If the political events of a single day can set the tone for what follows, the UK is on its last legs. Calling for another independence referendum at Bute House in Edinburgh on the morning of Monday 13 March, Scotland’s First Minister, Nicola Sturgeon, appeared typically poised and (apparently) in control of events, while from Downing Street that afternoon there was the distinct sound of flapping.

Brexit highlights the contradictions on both sides of the constitutional divide. There is an obvious flaw in the SNP leader’s argument that the UK extracting itself from an economically beneficial union – the EU – would prove “catastrophic” while Scotland leaving the UK will be fine. Equally, Theresa May cannot credibly talk up the benefits of UK “independence” while casting the Scottish equivalent as a calamity.

Yet the optics in Edinburgh and London don’t give the full picture. By any empirical measurement, the economic case for Scottish independence is weaker than it was in 2014. However, the trouble for unionists – as for Democrats in the US and Remainers in the UK – is that the political conversation is no longer taking place in the realm of balance sheets or, indeed, of objective reality.

Sturgeon probably knew that this was coming from the moment she put a second independence referendum back “on the table” the morning after a majority of UK voters (but not Scotland) chose to leave the European Union. Yet between then and Monday morning, she had to appear reasonable, as if she had exhausted every possible compromise. The British government’s inflexible response to the First Minister’s quixotic plans for a “differentiated” Scottish settlement strengthened her hand.

No one in the SNP expected Theresa May to deliver the requested compromise. And while many believe that Sturgeon got a little carried away on 24 June 2016 in her expectation that pro-European sentiment would boost support for independence significantly, Brexit has been a political gift. Not only did the differential outcome in Scotland reinforce long-standing arguments about the “democratic deficit”, it also enabled the SNP to recast Scottish nationalism as internationalist and cosmopolitan, in contrast to the “Little Englander” variety.

Nevertheless, the First Minister ended up taking the plunge slightly earlier than anticipated, probably because newspapers had suggested that Article 50 could be triggered on 14 March. Sturgeon will now get a second media “hit” at her party’s spring conference in Aberdeen this weekend. Forcing her hand was not Alex Salmond, as some spurious reports implied, but the realisation that circumstances would never be this good again. Yes, there is the backdrop of Brexit, but equally important are the existence of a pro-independence majority in the Scottish Parliament (which is unlikely to be sustained beyond the 2021 Holyrood elections) and the continuing dysfunction of the Labour Party. Jeremy Corbyn might go down in history as an unwitting facilitator of both Brexit and Scottish independence.

This time last year, Nicola Sturgeon was telling interviewers that she would pursue another referendum only if opinion polls showed a sustained lead for independence. Though two recent surveys suggest a modest tilt towards Yes, this has not transpired – at least not in public polling. It seems likely, however, that private polling tells a different story, which is another reason why the SNP leader felt able to move as she did.

Crucial to the next vote is the group that we might call “Yes-Leavers”. With a degree of intellectual consistency, its members want to regain “sovereignty” from both London and Brussels. In an attempt to keep hold of that constituency, the First Minister has attempted in recent months to detach a second referendum from Brexit, arguing that independence “transcends” this and almost every other political consideration. SNP advisers also floated the idea that an independent Scotland might settle for membership of the European Economic Area, like Iceland or Norway (the party’s favourite constitutional case study), rather than full-blooded membership of the EU.

The SNP is confident that, come the crunch, the majority of Yes-Leavers will end up backing independence. The tenuous claims, made during the last Scottish referendum campaign, that an independent Scotland would “automatically” become or remain an EU member are dead in the water. Instead, the Scottish government tacitly accepts – indeed, welcomes – the possibility that it will be outside the EU, at least for the time being.

