John Rentoul: Clegg has blown it, after all that

The Lib Dem leader has shown his hand too early and may have missed the chance of a lifetime

All three parties face a crisis after this election. In Labour's case, its response could decide whether or not the party survives at all. The crisis facing the other two parties is not so serious, but it is historic. David Cameron faces the Mervyn King Conundrum. The Governor of the Bank of England was alleged last week to have said that whoever wins this election will be "out of power for a whole generation because of how tough the fiscal austerity will have to be". Mind you, when I mentioned this on the campaign trail last week, a cabinet minister almost spat: "It is always better to be in government than not." And so it is.

It is Labour and the Liberal Democrats who are really poised on the edge of a cliff. It is all too easy to imagine a Labour Party, overtaken by the Lib Dems in the popular vote, collapsing into recrimination and ideological warfare after Thursday. It is almost possible to smell the longing in large parts of the party for the comforts of opposition. The only thing that might save the party would be the bursting of the Lib Dem bubble. Fortunately for Labour, that is almost inevitable.

As soon as the polls close on Thursday – and let us assume that it really is a hung parliament – Nick Clegg ceases to be the transcendent alternative to the tedium of reality and becomes a politician haggling over the spoils. Whatever he chooses to do will instantly offend about half of his supporters. Because, tediously, the overwhelming likelihood is that either Gordon Brown or David Cameron will emerge as prime minister, and it will probably be Clegg's party that makes that decision. Which brings us to Clegg's great big mistake.

All three party leaders have made historic errors. By allowing the televised debates to go ahead, Cameron made the mistake of giving Clegg what his predecessors have long craved: parity of status. Clegg seized the chance, and for a while achieved parity in the opinion polls, too. It is notable that Cameron in his interview with us today seems to accept that he acted against his own interest in allowing the debates to go ahead. "Don't ever let it be said that politicians don't have some principles," he said, sounding rather forlorn.

Brown's mistake – well, there's no need to go into that. But it is Clegg who has made the most extraordinary strategic error. He says that he would allow Cameron to be prime minister if the Conservatives fail to win a majority on Thursday. If the opinion polls are right, or nearly right, the Conservatives will be the largest party in a hung parliament. In that case, Clegg has said, and repeats to us today, Cameron would have the "moral right" to seek to form a government. Instead of seizing that historic moment, that "one chance in a generation" of which he speaks, Clegg has already said which way he will jump. Because if Cameron is "seeking" to form a government, to whom should he apply? To one N Clegg Esq, of Cowley Street. But if N Clegg has already set out his doctrine of the mandate and said that Brown has "written himself out of the script", Cameron is in the car to Buckingham Palace before the Federal Executive of the Liberal Democrat party can say "d'Hondt quota". Clegg's ambiguity about whether his doctrine of the mandate is defined by votes or seats is irrelevant because, if the Conservatives have the most seats they would also have the most votes.

That's fine by me. If the choice at this election is between Cameron as prime minister with a majority, or Cameron as prime minister in a hung parliament, I prefer the latter. But if there is a hung parliament, the Lib Dems would probably be in a position to choose between Brown and Cameron. And if that is the choice, I would prefer Cameron.

I think the Conservatives will take the reduction of government borrowing more seriously, and their schools policy is right. I think too that Cameron is more likely to act in the national interest and would be better suited to the important task of explaining why difficult choices will have to be made. Although I do not approve of inheritance tax cuts or the marriage tax allowance, which is why the restraining influence of the Lib Dems would be beneficial.

So Clegg's error is fine for people like me. I even disagree with proportional representation. But it could be catastrophic for his party, although I can see why he did it. He desperately needs to appeal to Tory-minded voters, and for them, the Conservative slogan "Vote Clegg, get Brown", is a genuine deterrent. Our ComRes poll today shows a sharp switch in preferences from a Con-Lib coalition to a Tory majority. Last week's campaign against a hung parliament, of which we in the commenting community made such fun, seems to have been effective.

But he has thrown away most of his bargaining power. If Cameron fails to secure a majority, Clegg is almost bound to agree to his Queen's Speech and George Osborne's Budget. The Conservatives would have to offer only a few token concessions to make Clegg an offer he cannot refuse. The Lib Dems may abstain in the key votes in the House of Commons, but it would be apparent that Cameron governed by their permission. I do not know if that is Clegg's plan: to allow Cameron to have a go for a year or two before switching to a David Miliband-led Labour Party offering proportional representation. If it is, it is a rubbish plan. As I say, who knows what will have happened to the Labour Party by then? And if Cameron impresses, as I suspect he has the capacity to do, he may go back to the voters to ask for a working majority.

The way the opinion polls look this weekend, it may be that Cameron scrapes in with a majority in any case, despite his mistake of conceding the debates. If not, however, it may be that history will conclude that Clegg was the one who made the really big error.

John Rentoul's blog is at www.independent.co.uk