NICOLA Sturgeon is building. Standing at the Bute House podium yesterday proclaiming Scotland’s right to hold a second independence referendum, she should have been wearing a hard hat.

The First Minister has a grand design in mind but knows it will collapse unless the foundations are sound.

So that’s what she is doing – laying the foundations for a Yes vote.

She appeared to be making the case for the right to a referendum, but the real purpose was to rally supporters to the independence cause.

Technical arguments about the mandate for another vote flowed seamlessly into an emotional pitch for Yes that appealed directly to voters’ sense of justice – how Boris Johnson was “remaking our future without our consent” and how things would be better if Scotland just went it alone. “Let’s imagine a country that is at the heart of Europe, a welcoming, outward-looking nation, a country where we get governments we vote for, a Scotland with full powers to lift children out of poverty, a fairer country and a more prosperous economy.”

The Yes campaign has already started and it’s aiming firmly for your heart. Why? Because Ms Sturgeon knows that the detailed arguments over what independence will mean – questions about the currency, fiscal constraint, depleted oil revenues and the English-Scottish border – are more complex and troublesome than she would like, even with Mr Johnson in Westminster. If she can avoid engaging with them until she has to, and use her time instead building principled support for independence (by exploiting unhappiness with Westminster) then it might make all the difference.

Because a Yes win in another referendum is far from certain. No matter when it is held, the campaign will turn the spotlight on tricky issues. Just how tricky was highlighted by an exchange yesterday between her Constitution Secretary Michael Russell and the BBC’s Justin Webb on the Today programme.

Now, Mr Russell is a very bright chap and I do not take lightly the decision to compare him to David Davis. The intellectually underwhelming Mr Davis was so unconvincing as Brexit Secretary he made his successor Dominic “Where’s Dover?” Raab seem like Henry Kissinger. But there is simply no denying the Davis-Russell parallel.

Mr Davis was famous for airily dismissing difficult technical questions about the UK’s trading relationships. And weirdly, Mr Russell adopted a similar tactic. Asked about the risk of a hard border between England and Scotland, Mr Russell tried first to deflect the question, saying he would have that debate as part of the independence referendum, then when forced to respond, asserted there would be no need for a hard border, in effect because Mr Johnson had said so. The Prime Minister, he said, had stated his intention to have an open, productive, tariff-free trading relationship with Europe and that it would also apply to an independent Scotland since Scotland would be in the EU.

This was such an exercise in contortionism it made your eyes cross. Only a few weeks ago Mr Russell wrote a piece assassinating Mr Johnson’s character (with ample justification) and describing him as “a man not averse to lying to get his own way”.

Now here he was holding up the Prime Minister’s words as evidence that an independent Scotland would have no hard border with the EU. I found myself doing my Manuel face. Qué?

To make matters still more surreal, his words appeared to contradict his Government’s long-standing position that Brexit will do serious damage to the UK’s trading relationship with the EU.

The exchange was telling because it highlighted the way in which the Scottish Government’s position of opposing membership of one union while supporting the membership of another, has set up trip wires for Scottish ministers. The motivations for Brexit and for Scottish independence are different but there is no denying that they throw up certain similar challenges for those who advocate them. “It’ll be fine,” was Mr Davis’s response to tricky questions about trading relationships and it sounded very much like Mr Russell was saying the same thing.

By the time another independence referendum actually happens, there may be more clarity about UK-EU trade. In fairness to Mr Russell, both the UK and the EU will want to minimise damage to their trading relationship, though we can safely assume that Brexit won’t turn out to be as cost-free and seamless as Mr Johnson promises. The real challenge for the Yes campaign is that Brexit will be both a driver of support for independence and a brake on it. In the event of a bad Brexit, independence offers voters a way to get the hell out of Dodge, but the negotiations will also demonstrate to Scottish voters that breaking up unions is messy and has costs as well as benefits. Will the UK’s damaged post-Brexit trade relationship with Europe make Scottish voters want to vote Yes so Scotland can rejoin the EU? Or will seeing the UK Government fail to achieve its hope of tariff-free trade make voters question the Scottish Government’s promises of border-free trade within the UK after independence?

No wonder Ms Sturgeon is trying to circumvent these tricky questions, by making a principled case for independence.

What the Yes side must hope is that there will come a tipping point when the adverse impacts of Brexit start feeding into those all-important emotional arguments of identity, a moment when trade with the EU is taking a hit, the UK Government has caved in to pressure from would-be trading partners to lower environmental and food safety standards, Tory backbenchers are leading an assault on human rights, and the NHS and care system are struggling to fill thousands of posts vacated by EU workers. Ms Sturgeon spoke yesterday about “the sort of country we want to be”. She wants Scottish voters to look at Brexit Britain and say “not one like that”.

Ms Sturgeon has barely been able to keep the smile off her face since last Friday. As long as Mr Johnson continues refusing her a referendum, she can continue building a majority for independence.

But if she succeeds, and any poll lead for Yes isn’t to melt away in the white heat of a campaign, then she’ll need more than “it’ll be all right” to convince the doubters. Scotland’s second independence referendum will be a battle royal and voters will want convincing answers to the difficult questions Scottish ministers would rather avoid.

Read more: Sturgeon demands permanent referendum power for Holyrood