We never voted for that. It has long been the lamentation of Scots that England has imposed alien governments on them. They rejected Margaret Thatcher, but had to endure the she-devil and her hated poll tax because England put the blue lady in Number 10. They didn’t want David Cameron, but he and his hated bedroom tax were forced on them because Sassenach voters installed him at Downing Street. Even when they did get Labour governments under Tony Blair and Gordon Brown, and even though both those prime ministers were born north of the Tweed, some Scots continued to complain, saying that their country’s interests were subordinated to the pursuit of swing voters in English marginals.

So I can quite see why Scots might think it poetic justice if the balance of power in the next House of Commons is decided north of the border. And that is looking increasingly likely. To the surprise of the SNP’s leadership, and to the enduring shock of the Labour party, the nationalists have been on a roll since the referendum. The “45”, as yes-voters like to call themselves, are still impassioned by the energy aroused during the campaign and unreconciled to losing. They have surged behind the SNP. The party’s membership has ballooned from around 24,000 at the time of the referendum to about 100,000 now.

Their poll numbers have also soared, hugely at the expense of their ancient and bitter Labour rivals. Jim Murphy, Labour’s new leader in Scotland, has got off to an energetic start that is attracting admiration from his colleagues, but years of neglect of the party’s supporters and lacklustre leadership from Labour in its former heartland will not be reversed in the short time left before the election. “Jim is playing a bad hand as best he can,” says one colleague.

Labour currently sends 41 MPs to Westminster from Scotland; the SNP has only six. The most dire projections for Labour would almost precisely reverse those numbers. That would make it highly likely that the SNP, not the Lib Dems, would be the third largest party in the Commons. Even if Labour isn’t slaughtered quite that dramatically, the party’s senior figures are pretty much reconciled to losing a slew of seats to the nationalists.

The prospect of being kingmaker in a hung parliament is emboldening the SNP to assert what it would demand in return for its support. The nationalists want the scrapping of the Trident nuclear deterrent. They want devolution going beyond that agreed by the parties when they signed up to the proposals of the Smith Commission. Nicola Sturgeon, the SNP’s smart and popular leader, recently added a further item to her growing list of demands when she declared that the nationalists would seek an additional £180bn of public spending over the course of the next parliament.

The nationalists’ strategy involves some double-talk. They electioneer by deriding Labour as Tory-lite. Yet they say that the SNP are natural partners for Labour who could make common cause after the election to form a “progressive alliance”. The suggestion is that voters should vote SNP to get a Labour government with a more leftish, tartan flavour. If the polls are right, this notion is proving alluring to a great many Scots.

It is also a source of huge delight to the Tories. They may say they love the United Kingdom, but the Tories are cheering on the SNP surge. A Tory attack ad designed to scare English audiences photoshops a grinning Alex Salmond outside Number 10 with his arm around Ed Miliband. The Labour leader’s face appears to have been manipulated to make him look chubbier. The tagline reads: “The SNP would prop up Ed Miliband – meaning chaos for Britain.” The Conservative propaganda machine doesn’t do subtle. They hope to frighten English voters with the idea that a Miliband government would be the SNP’s puppet jerking to nationalist tunes. Tory strategists claim this is “having traction” on English voters when they test the message on focus groups.

Labour thus finds itself pincered by a two-front assault: bashed in Scotland by the nationalists for being too like the Tories, and whacked in England by the Tories for being ready to jump into bed with the nationalists. In a speech in Edinburgh on 20 February, David Cameron described a Labour coalition with the SNP as the “ultimate nightmare scenario”. The obvious way for Labour to close down this line of attack is for Ed Miliband to declare unequivocally that he would never go into coalition with the nationalists. He’s not done that yet. When no one can be sure exactly what they will face on 8 May, there’s a reluctance to narrow down options. The Labour leader is also wary of opening any sort of conversation about what he might do in a hung parliament. A senior figure on the Labour campaign team says: “If we say no to the SNP, people will then ask, are you saying no to the Liberal Democrats?” There is also anxiety that ruling out a deal will come over as presumptuous. That might feed the nationalist narrative that Labour is part of the arrogant “Westminster establishment”.

Tories hope to frighten English voters with the idea that a Miliband government would be a puppet of the SNP

A growing number within the shadow cabinet are no longer convinced that this is the right strategy. They are urging a much more emphatically anti-SNP message. One of them says that Labour now needs to say “loud and clear” that it would not treat with the nationalists as “a party that wants to tear the United Kingdom apart”. Another agrees: “We’re going to have to say no deal with the SNP.” That, they think, would take the air out of the Tory attack. It is also necessary, they argue, to disabuse Scottish voters of the notion that voting SNP will give them a perfect world in which David Cameron is thrown out of Number 10 and Scots hold sway at Westminster. Senior Labour figures also contend that striking any sort of bargain with the SNP would be such a strategic mistake that they should never countenance doing one anyway. Says a member of the shadow cabinet: “If we do a deal with the nationalists, my fear is that it will not just be the end of the Labour party in Scotland, it will be the end of the Labour party in England.”

The nationalists are probably not sincerely interested in going into government at Westminster. They have prospered by riding the anti-politics wave and presenting themselves as the plucky “outsiders” battling the leviathan of the UK political establishment. That would be a harder act to perform if the SNP were in power both in Holyrood and Westminster. The way in which they are scaling up their demands also suggests that they don’t really want a deal. Ed Miliband’s colleagues believe he would never agree to casting aside the nuclear deterrent. And if he tried to, he would face massive opposition from within his own party at the most senior level. In the words of one member of the shadow cabinet: “It would be completely unsupportable to make a decision about the defence of the nation in a few days of scrambling for power.”

You don’t need a tremendous degree of political sophistication to spot that the SNP’s goal of independence would be best served by Mr Cameron winning another term in Downing Street. That would add potency to the nationalist argument that Scotland and England have become so politically divergent that the two nations ought to go their separate ways. It would also likely mean a referendum on British membership of the European Union. If the English voted to leave the EU and the Scots voted to stay, that would furnish the nationalists with perfect grounds to demand another vote on Scottish independence and a highly propitious context in which to win it.

Though they’d never admit this, it suits the nationalists’ central ambition for the Tories to do well. It equally suits the short-term electoral interests of the Tories, if not the long-term health of the union that Mr Cameron claims to care so much about, if the nationalists rob Labour of a lot of its seats north of the border.

That could very well make the difference between Ed Miliband emerging from the election with the most MPs and David Cameron doing so. If Mr Miliband were the runner-up in seats it would be a helluva a lot less likely that he could form and justify a Labour-led government. At every UK election for nearly a century, the party with the most seats has won the keys to power.

For their very different reasons, the Tories and SNP suggest that a vote for the nationalists will be a vote for a Labour government. The reverse is much more likely to be the case. What a mighty irony it would be if voting SNP were to put David Cameron back in Downing Street. That outcome might secretly delight the leadership of the SNP. It is rather more doubtful that it would please many of the Scots currently saying they plan to vote nationalist.