing so, throwing a lifeline to Alex Salmond.

With Devo Plus excluded from the ballot paper, voters’ minds should have been focused on the stark alternatives: the status quo or unaffordable independence. Defeat would have left Alex Salmond on the ropes, his party split amid bitter recriminations, his leadership untenable. Instead, after losing the referendum he will be able to tell his supporters: ‘I am sorry I could not deliver full sovereign independence — that was always a mountain to climb — but I have secured from our frightened opponents 90 per cent of what we want: de facto independence, minus flag and anthem.’

The coming defeat of Salmond should be the time to strengthen Britain. How tragic that, instead, a frightened Cameron wishes to throw Scottish Unionists to the wolves.