Looking back over the last half-century or so, from the 1960s until the summer of 2014, the popularity of nationalism in Scotland has been relatively static. When polled, between one-quarter and one-third of us consistently indicated that we wanted to be independent from the rest of the UK, with the rest of us wishing to remain in the Union.

So, when the Yes campaign polled 45% in the referendum, we had a good indication that the sands were shifting rapidly. Furthermore, although it seems to have escaped the attention both of Downing Street and the UK Labour leadership, we also knew that such a positive result for the nationalists meant that we inevitably remained on a journey to somewhere, rather than at a destination.

It was no surprise that, in the aftermath of an underwhelming Smith Commission, polling showed a continuation of the movement which was evident in the run-up to September 18th. Independence now leads, and similarly the SNP has, since the referendum, been polling deep into the 40s and sometimes over 50% in Westminster voting intention.

However, when those votes began to be translated into seats and we saw predictions of the SNP winning anything from 40-55 of the 59 available seats at Westminster, it seemed implausible. Ludicrous. Even those, like me, who have bemoaned the rank complacency of Westminster unionists thought that this was impossible. Indeed, backed up by private conversations with several nationalists, I criticised the SNP for failing to adequately manage expectations, feeling that if they returned, say, 25 MPs it would be seen as a failure rather than, deservedly, an overwhelming success from a base of 6.

But there is a new normal in Scottish politics. And that new normal is total domination by the SNP at the expense of everyone, especially the Labour party. People in all three unionist parties now tell me that they worry the surge is real. Labour door-knockers tell me that their support is haemorrhaging. “There’s just nothing”, one said to me. “They’ve gone”. The Lib Dems appear to face oblivion and the Tories are being squeezed to the point of suffocation.

So what if the surge is real? What if the SNP get 40, or even 50 seats? What if, as some polls predict, the Lib Dems unthinkably lose Orkney & Shetland, seat of the Scottish Secretary, and the Tories lose Dumfriesshire, Clydesdale and Tweeddale, seat of his Minister, leaving no MPs from the governing coalition in Scotland? (Although I will refuse to believe Alistair Carmichael can lose until the very point of declaration).

Well, that would trigger a sequence of events that would make the 2010-2020 decade surely the most constitutionally turbulent one for over 300 years. The balance of power in the UK would be held by a party whose primary aim is to end it. Quite properly give their raison d’etre, their strategy would be one of disorientation and destabilisation. We’d be giving the prison keys to the inmates.

Let’s examine what the SNP might do if Labour is the largest party. My view is that they will not enter a formal coalition. Don’t expect to see Alex Salmond as Deputy Prime Minister. That would be too stabilising, too cooperative. Instead, their best strategy would be to agree to support a Miliband government on a case-by-case basis. They might ease into in, ‘playing nice’. However, rapidly, they would demand the sort of concessions that Prime Minister Miliband simply cannot afford to agree to. They might start with corporation tax. More aspects of welfare. They just might find agreement on those. They’re jabs. But then come the bigger blows. Oil revenues (they’re still significant, folks!). National insurance. Pensions. And of course Trident.

Miliband says no, he loses his majority on that healthcare bill, or that austerity-busting bill, or that energy capping bill, and we know the rest of the story. Let us not underestimate the resentment which will be built in England towards the Scots who held the country to ransom and plunged it into a second general election. The late David McLetchie always wondered whether the greatest threat to the UK came not from Scottish nationalism but from English nationalism. Mark his words.

And what if the Conservatives are the largest party in the UK, most likely with a majority of seats in England? The SNP is in a different position then. It could be the party which puts Ed Miliband in Downing Street against the wishes of England. But I don’t think it would. Instead, I think it would use the backdrop of a Tory government with no Lib Dem ‘restraint’ at Westminster and with renewed vigour for the austerity agenda to underscore how supposedly ‘different’ Scotland is.

That’s why we have decided to put another referendum on independence front and centre of our 2016 Holyrood election manifesto, they will say. One chance to stay in the EU. A second chance to end austerity. A final chance to kick the Tories out of Scotland for good.

In my view, this latter outcome is what their strategists would prefer. A majority of Scottish seats at Westminster. A majority of seats at Holyrood. A clear mandate for another referendum. The backdrop of continuing austerity coupled with the prospect of pulling out of the EU.

In such circumstances, predicting a No vote in IndyRef 2 would be, as Sir Humphrey would say, very brave indeed.