In an age of multi-party politics, manifestos are academic exercises.

For their authors, they describe the best of possible worlds, not a reality in which compromise is king. Of no party is this more obviously true than the Liberal Democrats.

Opinion polls say Nick Clegg and his colleagues are in for a fraught time on May 7. Many will not return to Westminster. As things stand, some are liable to be humiliated. Barring a near-miracle, the LibDems are about to pay a very high price for five years of coalition.

So why bother with a manifesto? Why set out policies "for government" when the party's role is likely to be marginal and its influence debatable? Even if they are invited to help Labour or the Conservatives into office, the LibDems' cherished ideas will fall victim to inevitable horse-trading. Why persist with the ritual?

There are two answers, one less elevating than the other. Crude pragmatism says, first, that Mr Clegg's party could yet matter a great deal. If even their depleted numbers shape the next government, they will certainly have influence. Up to a point, they will be able to name a price. The manifesto launched yesterday does just that.

A second answer should be obvious: the LibDems have something to offer. David Cameron might want to claim a programme to raise personal tax allowances as his own, but the idea came from Mr Clegg's side. Ameliorating the condition of the worst-paid by lifting them out of tax is a deed more useful than all the words spoken about poverty. The LibDems intend, as they should, to press ahead with the idea.

They raise a voice, too, for green politics. All politicians pay lip service to issues such as the decarbonisation of electricity. Aside from the Green Party and the Scottish Greens, only the LibDems have been consistent - and persistent - in pursuing policies. And only Mr Clegg and his colleagues can harbour hopes of forcing through legislation.

The party's offering remains pragmatic, nevertheless. The manifesto is a message to suitors in waiting. The LibDems, says Mr Clegg, "would add a heart to a Conservative government and a brain to a Labour one". In practice, this means more spending on education, health and environmental measures, as Labour might desire, but also a balanced budget on the current account by 2017-18. That would not displease the Tories.

Significantly, the document is silent on Mr Cameron's promise of a referendum on EU membership. On the face of it, the omission is surprising. Perhaps the LibDems believe such a vote cannot be prevented. Perhaps they believe it can be won. If the latter, they might well be right.

Mr Clegg meanwhile says he will prevent the SNP or Ukip from holding the balance of power. In the case of the latter, his efforts might be unnecessary. Nigel Farage's presentation of his manifesto was both opportunistic and typically confused. Thumping tax cuts; something done (it is not clear what) about immigration; and, of course, withdrawal from the EU.

Mr Farage would pay for his tax "giveaway" by, among other things, cutting funding to Scotland. Clearly, he has no ambition to see his party live up to its name. Scottish voters will shed few tears over that.