Scotland’s first minister Nicola Sturgeon deftly trolled Boris Johnson by suggesting the UK premier’s visit north of the border on Thursday was not “essential” under Covid-19 rules. Supporters of the UK union might feel the journey was not only essential but that he or his colleagues should be venturing north far more frequently. For it is clear they now face an existential battle to save the 313-year-old union.

A string of opinion polls since last summer have, for the first time, found that a sustained majority of those Scots who expressed a view favour Scottish independence. Sentiment has been fuelled by Brexit — which 62 per cent of Scottish voters opposed in 2016 — and perceptions that Ms Sturgeon has been a more sure-footed pandemic leader than her UK counterpart. Her Scottish National party is on course for a majority in May’s Scottish parliament elections, which it will claim as a mandate to request a second independence referendum. It can argue that Brexit has transformed the constitutional backdrop since the last one in 2014.

The SNP has raised the stakes by indicating that if Mr Johnson did not give the required permission, it would legislate to hold an advisory vote, defying the prime minister to challenge the law in court. Some Tories want to tough things out. They believe enthusiasm for “Indyref2” will wane once the pandemic ends and the focus reverts to the SNP’s spotty record on Scottish education and health. But hoping for the best is not a strategy, and ignoring referendum demands is risky. The union is underpinned by an assumption that it is voluntary. Even Scottish unionists might reconsider if they felt the country was being barred from having another say. Neither side should seek a repeat of what happened in Catalonia.

Unionists, then, must win the argument. The government needs to come up with a positive, inclusive vision for the UK shorn of the Brexiter rhetoric that repels many Scots. It must emphasise the value-added that the union brings, without heavy-handedness. It also has to show a readiness to reform the union to make it more appealing.

An example of what not to do was the government’s Internal Market Act, in which London retook control of structural funds previously disbursed by the EU. Instead of handing money to devolved governments to spend, Westminster will fund its own chosen projects, hoping to burnish the union’s popularity. Many Scots see this instead as an attempt to bypass devolution.

An enlightened step, by contrast, would be House of Lords reform. An elected second chamber of members from the UK nations and regions, with greater power to block decisions affecting the whole nation, would respect Scots’ desire for a greater voice not just in their own affairs but at Westminster.

The UK government should heed the lesson of the Brexit poll: that appeals to sovereignty and identity can trump negative economic realities. But pro-union parties should press the SNP to begin seriously setting out its renewed case for independence — and the undoubtedly tough near-term implications. Scotland’s gap between spending and revenues has for years been larger than the UK’s. Losing the fiscal transfer from the UK would pose very hard choices on spending, for years to come. Fulfilling the SNP’s aim of rejoining the EU, post-Brexit, would impose a “hard” border with the rest of the UK, which takes more than 60 per cent of Scottish exports.

Many Scots like Ms Sturgeon because she is seen as a straight-talker. She may argue the hardships of independence would eventually be worthwhile. Yet Scotland’s first minister has a moral responsibility to level with her voters on exactly what those hardships are.