David Cameron and Ed Miliband refused even to entertain the idea of a coalition government during their appearance on Question Time a week before the 2015 election, but the studio audience was not fooled – and viewers at home probably weren’t either. Neither the Tories nor Labour looks set to win a majority, and one or the other will have to find another way of putting together a government.

It is still in the balance whether there will be a Conservative-led government, possibly with the Liberal Democrats on board again, or a Labour-led one. That would almost certainly be a minority government – particularly now Miliband has completely ruled out entering a coalition with the SNP or, indeed, doing any kind of deal with the party whatsoever.

If the SNP does win enough seats to give this loose centre-left bloc a legislative majority, there is no reason why such a minority government could not be stable. There is enough shared ground between the SNP and Labour for Miliband to have the confidence of the former’s support on important votes without needing to enter into a more formal arrangement.

Whether it would be seen as legitimate is another matter entirely.

Sliced and diced

The question of legitimacy goes beyond the rights and wrongs of minority governments, and whether they enjoy mandates from the voters. Minority governments are common in Europe, as well as in Canada and New Zealand. They were also common in the UK until the 1920s.

The big legitimacy problem for a minority Labour government could be the national question. If current predictions are correct, it’s more than likely that a Labour government will be formed despite having failed to emerge as the largest party in either England or Scotland in terms of votes or seats.

In fact, at the moment, it seems very likely. Going on the latest predictions, Labour is headed for 270 seats, the Conservatives 279 and the SNP 51. The number needed for a majority in the House of Commons is 325 (or more likely 323 given Sinn Fein’s abstentionist policy).

The Tories currently hold eight seats in Wales and one in Scotland, and are unlikely to improve on these tallies. They could win a slim overall majority of English seats, say 270-275 of the 533. Labour, meanwhile, could have 8-10 Scottish seats, together with up to 30 Welsh seats, leaving it with around 230 English seats not held by other parties.

This would mean Labour would finish second to the Conservatives in England and a very distant second to the SNP in Scotland. Labour does not field candidates in Northern Ireland, and so the only nation of the UK in which the country’s governing party was also the most popular party would be Wales. Meanwhile, neither of the parties that won respective majorities of seats in England or Scotland (or for that matter, in Northern Ireland) would be in government.

This outcome would be unique in modern British political history, and given the current salience of the unionist-nationalist cleavage in British politics, questions would inevitably arise over the legitimacy of a Labour government that was the largest party in neither England nor Scotland.

The big rift

The Conservatives would present themselves as the voice of England, while the SNP would claim to speak for Scotland – but it’s not clear who, exactly, Labour would represent. Miliband might argue, with good reason, that narrow appeals to national identities will undermine the strength of the union, and that what’s needed is a government for the whole of the UK, not factions representing the nations within it.

That’s fine as far as it goes, but it would be an easier sell if the Conservatives had fallen behind Labour in England. If the Tories win an English majority, every decision a Miliband government makes on constitutional matters affecting England could be denounced as illegitimate by its opponents. That includes extending more powers to Scotland as part of the unionist “vow” made on the eve of the Scottish independence referendum, especially if there’s no move towards English votes for English laws at Westminster.

Allowing the SNP to vote on policy issues that did not apply in Scotland would lead to howls of outrage from the Conservatives and their allies in the press, as well as from UKIP. The Tories in opposition would probably emerge more strongly as a party that sought to defend England’s interests within the union.

On the other hand, attempting to deny the SNP a second referendum on independence would result in accusations that Labour was frustrating the will of the Scottish nation, as expressed through its nationalist tribune. In these circumstances, a Miliband government might simply be unable to withstand the forces of nationalism or to drown out its loudest proponents.

Labour might argue that it alone was best placed to govern the UK. The SNP did not appear interested in joining a coalition government while the Conservatives have become an almost entirely English party, with little support in Scotland and not much more in Wales. Besides, if the Tories and their allies had performed better, they would have been able to form their own government. It would not be Labour’s fault that the parliamentary arithmetic ended up as it did.

But the terms of the union between the component parts of the UK seem to be changing. Nationalism – not only Scottish, but the English variety too – will play an increasing role in voting behaviour and government formation. And any major party that tries to govern the UK in 2015 will inevitably be judged on its response.

The danger for a minority Labour government, if we get one, is that it will be caught in a pincer movement between English and Scottish nationalism, with its very legitimacy called into question before it can even begin the task of rebalancing the constitution – a task that no government will be able to put off.

This article was originally published on The Conversation.