Ed Miliband is going to promise a referendum on Britain's membership of the European Union. And that means the referendum will almost certainly happen. Previously, I had assumed that David Cameron's promise of a referendum was more about managing the Conservative Party than about European policy – although that's not to say it is a bad idea. In fact, the Prime Minister's policy of renegotiating the terms of our membership of the EU and cementing the result with a referendum, with the aim of staying in the EU, is probably closest to what most British people want.
Cameron's chance of winning the next election with a Conservative majority is probably less than 50 per cent. I therefore thought it was more likely that the referendum wouldn't happen than that it would. Labour and the Liberal Democrats are currently opposed to the idea.
But things are changing. When Clegg stood in for Cameron at Prime Minister's Questions two weeks ago, he made a calculated aside. "My party has always believed there should be a referendum on Europe when the rules change," he said. "By the way, I think it is a question of when, not if, because the rules are bound to change."
That is the sound – "when, not if" – of a politician preparing to change course. Clegg can see the same logic that is closing in on Miliband, and he does not want to be the only party leader going into the election telling the voters that he won't let them have a say.
So why do I think that Ed Miliband will commit to a referendum? Partly, because I have some inside information. I understand that Ed Balls urged the Labour leader to go for it in the last few months of last year. It was known that Cameron intended to make a big speech about Europe, and word leaked out from Downing Street that in it he would promise an in-out referendum. Balls urged Miliband to get ahead of the Prime Minister and be the first to make that promise. Labour would assume the mantle of democracy – it could be the People's Party again; and it would be the pro-European party, because, Balls argued, the people would vote to stay in, and that would shut up the Eurosceptics for a generation. Above all, though, his argument was tactical: that we must not "allow ourselves to be caricatured as an anti-referendum party" – as he said publicly, once it was too late.
The Labour leader hesitated, however. His instinct – he is a pro-European who had always thought a referendum was a device of the antis – conflicted with his Brownite tactical politics. He was getting the opposite advice from Tony Blair, Peter Mandelson and his own brother. Promising a referendum would be a disaster, they told him. Blair said publicly on the day that Cameron finally made his big speech that it was "pretty mad" to open the way for Britain to leave "the biggest business market in the world, on our doorstep". Tactically, their argument was that Miliband should let the Tories tear themselves apart, a task in which they have proved themselves adept. By then, Miliband had decided by not deciding. By indecision, he had sided with the Blairites rather than with "Balls and the Brownite restorationists", as Michael Gove called them last week.
About half the Shadow Cabinet probably lean towards a referendum. As well as Balls, Jon Cruddas (policy) and Jim Murphy (defence) are advocates. The main voice against is Douglas Alexander, Shadow Foreign Secretary, who worries about fighting a referendum campaign in the early months – and it would have to be early on – of a Labour government, against a Tory party turned fanatically Eurosceptic in opposition.
Balls will continue to press for a referendum, but the main reason for thinking Ed Miliband will come out for a referendum is nothing to do with inside information. Outside the Shadow Cabinet, the Labour Party is stirring, slightly. John Prescott, who was its longest-serving deputy leader, came out for a referendum in his Sunday Mirror column last weekend. And this week we will see the formal launch of Labour for a Referendum, a campaign chaired by John Mills, the Labour donor (and brother-in-law of Tessa Jowell), and supported by 15 MPs.
But the main reason Miliband will change is that he can imagine an election campaign in which he tries to hold the line on being "not opposed in principle" but promising a referendum only if there are EU treaty changes.
The party has been through this before. In April 2004, Blair suddenly said the UK should have a referendum on the EU Constitution, the grand vision that was eventually watered down to become the Lisbon Treaty. However brave he may urge Miliband to be now, when he was a year away from the 2005 general election he was not prepared to tell the voters that they couldn't have a separate say.
Then, as now, few people probably cared enough for it to change their vote. But when the election comes closer the Labour leader will not want to put off even a handful. I predict that, within six months of the election, all four main parties will be committed to a referendum.