John Rentoul: The party voted for David Miliband but got the Panda instead

The result is the worst possible for the Labour Party. A decisive victory for Ed Miliband would have been one thing. The party would have made the wrong decision, but it would have done so with its eyes open and with the leader's legitimacy unchallenged.

Instead, Ed Miliband lost the election in two parts of Labour's electoral college, among MPs and among Labour Party members, but won thanks to a 60-40 margin the other way among trade union members on a very low turnout.

If he had won by a clear margin I would have said that Labour had chosen the Panda as its leader. He is soft, cuddly and panders to every oppositional instinct in the party. There has been no position taken by the Labour government of which he was a member that he was not prepared to trash if he thought Labour members would like it. Tuition fees, Iraq, the third runway for Heathrow: you name it, he disowned it. Even, in the case of the third runway, a policy for which he had direct ministerial responsibility. There has been no part of the manifesto, which he wrote, that he has not been prepared to ditch.

But that is not what the party did. Labour Party members chose David Miliband. Labour MPs chose David. The party itself voted for what Ed Miliband tried to dismiss as the "New Labour Establishment", in other words, the supposedly hated Blairites. The party read the signals from the voters correctly. It knows that, although it didn't do as badly as it feared in the general election, a lurch to the left was not the answer. Party members and MPs took on board Tony Blair's warning that Labour lost because Gordon Brown "moved a millimetre" to the left of New Labour. They did not fall for the argument that Labour did better than expected by moving a millimetre to the left and, therefore, to do better still they needed to move a couple of centimetres further to the left.

They did not think that, because Labour has pulled level in the opinion polls, the next election is in the bag. They know that the opposition party often moves ahead of the government in the opinion polls within months of an election. They also know that Labour's poll rating is artificially inflated by the absence from the fray of the Liberal Democrats as a protest party, and they know that this protest vote cannot be relied upon in a general election.

The Labour Party and Labour MPs got it right, and the election was won by an operation run by the trade union machines. That means that Ed Miliband, who would have struggled against David Cameron in the House of Commons in any event, is going to be roasted every week.

Every week, the Prime Minister is going to point out that the Leader of the Opposition does not have the support of the members on the benches behind him. Not only that, he does not have the support of the members of his party in the country. Every week, Cameron is going to portray Miliband as the place person of the trade union bosses and the beneficiary of a boss-politics stitch-up.

If Ed Miliband had won the party members but lost among MPs, which is what I expected, the Conservatives would have teased him in the Commons, but he would have had as much legitimacy as Iain Duncan Smith – the choice of the party activists against the wishes of his MPs. No wonder Ed was the "cork-popper" candidate in Conservative HQ. The way things have turned out, that's not just a phrase; it would be worth lifting the austerity ban on bubbly for. David Cameron, lucky general, faces an opponent with less of a mandate than IDS.

Even if Ed Miliband had won comfortably in all three sections of Labour's electoral college, I feel he would have struggled against Cameron. This way, I fear that he fights with both hands tied behind his back.