All three major political parties face a crisis after this UK 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.

Conservative leader 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. 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 Friday morning NZT - and let us assume it 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 the overwhelming likelihood is that either Gordon Brown or 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.

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 he would allow Cameron to be prime minister if the Conservatives fail to win a majority. 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, 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. 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 intervene. 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.

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.

Clegg's error 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.

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 shadow chancellor of the exchequer 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. 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.

- INDEPENDENT