A Labour/SNP coalition would damage both parties
Here's a quandary. As things stand, Labour are increasingly likely to need SNP support in order to form a government in May.
A hung parliament is by far the most likely outcome according to the betting markets, with some polls suggesting the SNP could overtake the Lib Dems as the third largest party at Westminster.
However, while a Labour/SNP coalition would be highly popular in Scotland, it would be deeply unpopular outside.
According to two new polls released this week, the possibility of a coalition between the two parties sharply divides opinion north and south of the border. On the one hand one new poll out today finds that such a coalition would be the first choice among a clear plurality of Scots, including almost one in five Labour supporters.
However, a separate poll released over the weekend suggests that such a coalition would cause deep resentment in the rest of the UK. According to Yougov, almost six out of ten people in the South of England and around half of those in the midlands and north of England think a Labour/SNP coalition would be a 'bad thing'.
So while it's perfectly possible to see Labour forming a government with the SNP, it would be at the expense of alienating large numbers of English and Welsh voters.
Other more practical obstacles also lie in the way of such a coalition. The SNP have repeatedly made it clear that they would only prop up a Labour government if Miliband agrees to remove the Trident missile system from Scotland. In order to underline this point, the SNP will later today hold a Commons debate calling for just that.
Labour are due to boycott what they describe as a "meaningless" debate. However, far from being meaningless, the debate over Trident is set to be crucial in deciding whether Ed Miliband becomes prime minister.
This exposes massive difficulties for Labour. While large numbers of Labour supporters and activists agree with the SNP's position, most senior Labour MPs see Trident as a totemic issue symbolising the modernisation of the party.
If Labour were forced to ditch support for Trident it would open a deep split in the party at exactly the wrong time for Miliband. Would he brave or stupid enough to risk it?
Risks for the SNP
A Labour/SNP coalition would also poses big problems for Nicola Sturgoen party as well. The spectacular rise of the SNP in recent few months was thanks in large part to dissatisfaction among Yes voters with the three main Westminster parties.
Much like the rise of the Lib Dems following the general election debates in 2010, this support is dependant on the SNP appearing distinctive from the 'old' parties. If the SNP formed a coalition with Labour, they would risk facing the same fate that Nick Clegg's party has since 2010. Of course, that outcome is not inevitable. The SNP could learn from the Lib Dem's mistakes and choose to offer a 'confidence and supply' arrangement instead, or they could make coalition dependent on certain other policy red lines.
But for every new red line drawn by Nicola Sturgeon, Labour would face new political difficulties south of the border. As the recent row over Jim Murphy's mansion tax comments revealed, a SNP/Labour collation would risk deep new divisions opening between voters in both countries, a fact which could end up hurting both Labour and the SNP.
Faced with so many obstacles Miliband may well find that an SNP/Labour coalition is impossible, in which case it could fall to the Conservatives to form a minority government instead.In those circumstance, the SNP would expect to pay a political price among Scottish voters as well.
It is these myriad difficulties which make the prospect of a coalition between the two parties both fascinating and highly difficult to imagine. Could such a coalition last a full term? Could it even be formed at all?
These questions are almost impossible to answer with confidence and yet they are exactly the questions both parties could be forced to answer in a few months' time.