John Rentoul: This is not a merger. It's a takeover

The Tories turn left to meet their new pals the Lib Dems, as Labour scoops up political refugees at the roadside

A Liberal-Conservative government is a bit like discovering that the Earth goes round the Sun. The assumptions of the ages are overthrown. In this case, that a Tory-led government would mean the end of the world. Instead, it is going to start cutting the deficit this year, cancel the third runway at Heathrow and bring in the free schools that Tony Blair wanted. What is not to like?

David Miliband, to whom now falls the task of opposing the new politics, once declared: "I'm not saying I'm the new Copernicus." It was one of those Milibandish things that he says, ineffectively self-deprecating, learned, left-field. He had been talking about how public services should revolve around people rather than the other way round. But now he faces the challenge of making sense of a more profound change.

I'm told that Miliband thinks that David Cameron has achieved "five years of modernisation in one day". If anything, that's downplaying the importance of what happened on Tuesday. Securing a coalition with the Liberal Democrats has been compared to Tony Blair's rewriting of Clause IV of Labour's constitution. Actually, it was bigger than that. It was Clause IV and The Full Monty rolled into one.

TFM, if you remember, was Paddy Ashdown's code in his diaries for a coalition between Labour and the Liberal Democrats. It was the goal for which he pressed, along with TGPB (Roy Jenkins, The Great Poo-Bah), with as much enthusiasm as last week he pressed for a coalition with the other side. The man whose ambition was once to put the Tories out of power for a generation, was last week instrumental in broking a deal that could put them in power for almost as long. One of the abiding images of Tuesday night was that of Lord Ashdown, emerging from the meeting of Lib Dem MPs and peers that approved the coalition with a one-word comment, "Hooray!"

Whatever Cameron's weaknesses and inconsistencies on policy, he showed himself in the early hours of the day after polling day to be a shrewd judge of the sweep of British political history. Hoping for a small majority, he instantly adjusted to the unfamiliar situation. No doubt he and George Osborne had discussed the coalition options beforehand, but the precise arithmetic of the new House of Commons was peculiar and not easy to calculate. After all, Gordon Brown, some of his advisers and many Labour supporters still thought a Lib-Lab-and-Leftovers rainbow coalition might be possible on Monday, when Brown made his first, pre-announced resignation speech on the steps of No 10. But Cameron had already seen how difficult that would have been. By making his "Boco" ("big, open and comprehensive offer") of a coalition on Friday afternoon, the Tory leader seized the initiative. Then all he had to do was wait until the adding machine of parliamentary arithmetic clanked to its pre-ordained conclusion. By Monday night it was obvious that Labour, with or without Gordon Brown, could not guarantee to get a referendum on changing the voting system through Parliament. The Tories could.

The Alternative Vote is not even the system the Lib Dems want, but it would work to their benefit. So they tried to pretend that it wasn't the most important thing, and that really they were interested in poor people and the planet. But it was the only thing, and it was the only big concession that Cameron made that he didn't want to make anyway. (Although the timing of the referendum remains a bit of a mystery.)

That is the simple story of the coalition deal: Cameron used the Lib Dems as an excuse to ditch the inheritance tax cut that he cannot afford, but the rest of the so-called concessions were cosmetic. Nick Clegg gets to be Deputy Prime Minister, but he is based in the Cabinet Office backing on to No 10. As John Prescott discovered, that is not where they keep the levers of power. At some point in what could turn out to be a three-term Cameron administration, Clegg's status will begin to resemble that of a day boy in a school of full boarders. George Osborne kept Vince Cable – the only other Lib Dem with the authority that comes from being a television personality rather than a politician – out of the Treasury, while keeping what he calls Lib Dem "cover on cuts" in the form of the equally clever but less well-known David Laws.

This is a takeover, not a coalition. The Liberal Democrats are being used in an even bigger rebranding exercise than New Labour. This is, unless you are a partisan Lib Dem, a good thing. Britain needs a liberal Conservative Party that is concerned about equality. By its creation, this Government has strengthened the Conservatives' claim to the centre ground. And, paradoxically, it has strengthened Labour, because a chunk of left-wing Lib Dem support has defected. That return to two-party politics would be reinforced by the Alternative Vote, if it passes the referendum, despite giving a one-off boost to the third party. In Australia, it has promoted a strong two-party system. I think this is a good thing, but others might wonder.

Labour is, therefore, in a stronger position than many of its supporters thought during the election campaign, when it looked as if it might be overtaken by the Lib Dems. And it now faces the curiosity of a leadership election contested by two Milibands, or 0.2 Centibands. Our ComRes poll today suggests that David has a commanding lead in a party that, unlike the Tories, tends to elect the front-runner. But whoever emerges as Labour leader faces the terrible danger of being sucked into the negative politics of saying "no to cuts".

Yet the new universe will be defined by fiscal austerity, and there is only one place for the opposition to go: it has to say that it would not cut as quickly or as deeply as the Government. It has to say that it would allow borrowing to be higher, or that it would raise taxes. Go to it, Copernicus.

John Rentoul blogs at www.independent.co.uk/jrentoul