Scotland will be gone: it feels almost gone already. In the hall of the SNP conference in Glasgow, the full force of this political tsunami rolled out in every speech at the weekend. Soaring SNP membership, at 103,000, would be equivalent to a UK-wide Labour or Tory party garnering 1.2 million supporters.

Burning with energy, blessed with an enviably able new leader, the SNP feels like the party of most Labour activists’ secret dreams. On the day Labour started selling £5 red mugs emblazoned with “controls on immigration – I’m voting Labour 7 May”, the SNP voted against “discriminatory” immigration laws that “rip families apart”, wanting a welcoming Scottish policy “driven by compassion and common sense”. With motions on more generous benefits, land reform, no fracking, no austerity, no Trident, when Nicola Sturgeon says SNP support would give Labour “backbone and guts”, a good many English Labour party members might nod in agreement.

But English Labour members should be angered by the SNP’s gross misrepresentations of Ed Miliband’s policy. Speech after speech said “Labour and Tories are joined at the hip” on billions of extra cuts, ignoring the Institute for Fiscal Studies report that Labour plans would cut £50bn less than the Tories – with scant need for any cuts. The SNP is often running to catch up with Labour, only now agreeing with Labour’s 50p top tax rate and dropping a beggar-thy-neighbour cut in corporation tax, while boasting of an SNP freeze in council tax since 2008. Nor has the SNP ever dared use the 3p leeway it has to raise income tax. Why not? Because, though the Scots like their closeness to Scandinavia, they are no keener on Swedish taxes than the rest of the UK.

Alex Salmond doing an interview for Dutch TV.

That’s the great quandary. The SNP asserts an elemental Scottish difference in political psyche – more collectivist, more egalitarian. More Scots have a cultural detestation of Tories, but attitudes don’t turn red on crossing the border. Right after that immigration debate, I had a Glasgow taxi driver launching unprompted into a tirade against immigrants stealing Scottish jobs and homes, living on benefits. “It’s not our country any more,” he said. Out canvassing in Glasgow North with Ann McKechin, a good Labour MP hanging by a thread, the first man we talked to, a lifelong Labour voter, said his son couldn’t find work because of the Poles – though Scotland has fewer migrants than England.

Will some day of reckoning come when the SNP finds that Scots are quite British after all? Or can the SNP show Labour that leading boldly from the front, instead of following focus groups, can sway voters leftwards? British Social Attitudes last week found 47.7% of Scots want tax and spend to stay the same, much like 52% of the English and Welsh. Just 7% more Scots are willing to see tax and spending rise. Oddly, in no-fees Scotland, a majority think some or all students should pay tuition fees. Scottish distribution of income is much the same as south of the border.

The SNP faces the election with ebullient confidence: either Labour or Conservative walking into Downing Street is win-win for separatists. A minority Labour government will need SNP acquiescence: it can extract a price. But David Cameron returning to swing his £50bn austerity axe is most likely to propel angry Scots faster to the exit.

Cameron never ruled for a united kingdom, unknowing and uninterested in the effect of his policies on unfamiliar regions. The Welsh are beyond his ken, except when he wants to heap political scorn on their NHS at Prime Minister’s Questions. His southern government has generated pressure for home rule from all those regions outside his comfort zone. Back in his early days, Cameron claimed Harold Macmillan as his model, with a picture of him on his desk. But Macmillan sat for Stockton-on-Tees, with an understanding of Britain learned from that northern town’s suffering in the depression. Cameron’s legacy will not be one nation, but a breaking nation.

Cameron never ruled for a united kingdom, unknowing and uninterested in the effect of his policies on unfamiliar regions

Scotland’s separation is speeded by the reckless opportunism of his response to the SNP surge. That early morning after the no result, a unionist would have embraced the Scots with warmth and solidarity. Instead, he drove a chisel into the rift with his rebarbative threat to punish Scottish MPs: English votes for English laws is a prescription for fragmentation.

The right is virtually ejecting the Scots before they march off. At a Write On! debate in Glasgow’s Mitchell library, one audience member spoke of the outbreak of English Jockophobia, to nods all round. The Tory right and Ukip berate Scots as subsidy junkies, attacking the Barnet formula that shares funds with Scotland, a redistributive glue binding them to the UK.

The most dangerous assault is in those Tory posters showing Ed Miliband in Alex Salmond’s pocket, a line of argument suggesting Scottish MPs have less legitimacy at Westminster. Salmond says they would “block the Tories” out of government but the SNP has a right to do so if it gets the numbers. It has every right to sustain a minority Labour party in power. If Labour has fewer seats than the Tories, that may feel illegitimate: expect a great howl of Tory rage. But the government’s own fixed-term parliament act obliges any party that can cluster enough MPs to pass its Queen’s speech to take command. To suggest that Scottish votes and Scottish MPs don’t count in that arithmetic is to expel them from Westminster and turbo-charge their case for independence.

Either the Scots are wanted at Westminster – whoever they send as their MPs – or not. It looks now as if the Tories relish their departure, as they so rarely win any votes in Scotland. Sturgeon’s incendiary talk of smashing the system is entirely democratic – and many who vote SNP are not separatists.

Labour expects to haemorrhage Scottish MPs. Jim Murphy earns respect for his plucky fight to defend a third of seats that might be held, but Labour’s Scots identity crisis runs deep. The SNP’s malicious caricature brands every Labour MP equally as a corrupt time-server. Authentic Scottish Labour, no longer a “branch office” of the party in Westminster, needs to keep top talent at Holyrood, after eons of brain-drain southwards.

Those of us begging the Scots to stay may face a bitter truth: in a new world of identity politics, one party can no longer stretch its nature to represent everyone from the Isle of Skye to the Isle of Wight.