In principle, there is no problem whatsoever with a Labour and SNP combination governing England, Wales and Northern Ireland after tomorrow’s vote.  That Labour could do so despite having fewer seats than the Conservatives is of no constitutional significance if Ed Miliband is capable of leading a government with a majority in the Commons.  That the SNP is a separatist, anti-British Party is no constitutional bar, either: its MPs are no more or less entitled to sit and vote in the House than those of any other party.

In practice, however, matters are not quite so straightforward.  At present, MPs from Scotland can vote on the business of the rest of the United Kingdom, but MPs from the rest of the United Kingdom cannot vote on the business of Scotland, under the terms of the rolling devolution settlement.  This has not proved intolerable to voters in English constituencies since the Blair/Brown devolution settlement (despite some predictions to the contrary).  And if Labour forms a government after Thursday, they may bear with it again – but watch the SNP.

During this election campaign, Nicola Sturgeon has paid lip service to the SNP’s policy of not voting on matters that affect England only.  But the purpose of her words has been to signal that, in the coming Parliament, SNP MPs will cast their voting net more widely than previously – on the NHS south of the border, for example.  It might be that voters in England would take such a change in the same phlegmatic spirit that they have taken the lack of constitutional balance since 1997.  Then again, it might be that they would not.

One of the main purposes of the SNP is to sicken the English of Scotland – thus helping to gain the independence that democracy has, to date, set its face against.  They want Scottish nationalism to pull one way, English nationalism to pull the other…and the United Kingdom to be torn apart by these competing pressures. Which returns us to legitimacy.  Some will offer one definition of it and others another, but ultimately legitimacy resides “where people believe it resides”, as Varys puts it in Game of Thrones: “It’s a trick,” he adds, “a shadow on the wall”.

Where will voters south of the border believe it resides if, after Thursday, the SNP begins to impose its will on a Tory-voting England by voting on English business?  Whatever the result, it is very unlikely that any one party will have a majority in the coming Commons.  A new and enlarged SNP contingent is poised to come south to Westminster.  No party is yet advancing the policy best place to stop the break-up of the UK – that’s to say, the federal solution set out in the ConservativeHome Manifesto.  The Union is in a perilous place.