On 13 March the First Minister said the Yes side would “be frank about the challenges we face”, yet another indication that the independence proposition will be less Pollyannaish than it was in 2014. Its advocates have little choice. Not only have North Sea oil revenues dwindled, but the sizeable gap between what Scotland raises in taxation and what it spends on public services – somewhere between £9bn and £15bn a year – is given an annual airing with the publication of the Government Expenditure and Revenue Scotland (GERS) figures.

Just as the SNP reversed its opposition to membership of Nato in 2012, the party is now closing down potential lines of opposition attack. The benefit of having fought a referendum just a few years ago is that nationalist strategists know where their weaknesses lie. Central to this process is a “growth commission”, led by the former SNP MSP Andrew Wilson.

Wilson has said that oil revenues will no longer be “baked” into the economic case for independence. His remarks were not intentional but proved useful, neutralising the oil issue early on, but the twin challenges of currency and the deficit remain. Last time, the SNP adopted the least bad option of a “currency union” with the rest of the UK, but since then opinion within the SNP has shifted in favour of a separate Scottish currency. Whether that becomes policy, however, is not yet clear.

There has also been a change of tone regarding the deficit, if not a wholehearted acceptance that the early years of independence would necessitate both steep tax rises and deep cuts to public spending. “It’s going to be tough for the first few years,” one Salmond-era adviser admits, but how frank the SNP is about that in public will be a test of the new realism.

Like Jeremy Corbyn’s Labour Party, the SNP has been better at calling for an alternative economic model than articulating what it would be. That won’t matter much in the heat of another referendum battle. The meta-narrative remains strong, and as the EU referendum and US presidential election demonstrated, a beguiling story of apparently easy solutions to difficult problems – even in the absence of any details – can prove a winning formula.

The central role of Andrew Wilson in the SNP’s pivot away from land-of-milk-and-honey predictions is also interesting. He and Sturgeon were colleagues in the first Scottish Parliament between 1999 and 2003, but they were far from close, and Wilson is typical of the Salmondista nationalists who once thought the idea of her leading the party was a bad joke but now view her with increasing admiration, not least for her willingness to gamble her career on a second referendum. The First Minister’s kitchen cabinet is small, but over the past few months, as a source puts it, “there’s been some reaching out” to Salmond-era advisers. A divided movement is not in any nationalist’s interest.

So where does that leave those who want to preserve the United Kingdom? Not in a good place, as the initial response demonstrated. The carrot-and-stick approach of the 2014 referendum is subject to the law of diminishing returns; offering yet “more powers” is difficult, now that the low-hanging fruit has been picked, and Project Fear II would likely suffer the same fate as last year’s Remain campaign. Organisationally, each of the three unionist parties – the Conservatives, Labour and the Liberal Democrats – will fight its own anti-independence campaign, thus appearing disunited (the Yes campaign will probably be much more disciplined than in 2012-14).

More to the point, with Northern Ireland once again in tumult, what precisely is it that binds Ulster, Wales and Scotland to England, beyond a balance sheet? In recent weeks, everyone from the Prime Minister to the Scottish Lib Dem leader, Willie Rennie, has attempted to articulate the Holy Grail of a “positive” case for the Union. None has got beyond the usual platitudes about past (the tense is revealing) British greatness and fuzzy rhetoric about “solidarity”. There is also English public opinion to factor in. A few years ago, the English, on balance, wanted Scotland to stay, but who can say if that sentiment will survive Brexit and a second independence referendum?

As Europhiles know all too well, defending a union that can appear harsh and remote is no easy task. It doesn’t matter that independence is a conclusion in search of an argument – oil in the 1970s, Margaret Thatcher in the 1980s, the Iraq War in the 2000s and now Brexit – or that economic reality favours the status quo. Success in 2019 (or perhaps even later) will come down to who tells the better story. Brexit gives the Yes side a more compelling good v evil tale than it had in 2014. If the No campaign relies on the same old boring story of economic woe (what else is there?), a second indepen­dence referendum is the SNP’s to lose.

David Torrance has written biographies of Alex Salmond and Nicola Sturgeon