Deal or no deal? The parties are actively preparing for talks on the fiscal framework to fail

Answering questions on the process at Holyrood on Thursday, Nicola Sturgeon appeared to be rehearsing the lines she would use if/when it all comes crashing down.

Adding to the sense the SNP is now actively preparing for that moment, Bruce Crawford, convener of the Holyrood committee which must scrutinise the fiscal framework before it can be approved by MSPs, underlined the need for an early deadline.

A strange move, on the face of it, if the SNP really does want to give the talks the best chance of success.

It has long been assumed by the UK Government that the fiscal framework, and with it the Scotland Bill, would be passed without too much difficulty.

But it is time to start thinking the previously unthinkable and to consider what will happen if/when Holyrood's extensive new tax and welfare powers are kicked unceremoniously into the long grass.

First, though, the dispute.

The two governments are deadlocked over how to reduce the Scottish Government's annual block grant to take account of the fact it will raise and retain income tax - worth about £11billion -

if/when the new powers are devolved.

John Swinney favours a mechanism known as "per capita indexation" which would protect Scotland's funding in the likely event its population grows less quickly than England's.

Economists generally agree this would be the most advantageous arrangement for Scotland, potentially worth hundreds of millions of pounds more per year, within a few years, than alternative methods for adjusting the block grant.

It has been claimed, however, it would be unfair on English income taxpayers, who would contribute to services in Scotland to a larger extent than under alternative systems, despite the tax being devolved.

As economist David Bell, of Stirling University, concluded in one of the most thorough analyses of the options, it is almost impossible to square two of the key principles set out by the Smith Commission when it devised the package of new powers: its "no detriment" rule, which says neither government should lose out as a result of devolution itself, and its insistence the arrangements should be fair to taxpayers across the UK.

There are other nuances, lots of them, in fact, in such a complex negotiation.

But it is starting to feel as if the time for subtlety and compromise has passed. The parties are already going to war.

If the talks collapse, Mr Swinney can expect a ferocious backlash from Labour, the Conservatives and the Lib Dems, the parties that have been most enthusiastic about the Scotland Bill.

He will be accused of deliberately scuppering the deal, of making unrealistic demands, of preferring a war with Westminster to the hard reality of using the new powers.

Opponents will also highlight a glaring contradiction in his approach. Mr Swinney, they will point out, is intent on securing the system which offers the greatest protection from the UK while simultaneously campaigning to leave the UK.

But in the explosive political battle that would follow failure in the talks, I predict only one winner.

Mr Swinney will have a simple and powerful message. He will accuse the Treasury of trying to grab hundreds of millions of pounds from Scotland. He will be lauded by his supporters for standing up for Scotland.

There is no shortage of reasons why all that might appeal to the SNP in the run-up to the Holyrood election.

For a start, their opponents would have to rip up a whole series of manifesto pledges based on using the new tax powers to raise or save money.

It would also allow the SNP to seek a mandate for a new deal on devolution. That might be an appealing way for the Nationalists to frame the election, especially if Nicola Sturgeon is not planning to offer an explicit promise of a second independence referendum.

Right now, that looks how things will unfold. Unless, of course, something unexpected happens on Monday.