In the aftermath of the 2010 general election, it was clear that the Conservative leadership had made plans for a hung parliament. This time round, we can be sure that all the main parties are preparing themselves – but what for exactly?  With so many jokers in the pack (Lib Dem decline and the rise of UKIP, the SNP and the Greens), the permutations have multiplied, as noted by Ian Birrell in the Guardian:

“Party leaders, however, may have to build broader coalitions than our current two-party version. I have heard one Downing Street insider punt the concept of a Tory-Lib Dem-Green coalition, a senior Tory suggest a Conservative-SNP deal based on faster devolution, and a Labour figure float a Labour-Lib Dem-SNP-Plaid Cymru agreement reliant on big tax rises and slower spending cuts. Any of these fragile groupings could be held hostage by single-minded militants or single-issue obsessives capable of collapsing the government.”

As various pundits have anticipated, it’s quite possible that a conventional coalition of the largest party plus the Lib Dems would not have a majority. Yet there is one two-party combination that would – which prompts Birrell to think the unthinkable:

“…as Labour and Tory leaders explore the post-electoral landscape, they may find coalition deals with insurgent forces too costly, minority government too impractical and a far better solution staring them in the face.

“A government of national unity between Labour and the Conservatives may sound far-fetched, especially amid the froth and fury of a nascent election campaign. It would certainly be tricky, exacerbating internal divisions and leading to more defections. Yet, while there are serious disagreements, the two parties have more in common with each other than with the insurgents on many key issues – especially if David Cameron survived and Miliband was replaced by someone such as Chuka Umunna.”

Birrell has received a lot of stick for even raising this possibility – which is widely dismissed as a blatant impossibility.

On the face of it, the sceptics are right. The only way that David Cameron can survive as Conservative leader is if he survives as Prime Minister – which, barring an overwhelming national emergency, is not an arrangement that Labour MPs would ever support. Instead, following an election in which they came second, they’d dump Ed Miliband, get a new leader, and then seek the first opportunity to trigger a second general election.

However, if Labour comes first and Ed Miliband (or a Labour successor) becomes Prime Minister, then something approaching Birrell’s scenario could unfold.

Imagine a Labour-led government that relied on the SNP to stay in office. This is unlikely to take the form of a formal coalition, as the Nats have no interest in (a) governing a country they don’t believe in and (b) sharing the fate of the Lib Dems. Labour might be able to rely on the SNP to abstain in a vote of no confidence, but not in regard to particular spending cuts. For those, they’d have to turn to the Tories.

One might ask why the latter would be willing to help out in this way – to which the answer is two-fold: firstly, it would be difficult for the Conservatives to credibly oppose austerity measures that they clearly believed to be necessary; and, secondly, the opportunities for stoking discord within the Labour Party would be too good to pass-up.

Note that this wouldn’t entail a formal Lab-Con coalition – but rather a series of ongoing negotiations on economic policy. Nor would this be a ‘confidence and supply’ arrangement – as the Conservatives would still try to bring down the government in the event of a vote of no confidence. Rather, Labour would be reliant on the SNP for the ‘confidence’ and the Conservatives for the ‘supply’.

No doubt, the Conservatives would demand a price for this limited form of cooperation – most obviously in the form of a promise to hold an in/out referendum. Indeed, the possible terms of engagement with a dependent Labour-led government would become a pivotal issue in a post-election Conservative leadership contest.

Complicated, isn’t it?