IN 2015, the politics of the United Kingdom could be turned upside down.

The Scottish National Party, which has hitherto declined even to vote on English matters in the Commons, could decide the government of England after the May General Election.

It is an intoxicating dream: Alex Salmond, the new MP for Gordon, laying down the law to Ed Miliband, blocking David Cameron from Number 10, demanding a new constitutional settlement. And make no mistake, it could happen.

The recent string of opinion polls showing the SNP in line to win 40 or even 50 seats in May are hard to credit, but credit them we must. Two major polls in as many weeks have demonstrated that the much anticipated "bounce" from the appointment of new Labour leader Jim Murphy has yet to happen. The SNP roll is continuing, and might even be gathering momentum.

Survation's poll in the Daily Record, conducted shortly before Christmas, indicated that the SNP had actually increased its lead in Westminster voting intentions by two points, taking it to 48% against Labour on 24%. This was after a month in which Murphy had been all over the front pages promising to liberate Scottish Labour from London control. If that swing were to be reflected in May, the SNP would return 54 of Scotland's 59 seats and Labour only four.

Then, hardly had we finished opening our presents than The Guardian broke its authoritative ICM poll on Boxing Day showing another huge SNP lead. At 17%, it was smaller than that predicted by in Survation. However, the ICM poll indicated that the swing against Labour was actually more pronounced in the safest Labour seats than in more marginal ones. Taking this into account, the ICM numbers suggest the SNP are in line to return 53 seats against three for Labour and three Liberal Democrats.

Professor John Curtice, ­analysing these polls, concluded that a sea change had taken place in Scottish voting intentions. "In short," he writes in his blog What Scotland Thinks, "pretty much every Labour seat in Scotland has to be regarded as currently at risk of being lost to the SNP." The UK's leading electoral analyst does not make pronouncements like that lightly. We really are in uncharted territory.

Do I believe the SNP will win more than 50 seats in May? No, I don't. It is just too much of a ­paradigm shift for an old hack like me to grasp. The SNP returned only six MPs in 2010. There is not a single Scottish Labour seat in which the SNP is less than 10 points behind, but thanks to our first-past-the-post voting system, if the Nationalists get a swing larger than 10% they start to take Labour seats by the bucket-load. And this shouldn't be so hard for me to grasp, since something very similar has happened before in Scotland in my lifetime.

The Conservatives used to dominate Scotland's electoral landscape, at least in the days when they were still the Unionist Party, and they remain the only party since the Second World War to have gained more than 50% of the votes and seats in a Scottish election. But in the 1980s, Labour tipped the balance against them, and within a decade the Conservatives were wiped out as a political force. In the 1997 General Election, the Tories lost every single Scottish MP. We have to consider the possibility that something similar is now happening to Labour.

Labour's unique selling point in General Elections has always been: "Vote Labour for Westminster to keep the Tories out." But if the SNP start polling above 40% then this becomes: "Vote SNP to put Scotland in." There is no longer the old fear that an SNP vote is a wasted vote or, worse, a Tory vote, because the SNP would have sufficient seats to become a political force in Westminster in their own right. Scottish voters can keep the Tories out and play the Scottish card. In their present post-referendum mood of rolling rebellion, the 45% who voted Yes might find this an attractive offer.

So, we may be seeing Holyrood voting patterns being translated to Westminster. And reinforcing this nationalisation of the General Election will be two constitutional issues of great interest to Scotland, which will dominate politics in 2015. First, the commitment to English Votes for English Laws (Evel), which the Conservatives are determined to place high on the agenda for May. And second, the prospect of a referendum on Britain's continued membership of the EU.

In fear of Ukip, the Conservatives have been ramping up their anti-European rhetoric in recent months, with David Cameron demanding an end to free movement of Labour within the Union and threatening to leave the EU if significant powers are not repatriated to the UK. This anti-Brussels tone can only grow in intensity in 2015, along with further restrictions on immigrants.

The 2010 Conservative pledge to cut immigration from the hundreds of thousands to the tens of thousands has been a ­failure, largely because of migrants coming from the EU. Ironically, it is the relative success of the UK economy in generating jobs under the Tories that is sucking in workers from unemployment blackspots in Europe, such as Spain and Italy. You may have noticed a marked increase in the numbers of Spanish waiters - though nowadays they aren't figures of fun called Manuel, but hipster baristas with impeccable English.

To compensate for this influx, the Conservatives will be making a big play of their commitment to an in-out referendum on Europe in 2017. It seems likely that Labour, which according to some analysts is even more vulnerable to the Ukip threat than the Tories, will have to follow suit. Many working-class voters in English marginals are fed up with Ed Miliband and rather like Nigel Farage's populist pitch. Miliband has already promised a referendum if Brussels seeks more powers.

This will naturally be of concern to Scots who, on the whole, are supportive of Europe - or at any rate much less interested in withdrawal. Northern Ireland has the opportunity, under the Good Friday Agreement, to vote on whether to remain in the UK if the UK leaves the EU. Scotland does not. This could emerge as a major reason for Scots to vote SNP rather than Labour in May - to ensure that Scotland's voice is heard clearly in Westminster at a crucial moment in UK history.

