Boris Johnson’s ruling Conservatives have an important choice to make. The prime minister’s party can give itself a modest shot at persuading Scotland to remain within the Union of Great Britain and Northern Ireland. Or it can stick with Mr Johnson as leader.

The prime minister’s latest visit to Scotland underscored the challenge. Nicola Sturgeon’s Scottish National party had a field day. Ms Sturgeon first feigned outrage, charging that the trip breached Covid-19 restrictions on all but essential travel. She then held up his presence as proof of the political chasm between Westminster and Edinburgh.

Mr Johnson is unpopular in Scotland. A recent poll gave him a minus 42 per cent rating against a positive 36 per cent for Ms Sturgeon. The UK’s vaccine success apart, Mr Johnson’s response to the pandemic has been bluster and costly hesitation. The SNP leader has also struggled but has looked calm and decisive throughout.

Behind the clash of temperaments lies a more visceral collision. The prime minister speaks for a reactionary strand of English Toryism. As first minister, Ms Sturgeon embraces the European-style centrism that sits comfortably in the Edinburgh parliament. Mr Johnson is also Brexit’s lead author — and this has taken Scotland out of the EU against the wishes of 62 per cent of Scots who voted in the 2016 referendum.

Polls predict the SNP will win a landslide victory in this May's elections to the Edinburgh parliament. With it will come a mandate for an independence referendum. A procession of surveys since the Brexit vote point to a majority in favour of separation.

Mr Johnson insists that the 2014 plebiscite rejecting separation settled the matter for a generation. But Brexit rendered it null and void. Six years ago a vote for the UK Union was also a vote for the EU. Brexit has forced a choice between Mr Johnson’s England and engagement in Europe.

The prime minister has rarely shown any affection for the Union. His bearing resembles that of a colonial master. He has called the devolution of power to Edinburgh a “disaster”.

The reason? Scots have had the impertinence to back the SNP instead of deferring to Westminster. Mr Johnson likewise brushed aside warnings that Brexit would drive separation, and subsequently denied the Scottish government a say in negotiations with the EU27.

Scotland’s future is not settled. Infighting within the SNP about how hard to force the pace on a referendum, and a bitter feud between Ms Sturgeon and her predecessor Alex Salmond, show the nationalists have their own weaknesses. Unbiased economic analysis suggests that Scotland would pay a heavy price for break-up. An imaginative UK prime minister could change the terms of debate by widening the options for Scottish voters by devolving more power in a federal constitutional settlement.

Given the outcome of the Brexit referendum, however, Mr Johnson should know better than anyone that arguments about economics fare badly against the emotional pull of identity politics. And he shows no inclination to consider the constitutional changes needed to alter the debate. Instead, he has used Brexit to tighten control from Westminster.

His answer to the SNP’s demand for a second referendum is that he will refuse. But a big win for Ms Sturgeon at the May elections would render this stance politically unsustainable, and might anyway lead nationalists to conduct their own, unofficial poll. To the extent that Mr Johnson might succeed in delaying a second vote, it would be at the expense of adding substance to nationalist grievances.

It is too late for Mr Johnson to reinvent himself. Senior figures in the SNP call him their not-so-secret weapon. Even cabinet colleagues can be heard privately to admit that in character and temperament Ms Sturgeon could not want for a more helpful opponent. In a tight referendum contest — and the polls still point to a relatively close-run race — the prime minister slips perfectly into the SNP narrative of a Scotland trapped in English vassalage.

Many Conservatives may not much care. One poll, in 2019, recorded that over three-quarters of those English Tories backing Brexit gave greater weight to leaving the EU than to the preservation of the UK Union. Mr Johnson also echoes a view commonplace in his party that Scotland is an ungrateful drain on England’s finances. But the party should be clear about the choice. It can join Mr Johnson on the road to little England, or it can fight to save the Union. There is no third way.