Nicola Sturgeon has just kebabed Kezia Dugdale in the STV debate

For much of last night’s STV debate, the last such contest in Scotland in this election, it looked as though the headline story would be the manner in which Ruth Davidson was hammered by all the other Scottish party leaders. The Tory leader was taken to task over the government’s changes to tax credits and, in particular, the so-called ‘rape clause’. Valiantly as she tried to defend herself she was unavoidably on the back foot.

And then Nicola Sturgeon changed the subject. According to the first minister, in a private conversation just after the Brexit referendum, Kezia Dugdale, the Labour leader, told Sturgeon that she and her party were now open to the idea of a second referendum on independence.

That, children, is what a nationalist dead cat looks like. Lies, screamed Labour. The truth will out, roared the Tories. And all the while, the nationalists smiled quietly, admiring the neatness of Nicola’s knife work and enjoying the manner in which she had filleted poor Kezia Dugdale.

Did you really ever think this election had nothing to do with independence? If you did, you’re credulous enough to write editorials for the Guardian. The rest of us have long known better than that.

Is Sturgeon’s claim true? In truth no-one save the two women involved can know, but Sturgeon had not kept the conversation secret from her team. Nor was it any accident this revelation was dropped tonight. But first, some background.

Two years ago you will remember that the Daily Telegraph reported that Nicola Sturgeon had told the French Ambassador she’d be happier if David Cameron won the general election than if Ed Miliband did so. An almighty stushie duly ensued and whether you believed that Ms Sturgeon really said this was dictated by whether you liked the leader of the SNP in the first place.

If you did, you were furious with the Telegraph’s ‘gutter journalism’; if you weren’t one of Sturgeon’s fans – and especially if you were a supporter of the Labour party – you thought that the Telegraph’s reporting had the whiff of truth about it. More than that, actually, because it confirmed what you’d long suspected: the SNP have an interest in talking up the Tories and doing their utmost to destroy Labour as a functioning party of government.

The details of what Sturgeon said, or did not say, are one thing; the reaction to the story quite another. For many, it had the ring of truth even if it might also be not much more than truthiness; Sturgeon’s claims about her conversation with Dugdale do likewise.

Even a bear of no great brain can see that while the nationalists might find Tory rule deplorable they are also alive to its upside. Sturgeon herself has subsequently observed that one argument for independence, or at least for the right to hold a second independence referendum, is the grim prospect of permanent or near-permanent Tory rule in London.

In other words, the Tories help make the case for independence. There is polling evidence to support this too, demonstrating that Scots are more likely to support independence if it means no more Tory governments. Or, at any rate, if the alternative is a decade of more of unpopular Tory government across the UK.

So, never, ever, under-estimate the terror of the Tory bogeyman.

But if the Tories are the SNP’s useful enemies, so the reverse is true too. The Scottish Tory revival is predicated on being a muscular alternative to the SNP. Like the nationalists, the Tories quite enjoyed the independence referendum. At last they had a cause in which they believed and a song to sing.

Without the SNP, there is no Tory revival in Scotland. Without Nicola Sturgeon, Ruth Davidson is a significantly lesser figure. Without the permanent threat of a second referendum, the Tories have much less to say and, significantly, many fewer people to whom to say it. At a stroke, the referendum gave them a larger pool of voters in which to fish. Fifty five percent of Scots voted No in 2014 and at least a plurality of those still around to vote on Thursday will endorse Conservative candidates.

All of which is to say that there is a certain unacknowledged but real co-dependency between the Tories and the SNP.

Which is also why, as has become traditional in the new era of Scottish politics, Labour are left as the poor relations, noses pressed up against the glass as they peer in through the window at the feasting nationalists and Tories inside. And what they see, when they look at this spectacle, is that the SNP and the Tories are feasting on Labour.

True, opinion polls report that there has been an uptick in Labour support in Scotland in recent weeks. But even this ‘surge’ (more Jez than Kez, I’ll allow) only takes Labour back to where they were in 2015. When, you will recall, they lost 41 of the 42 seats they previously held in Scotland.

This, then, is the background to the First Minister’s remarks about her private conversations with the Labour leader.

SNP spinners – who seemed suspiciously prepared for this moment – immediately cited a Times report from last summer in which Dugdale said that she and Sturgeon had talked ‘at length about how horrified we were at the result of the referendum. And I pledged at that point to do everything I could, with the powers I had, to support Scotland having as strong a relationship with the rest of Europe as possible.’

It was at this point, nationalists argue, that she countenanced the idea of a second independence referendum, ‘Kezia just left that bit out’ of her account of her conversation with the first minister, said one SNP insider.

‘This is a lie’ a Labour spokesman said last night and ‘nothing but a final act of desperation from an SNP leader who knows the public has turned against her. It shows how far she is prepared to go in the hope of electing a Tory government.’

The Tories certainly hope so, leaping with glee upon Sturgeon’s claim. This just goes to show, they crowed, that only the Tories can be trusted with the Union. In seats across Scotland, if you don’t want another referendum, you have to vote Tory. You can’t put your faith in Labour, not when Dugdale’s revealed to be as weak on the issue as Jeremy Corbyn who is happy to ‘open discussions’ with the SNP over the timing and framing of a second referendum.

It is true that almost no-one really believes Corbyn is truly, deeply, opposed to a second referendum and it is also true that Dugdale has also been uncertain on the matter declaring herself, at various points, open to the idea or happy to allow her parliamentary party a free vote on the matter.

That has changed in recent months; months in which Dugdale has held to a noticeably tougher and more consistent line. The lesson of last year’s disastrous Holyrood campaign was learnt and since then Scottish Labour at least has been more disciplined on the constitutional question. Not coincidentally, Dugdale has seemed happier, and more persuasive, now that she has a fixed position.

Labour’s agony is that it is the only major party that needs the votes of people who voted Yes as well as the votes of people who voted No in 2014. Hence the desperate pleas for Scotland to ‘move on’ from the referendum and put all this appalling ‘division’ behind us.

But the country is not ready to move on. Most polls suggest that while support for the SNP may have slipped, support for independence remains around the 45 percent it was in 2014. And it is the only issue upon which everyone has a firm opinion. An Ipsos MORI poll last week reported that just three percent of Scots don’t know whether they favour an independent Scotland or a Union Scotland.

And Labour, now a more Unionist party than it has ever been, cannot win back Yes voters without alienating No voters. Independence, Dugdale says, is the issue raised most frequently on Scotland’s well-trodden doorsteps. And her voters don’t want another referendum.

Nicola Sturgeon knows this. Which is why she was happy to torpedo Dugdale this evening. It was a remark and a moment that cannot possibly help Labour but could easily help the Conservatives. The first minister is not a stupid or a foolish lady. She knew what she was doing.

The complete and final destruction of the Scottish Labour movement might not be an essential precursor to independence but making Scottish politics a fight between the SNP and the Tories is something that the nationalists believe helps them in the long-run.

The Tories, of course, are happy for play this game too, sharpening the divide between Unionist and nationalist until such point as Scottish politics becomes just a two horse town. That’s helpful in the short-term for the Conservatives but a risky gamble or proposition looking further down the line.

Meanwhile, having spent much of the election campaign making the laughable suggestion Thursday’s vote has nothing to do with independence, Nicola Sturgeon has ensured that it finishes with a rammy that’s entirely about independence. Fancy that.

Not for the first or, I fear, the last time I find myself thinking ‘poor Kezia Dugdale’.