But even more than deliverance from Ukip, voters are likely to want deliverance from Evel. The Conservatives' proposals, unveiled before Christmas, involve English MPs having, at the very least, some kind of veto on domestic legislation in Westminster. Scottish MPs would be barred from votes on nominally English affairs, possibly including income tax if you believe the claims made by Gordon Brown.

This would not only undermine the principle that all MPs are equal in the Commons, but could make Cabinet Government as we understand it impossible. A Labour administration would be in office but not in power across 85% of the United Kingdom.

Imagine the scenario: Prime Minister Ed Miliband tries to reverse the Tory privatisation of the NHS in England by ­repealing the 2012 Health and Social Care Act. The Conservatives in the proposed English Grand Committee reject this on constitutional grounds as ultra vires. There is a prolonged struggle between what have effectively become two legislatures in one parliament. It might be similar to the confrontation between the House of Lords and the House of Commons before the 2011 Parliament Act asserted the primacy of the elected chamber. Only this time there could be no resolution.

However, throw into this mix a phalanx of 40-plus Scottish MPs, led by Alex Salmond, and a different picture emerges. Evel has liberated Salmond from his self-denying ordinance not to vote on exclusively English matters in the Commons. This is because the future of English legislation now directly affects the governance and stability of the entire United Kingdom - at least this is how he will justify his U-turn on English votes. It means that the SNP is able to contemplate an electoral alliance with Labour to keep the Conservatives out of government. Such a coalition would have been impossible if the SNP had stuck to its policy of not voting on "English" bills.

There is nothing in the UK constitution that says the largest party after any General Election has to be the Government, or even part of the governing Coalition. Westminster is now, like Holyrood used to be, a parliament of minorities. It is possible that the Conservatives could win the largest number of seats in May but lose power to a coalition of Labour, Liberal Democrats and SNP, or even Labour and the SNP on their own if the LibDems lose most of their seats, as seems likely.

But could Tory voters, who would be in a majority in England, tolerate the abandonment of Evel and the imposition of a Labour government on the strength of Scottish ­Nationalist votes? Well, the short answer is that they would have to, because that is the logic of the first-past-the-post voting system in a unitary UK. If SNP MPs have a right to sit in the Commons then they have a right to become part of the government.

Alex Salmond will be enjoying all this hugely. However, there is a snag. Ed Miliband might finally balk at the terms demanded by Alex Salmond for a coalition. Not just devo max, not just an end to austerity, not just a Scottish veto on EU withdrawal, not just removal of nuclear weapons from Scotland, but cancellation of the very Trident renewal project itself. The prospect of, as the press would put it, ­leaving England defenceless to appease Scotland might prove too much for Labour in its current febrile state.

But if Miliband rejects a coalition with the SNP he could be accused of allowing the Conservatives to return to government as a minority. And that would anyway lead to a further period of instability in ­Westminster followed probably by another General Election as early as October. How would Scots then vote if Ed had preferred Tory rule to a coalition with the SNP?

HM the Queen might even be forced to play a decisive role after a second indecisive General Election. As head of state, she has the constitutional responsibility of selecting the prime minister of the UK. And no - it couldn't be Alex Salmond, in case you were wondering. But he could be involved in weeks of comings and goings between Buckingham Palace and the Palace of Westminster trying to find a stable UK Government.

It would be a constitutional bourach of epic proportions that could lead to unexpected alliances. It is assumed that the SNP could never form any coalition with the Conservatives. But no-one has asked what would happen if the Tories offered the SNP true devo max - everything except defence and foreign affairs - in exchange for a confidence-and-supply arrangement in Westminster. This would mean the SNP would not have to vote for every Tory bill, only ones that could lead to a minority Tory government being defeated on a confidence motion in the Commons. Could the SNP refuse an offer like that?

An SNP conference passed a resolution some years ago that the party would never form a coalition with the Conservatives, but there is nothing in the party constitution formally ruling out a pact, like the Lib-Lab pacts in the Commons in the 1970s. Alex Salmond has never ruled out any electoral arrangements with the Tories. How could he? Salmond relied on an informal alliance with the Conservatives in Holyrood to become First Minister in 2007. The SNP have formed alliances with the Conservatives in local councils.

An SNP/Tory pact would cause deep divisions in the Nationalist movement and probably resignations. Labour would be incandescent, calling them the Tories' little helpers. But what if the Tories offered to allow Scotland to become autonomous under the Crown, like Canada or Australia in the 1930s or like the Irish state might have been under Home Rule? Might that be attractive enough for Salmond to "deal with the devil"? It very well might.

Of course, we are straying far into the realms of political fantasy here. All we can say for certain is that Westminster politics look more unstable now than at any time since the days of Irish Home Rule before the First World War. And that was a time of constitutional uncertainty that ultimately led to the departure of Ireland from the UK.