Let's start by acknowledging that nobody knows what is going to happen in the event of a hung parliament, and that what ought to happen - as viewed in terms of partisan advantage - is no less difficult to work out.
Like many in the Labour party I am receptive (in my case, that's a radical understatement) to the idea of some sort of Lib Dem-Labour arrangement centred on serious reform of the political system.
But I keep encountering an argument against any such arrangement which goes something like this: 'If Labour comes third in the popular vote, it will lack the democratic legitimacy to cling on to power with the support of the Liberal Democrats. If Labour did this, it would get penalised all the more severely at any following election. So, in such circumstances, Labour should gracefully withdraw from government.' Seumas Milne echoes the first step of this argument when he says:
'If Labour were to continue to trail in third place, the Liberal Democrats could scarcely keep it in power, whether Labour ended up the largest party...or not.'
This argument - if I have got it right - is a helpful challenge to complacency about Lib-Labbery (or Lab-Libbery). But I am not convinced.
To begin with, the argument doesn't seem to grasp the difference between a genuine coalition or similar arrangement and 'Labour clinging on to power with support from the Lib Dems'.
Of course, the latter is out of the question. The Lib Dems will not agree to any arrangement which looked like they were just sustaining 'Labour in power'. But if the arrangement involves genuine sharing of power, why should or would it be seen as 'Labour clinging to power'? Taken together, Labour and the Lib Dems might well command a majority of seats and a majority of the popular vote, so why would there be a problem of democratic legitimacy for a government of this character?
There are at least two further ways such an arrangement could be put together so as to dispel the air of Labour 'clinging to power'.
First, it could be an arrangement for a clearly-stated short period of office and, perhaps, specific tasks. So the arrangement might last to hold a referendum on electoral reform and then govern for a short period prior to fresh elections to be held under the new electoral system. In this way, the arrangement could be presented, quite appropriately, as an 'alliance for change'.
Second, Labour could signal humility, in view of its low share of the popular vote, by insisting that Nick Clegg take up the position of PM. The idea of Clegg as PM is one also considered in Seumas Milne's recent, very interesting article. Sunder identified it as a possible outcome earlier this week. But Milne quickly backs away from the idea on the grounds that it is 'still implausible'. But a week ago a 10% jump in the Lib Dem poll ratings was implausible. Implausible is the name of the game right now. (A quick look at the Comments thread to Milne's article shows that some are accustoming themselves quite quickly to these 'implausible' thoughts.)
What would Labour get out of this? Well, presumably in return for proper electoral reform etc. Labour could get a major say in economic and social policy. There is common ground here with the Lib Dems, but Labour might usefully block some of the sillier Lib Dem ideas like spending £17 bn on an income tax cut that mostly benefits the affluent and scrapping the Child Trust Fund. More generally, the government's response to the budget deficit could be shaped by Labour's values and thinking which consistently offer better protection for the most economically vulnerable than those of the Lib Dems.
Now maybe there are other reasons why this would be a bad idea - either a bad idea for the country or a bad idea in terms of Labour's long-term partisan interest. I don't know. (Actually, I am pretty confident it would be in the best interests of the country...)
But the idea that any such arrangement is impossible to consider because Labour, if it came third in the popular vote, would lack the democratic legltimacy to continue in office - even as part of a coalition government - is surely wrong.
As we political science examiners like to say: 'Discuss'.
Stuart White is a Fellow in Politics at Jesus College, Oxford, and Director of the Public Policy Unit at Oxford University. He blogs regularly at Next Left in an independent capacity and is not a spokesperson for the Fabian Society.