Donald Macintyre: The Lib Dems can talk to Cameron, but their future is surely with Labour

The disappointing result for the Liberal Democrats is only one of several confounded predictions about this extraordinary election – of which we are all guilty. But unless Nick Clegg holds his nerve, it is the one which threatens to be the most immediately relevant in the days ahead, since Mr Clegg would hardly be human if he did not feel somewhat deflated by the outcome. And that may not be the best mood in which to negotiate.

There will be plenty of time to analyse the reasons why his sudden surge to stardom did not translate consistently enough into seats to strengthen his party's parliamentary presence. Rather than mourn, the Liberal Democrats' leader would do well for now to remind himself, as so many of his predecessors have done before, of the woeful unfairness of a system which has given him well under a tenth of the seats with well over a fifth of the national vote.

For the days ahead are likely to test Mr Clegg's leadership even more rigorously than the campaign he has just fought with such panache. The overtures made to him in David Cameron's skilful speech yesterday, not to mention the unequivocal anointing of the Tory leader as the prime minister-in-waiting by a supportive press, and the widely perceived urgency of forming a government in times of economic crisis, will make it all the harder for him to pursue his party's interests with anything like the ruthlessness that comes naturally to the two biggest parties – a ruthlessness born of the hard experience of what it's like to wield power and lose it.

However disappointed the Liberal Democrats may be in their own performance – despite stunning successes in some constituencies – the parliamentary arithmetic, for the first time since 1974, has dealt them a uniquely powerful and possibly unrepeatable opportunity to reshape British politics in precisely the way Mr Clegg has consistently argued he wants to do.

Mr Clegg is, of course, right to enter into serious talks with the Conservatives, just as, for that matter, Gordon Brown was right yesterday to offer them in his own short but still distinctly prime ministerial speech in Downing Street the space to do so. But it is difficult, deconstructing the Cameron speech, to see how it offers them much that is truly tempting. In testing the terms to the limit, Mr Clegg might tease more out of them.

Maybe Mr Cameron would, for example, agree to keep the lifting of the threshold on inheritance tax or even the allowances for married couples out of his planned emergency Budget. But for the most part, the Tory leader, who was as concerned yesterday to reassure elements in his own party restive about too many compromises as the Liberal Democrats themselves, appears to be saying in his opening offer: "We shall prioritise areas on which we both agree, and for the rest, you can support Conservative policy."

Nor will the offer of a cross-party commission on electoral reform excite, given that it joins a long line of talking shops going back to the Ullswater committee in the 1920s, the Speaker's Conference offered by Edward Heath to Jeremy Thorpe and the Jenkins committee established by Tony Blair. In reality, there is no clear sign as yet that even if Mr Cameron was inclined to make a more serious offer on electoral reform, his MPs, and perhaps some members of his Shadow Cabinet, would stand for it. The big question is whether he will offer a referendum.

Moreover, if the offer were to harden, as Mr Cameron hinted it could, into a semi-formal parliamentary pact let alone the full-blown coalition suggested by John Major yesterday, it could cause severe strains within the Liberal Democrat party itself. Of course, there are many Liberal Democrat seats in which Labour is the immediate enemy. And the Liberal Democrats might seek a guarantee that Mr Cameron would not call a second election at a time calculated to embarrass them electorally. But there are easily enough Liberal Democrats who did not come into politics to sustain a Conservative government for such an accord to risk a split in the party.

All this is surely well understood among senior Liberal Democrats. Which is why there is another different and widely canvassed possibility: that the talks will have a far more minimal outcome, that in the end Mr Clegg will merely agree not to vote against the all-important Queen's Speech of a Cameron minority government and make few, if any, commitments beyond that. That chimes with the mood, if not of fatalism, at least of resignation among some in the party about an outcome of an election in which the party performed below expectations and in which Mr Clegg, whether wisely or not, talked about the "mandate" of a party which won the most seats and votes.

Which is what indeed may happen. Yet when the party's MPs meet today it's hard to believe that the Labour Party will not also be on the minds of many. Labour has not been eclipsed after all. Its result may be historically bad, but after 13 years in power it, and not the Liberal Democrats, has reasserted itself as the main progressive force in politics. So much so that there may even be those in the party who would be relatively happy to see the Conservatives take the dauntingly tough decisions needed to tackle the deficit, now rather safer in the belief that they could come back to fight another election from a position of some strength.

That isn't the view at the party's nerve centre, which is that it would be prepared to make the Liberal Democrats an offer that they could hardly refuse, including a referendum on PR. Mr Brown gave few details yesterday, but if he entered serious negotiations with him, Mr Brown would presumably promise Mr Clegg that the referendum was on genuine proportionality – at the least, Jenkins's AV-plus – and an offer of more than half a dozen Cabinet posts.

Although Labour and the Liberal Democrats have together more seats – and much greater vote share – than the Conservatives, they have failed to secure enough seats to form an overall majority on their own. And for the Liberal Democrats, there remains the counter-argument that it would be seen to be propping up a Prime Minister who by normal indices had lost the election.

Mr Clegg anyway might secure a promise from Mr Brown that he would, if not go immediately, at least set a timetable for his departure, perhaps after the PR referendum and perhaps after a successor as leader had been elected. It cannot be certain that Mr Brown would agree, though he would be bequeathing a historic legacy. Yes, the Conservatives are the biggest single party. But they, as well as Labour and the Liberal Democrats, failed to meet their objectives in this election. There is far less ideological difference between the two parties than between the Liberal Democrats and the Tories. A Lib-Lab coalition would not only be a historic opportunity to achieve the Liberal Democrats' objective of electoral reform; it would also identify the third party at long last in the progressive, non-Conservative tradition to which it truly belongs.