INDEPENDENCE, not Brexit, will dominate the election campaign north of the border. Although Theresa May has tried to frame the vote in England as a chance to strengthen her negotiating mandate in Brussels, the best chance that her Conservatives and other unionist parties have of unseating the dominant Scottish National Party is to play on fears that a strong showing by the SNP would speed Scotland’s exit from Britain.

On the face of it, things might look tricky for the nationalists. Having scored a stunning victory in 2015, winning 50% of the vote and all but three of Scotland’s 59 seats at Westminster, the SNP has little to gain and much to lose. And a year after it won a third term in power in the devolved Scottish government, there is plenty for voters to complain about. Health-service standards are slipping. The once-excellent Scottish education system has slid below that of England in international rankings. Meanwhile the Scottish economy contracted by 0.2% in the last quarter of 2016 and may soon tip into recession. Scottish ministers blamed the fall on Brexit—but Britain’s economy expanded by 0.7% in the same quarter, and by 1.8% over the calendar year compared with Scottish yearly growth of just 0.4%.

Yet the Conservatives and other pro-union parties are unlikely to unseat more than a handful of the nationalists’ MPs. The SNP’s strongest defence is not its policy platform, but simple arithmetic. The size of most of its MPs’ majorities is crushing: in only six of the 56 seats that it won in 2015 did it have a lead of less than ten percentage points over its nearest rival (see chart). The Conservatives hope at least to take the seat of Berwickshire, Roxburgh and Selkirk, which would require only a 0.3% swing. A few others may fall. But in most of its seats, mainly former Labour turf, the SNP looks unassailable for now. The unionist parties intend to chip away at the nationalists’ lead by focusing on independence. The SNP’s leader, Nicola Sturgeon, has pronounced the Brexit vote a “material change in circumstances” justifying a second independence referendum, aiming to overturn the 2014 ballot in which 55% rejected breaking from Britain. Because preparing for this ballot is one of the few pieces of significant government legislation the Scottish Parliament has approved in the past year, opponents accuse her of obsessing about break-up rather than mending public services. They hope that the campaign will flush out more of the SNP’s independence plans, such as what Scotland’s currency would be.

Ms Sturgeon, by contrast, is determined to make the election a campaign against Mrs May’s “hardest possible Brexit”, meaning “more austerity and deeper cuts”. But Brexit presents problems for her party, too. A prominent nationalist MSP, Alex Neil, has revealed that he was among half a dozen of the party’s MSPs who voted to Leave, defying the party’s policy to seek EU membership. This reflects divisions in the party’s supporters: about a third of those who voted for independence are reckoned to have voted for Brexit. Ms Sturgeon may feel compelled to de-couple independence from EU membership by promising separate referendums on each.

The Scottish Conservatives look best placed to gain. Their leader, Ruth Davidson, secured an increase in the Scottish Tory vote from 14% in 2011 to 22% last year in Scottish parliamentary elections. That was its greatest share since 1992 and enough to displace Labour as Scotland’s second party and the official opposition. She will hope to build on that in June. But the scale of the SNP’s lead in most of its seats means that Ms Davidson, a Thai kick-boxer, will need to conjure some extraordinary moves if she is to dent Ms Sturgeon’s independence ambitions.