THE general election that will be held in Britain on May 7th is so finely balanced that predictions are foolhardy. Save one: the Scottish National Party (SNP) will triumph. And that spells grave danger for the United Kingdom, including—indeed, especially—for Scotland itself.

After failing to win last year’s independence referendum, the SNP might have been expected to collapse. Astonishingly, it has roared back. Through relentless campaigning and exemplary use of social media, the SNP has made fervent supporters out of nationalist sympathisers, many of them working-class Scots who always voted Labour. As a result, it now has 100,000 members from among 5m Scots; the Conservative Party, which draws from all 64m Britons, has only about 150,000. Nicola Sturgeon, the SNP leader, has been the star of the campaign’s televised debates. Polls suggest that the “Nats” may win as many as 50 of Scotland’s 59 seats in the House of Commons, up from just six in 2010. If (as seems likely) no clear winner emerges, they could well hold the balance of power.

This is a big problem for the Labour Party, and not just because its MPs occupy most of the seats the SNP is poised to grab. The nationalists could also drag it leftward. Ms Sturgeon, who is more straightforwardly left-wing than her predecessor, Alex Salmond, will not prop up a Tory government, and indeed promises to do her utmost to bring one down. She would support a Labour administration. Part of the price, suggested in her party’s manifesto, unveiled on April 20th, would be higher spending, particularly on welfare. Part would be the constant demand for favours to Scotland (see article).

Such informal dealmaking will threaten the legitimacy of government and the future of the union. When the Tories warn of the danger of a separatist party with a finger on the wheel of British government, they are trying to scare English voters away from Labour. But they are also right—and the appearance of nationalist influence could be as damaging as the reality. If Labour holds power with SNP support, every policy will be scrutinised for signs of Scottish favouritism. Labour wants a tax on homes worth more than £2m ($3m). Since there aren’t many of those in Scotland, this will be seen as a raid on the English, with some of the proceeds going to the canny Scots—who already receive much more state cash per head than the English or the Welsh. Repeat that complaint a few times, and you have the makings of a constitutional crisis.

Paradoxically, a resurgent SNP chafing under another Conservative-led government could imperil the union even sooner. Today the Tories have just one seat north of the border. If David Cameron remains in Downing Street, leading an almost entirely English party, the SNP will noisily question his legitimacy. A Cameron government would also raise the spectre of a British exit from the EU after the “in-out” referendum the prime minister promises. Nationalists already insist that Scotland’s views on this matter should be considered separately. If Britain does vote to leave, there will surely be another independence referendum, which the nationalists would win.

Just one good thing has come of the SNP’s surge. Belatedly, unionist politicians are paying attention to Scottish politics. They have long ignored it, leaving their B teams in Edinburgh while their best operators take top jobs in Westminster. Yet the SNP’s record in Scotland offers much to attack. The country’s schools are mediocre; the hospitals are protected from competition—and all the worse for that. The Nats boast about free university tuition, yet fewer working-class teenagers go straight to university in Scotland than in England, where they pay.

Not as tough as they look

Most damning, Scotland cannot afford the SNP’s plans. This newspaper argued three years ago that Scots would probably be poorer outside the United Kingdom, even with handsome earnings from North Sea oil and gas. Cheap oil has removed the uncertainty. The Scots would now unquestionably be much worse off: the Institute for Fiscal Studies, a think-tank, reckons that this year Scotland’s deficit will be roughly double Britain’s. Either independence or fiscal autonomy (the SNP’s next-best option) would be utterly ruinous. The nationalists are rampant, yet their case is crumbling. Exposing their fallacies is the best way to limit their sway over the next government, whatever its complexion—and to save the union.