Barring some unexpected change in the next two months, the 2015 general election is likely to end in an unholy mess. While the last election produced a hung parliament, it was the size of the LibDems in Westminster that made the "ConLib" coalition possible.
This time around, that is extremely unlikely to be possible - and is largely due to the SNP.
Coalitions of the unwilling
With the LibDems expected to lose anywhere between twenty or thirty-odd MPs (and even Nick Clegg under threat of losing his Sheffield seat), their losses look to be gains for the Conservatives and Labour in England and the SNP in Scotland (see the excellent explainer and graphic here). And with the Conservatives highly unlikely to retain their current numbers in parliament, a "Tory-led" coalition looks mathematically unworkable, even with the support of the DUP and UKIP.
What looks certain is that, even if the Tories somehow were able to hang on as a minority government with LibDem and others' support, the "anti-Tory" MPs (i.e. Labour and the "Celtic Fringe") would have more MPs to vote against any legislation. When it came to passing the Queen's speech later in May, the "anti-Tory" MPs could veto it. Ergo, government wouldn't be able to function.
Of course, that assumes that the LibDems would support the Conservatives the second time around. That assumption is also complacent, as it would depend on the internal politics of the LibDems post-election. For example, if (in the worst-case scenario) Nick Clegg lost his seat, it would probably put the more left-leaning party figures in more prominent positions. It is already likely that Danny Alexander will lose his seat to the SNP, leaving Clegg without one prominent supporter. In the event of negotiations with the Tories on another coalition (or "case-by-case" support), the more leftish parts of the LibDems are much more likely to make their voice heard, given the mauling the party is likely to have faced after being with the Tories in government for five years. So the LibDem leadership may well be under pressure to keep their distance from being too closely linked to anything the Conservatives do after the election.
For these reasons, it would be foolishly-arrogant for the Tories to take LibDem support for granted after the election. The LibDems may well choose to support Labour, especially if they have the larger number of MPs (which was officially the central reason for Clegg supporting the Tories in 2010). As the LibDems like to say of themselves, when in a coalition they would give a heart to a Tory government, and a backbone to a Labour one. This sentiment can be easily mocked, but that is more-or-less how the party sees itself.
One of the most outlandish options - which hardly anyone has spoke of - is a "grand coalition" of the two main parties. The German government is a "grand coalition" at the moment, and was in Merkel's first term (currently in her third).
However, there are reasons why this option is outlandish. It seems highly implausible for several reasons.
First, while this would guarantee a huge majority in parliament, it would cause massive ruptures within the two parties. Defections would be likely from both parties (e.g. to UKIP).
Second, it would look like - and would be - a stitch-up. The support for the other parties - UKIP, in particular - would likely surge, and cause problems within both parties (see point one). There would likely be strong calls for a change to the voting system (again), and this time they may have to be heeded.
Third, while the other parties - such as UKIP in particular - lump the three parties together as "LibLabCon", there are still large areas of disagreement between Labour and the Tories: austerity and Europe, to name just two. While the LibDems also had disagreements with the Tories when they entered the coalition, they at least both agreed on the idea of austerity. Labour's view is different from the LibDems, and thus very hard to match with the Tories' vision. It seems almost impossible to imagine the leading personalities of these two parties in the same government.
Which leads to the last point: the clash of personalities. The leading lights in the Tories and Labour can't stand each other (or at least appear to). How day-to-day running of government would be possible with such a group of opposing personalities with mutually-exclusive visions (Osborne and Balls fighting over the Treasury?) is almost beyond comprehension.
Scratch my back...
So far, we've looked at coalition options, but haven't mentioned the elephant in the room: the SNP. The Tories are keen to emphasize the "nightmare scenario" of a Labour-SNP coalition of the UK, making the Tories as the "anti-Scottish" party. Certainly, it is easy to see why this scenario would be unpopular in England, with the threat of the tail wagging the dog.
However, it looks like both Labour and the SNP are well aware of this perception as well. The SNP have made repeated statements that they would not enter a coalition with Labour (as well as refusing any deals with the Tories). The rationale for this appears to be one of "fear of association". Looking at what's happened to the LibDems in government with the Tories, the SNP seem to have decided that they would rather have their influence from afar, than be intimately-bound with any major party, and thus tarred with the same brush. If that is indeed the case, then they have learned from observing the LibDems' painful experience. While there will likely be no SNP ministers in government (though the thought of Alex Salmond as Scottish secretary is amusing), the question is what indirect influence could the SNP have?
While Labour have refused to rule out a coalition in principle, the SNP seem to have made their decision for them. As things stand, the most likely post-election scenario is a minority Labour government supported by the SNP (and others?) on a case-by-case basis. This is, remember, what Cameron first offered to the LibDems as his preferred governing option in 2010; and now the LibDems may well rue the decision not to have taken it. But is this scenario feasible in the long-term?
While it's true that the SNP did rule Holyrood as a minority government for a full term, Westminster politics lacks the more deliberative and co-operative aspects seen in the devolved parliaments in Scotland and Wales (and NI). Westminster is a bear-pit, as we see in PMQs each week.
It is hard to imagine a minority government of any colour surviving for a full term. Labour would be under pressure for an EU referendum from the Tories. Although Labour could theoretically ignore this, the threat of a Tory-UKIP alliance to lobby for a referendum would be hard to ignore - especially considering how UKIP are effectively the second party in parts of the North of England. This would put Labour MPs under pressure to heed to public opinion.
Secondly, assuming that the SNP gain the majority of MPs in Scotland, this would give them a legitimate right to demand another independence referendum (regardless of the fact they'd only just had one). While Scotland and the SNP may not really want independence outright, the SNPs large cohort of MPs could effectively act as leverage to campaign for "devo-max", with the emphasis on "max": to ensure that the famous "vow" for Home Rule is implemented in full. This is their real - and realistic - aim, and will likely have the support of the Tories (who have their own plans for EVEL). In this, a minority Labour government could be forced into an unlikely fait accompli between the SNP and the Tories to divide power between them. This would lead to a whole new constitutional can of worms.
Whatever the result of the election, things in the UK are unlikely to be the same again.