Sometimes in politics, the worst option in the short term proves to be the best in the long run. Gordon Brown is an intelligent man and he must be pondering this point right now.
Let's say that David Cameron and Nick Clegg fail to reach an agreement. The main obstacles will be opposition from within their respective parties, rather than any lack of pragmatism on the part of the leaders. It's possible something temporary may be cobbled together, but I wouldn't bet on it.
Gordon Brown's offer to Clegg is still on the table. An immediate referendum on PR in exchange for support in Parliament. Sounds good in theory and it could lead to the 'progressive' anti-Tory coalition that many on the centre-left have championed for generations. But there is a fundamental problem which goes way beyond the inadequate arithmetic of the Lib-Lab deal in Parliament.
An attempt by the incumbent Prime Minister to remain in power is not an option, because there is just as much an anti-Brown sentiment in the country as there is an anti-Tory one. If he were to cling on at Downing Street in the short-term it would do unimaginable damage to Labour over the coming years. He has to accept defeat gracefully and allow party members to elect a new leader, such as David Miliband.
Miliband - or whoever replaces Brown - cannot become Prime Minister immediately, because such an outcome would be utterly unacceptable to the general public. We thought we were choosing between three presidential candidates who paraded before us on the television. To be lumbered with someone, however able, who wasn't on offer previously would confirm everyone's worst suspicions about the political process. What's more, it would undermine the case for electoral reform that the new Lib-Lab coalition would presumably be making to the country. People would say that if the horse-trading under first-past-the-post produced such a perverse outcome, wouldn't the deals under PR prove far worse?
This leaves only one favourable option for the Labour Party. Allow the Tories to form a minority government and watch as they stumble from crisis to crisis. George Osborne will, of course, be hopelessly out of his depth. Cameron will be left cosying up to the DUP in Northern Ireland and forming ragtag and bobtail coalitions on an ad hoc basis to get his legislation through. We could be pretty sure that the more extreme elements of the Conservative manifesto would remain unimplemented.
Meanwhile, Labour would elect a new leader in an open contest. And this person would lay the ground for a victory in the next general election - almost certainly within the next six to eighteen months.