One of the conventional wisdoms since the great changeover on British political attitudes toward Europe 25 years ago when Labour became the party of Europe just as the Conservatives set off in the opposite direction is that the Labour Yes to EU vote in the forthcoming referendum is in the bag.
As he made his calculations in conceding a Brexit referendum in 2013 David Cameron assumed that accepting the first of the two main UKIP demands (the second being withdrawal from Europe) would help him in the 2015 election. Thereafter Labour, the LibDems, Greens, and enough Tories would vote to stop Brexit.
Events however have moved on. The LibDems have all but disappeared. Such Tory voices as are to be heard all seem keener on bashing the EU than advancing a pro-European case. UKUP's Mark Reckless, Tory MP Bernard Jenkin, and Business for Britain's Matthew Elliot appear on joint anti-EU platforms and George Monbiot, the Green guru calls for an Out vote.
And now it is far from clear that Cameron can rely on Labour and their allies to pull his chestnuts out of the fire and prevent a Brexit vote. The prime minister faces the same problem that confronted President Jacques Chirac of France when he insouciantly called a referendum on the EU constitution in 2005 assuming the Socialist Party of Jacques Delors, François Mitterrand, François Hollande and Dominique Strauss-Kahn would stay true to its European faith.
Instead, the Socialists split down the middle. Laurent Fabius, a former prime minister and Arnaud Montbourg, an ambitious young rising star led a Non campaign for the mainstream left. They won and the EU constitution was sunk.
So far, the Labour MPs calling for Brexit are septuagenarian veterans of anti-EU politics going back to the days of Michael Foot, Tony Benn and Peter Shore. They have a powerful ally in the journalist Lochinvar of the left, Owen Jones, who has coined the phrase 'Lexit' for left exit from Europe. In the current London Review of Books, Tariq Ali also calls for No vote.
They like many on the left, are disgusted at the way Greece has been treated by the conservative government in Germany and its conservative allies European People's Party in Brussels, effectively the ruling political formation in the EU.
Also hovering in the wings are the trade unions. Moderate pro-EU union leaders like the TUC's Frances O'Grady and Sir Paul Kenny of the GMB have made clear that if Cameron's negotiations with the EU result in the watering down of the largely symbolic Social Europe provisions then they expect Britain's 30 million workers to say No to any such deal and they won't be able to deliver trade union support for a Yes vote.
The EU has hardly featured in the Labour leadership debate. Andy Burnham, Yvette Cooper and Liz Kendall are like most of Labour since the Blair era ended - neither Europhiles nor Eurosceptics. They don't support withdrawal. Like Cameron they call for a reformed EU. But they know core Labour voters are hostile and attracted by Ukip. Labour gave up thinking about Europe a decade ago and few Labour MPs have found language to be positive about the EU and especially free movement of Europeans.
Jeremy Corbyn has denounced the treatment of Greece but in the past ten years he has never spoken in the Commons in any European debates on the principle of EU membership. Corbyn's internationalism is global rather than European, based on moralising rather than political alliance building. His Commons interventions focus on Palestine, the arms trade, refugees, and against military interventions in Africa, the Middle East and Ukraine.
Corbyn has now said he is not in favour of Brexit though in a grudging statement to the New Statesman full of complaints about the Single Market and the European Central Bank he manages to convey the opposite of enthusiasm for European unity and integration.
Whoever emerges as leader is, at best, likely to allow the Labour Party a conscience or free vote on Brexit. Alan Johnson will lead a pro-EU charge but the chances of Labour lining up enthusiastically behind whatever package Cameron brings back are slim. Labour as a party will end up where it wont say Yes, wont say No, won't say Stay, won't say Go. Instead Cameron will be left like Jacques Chirac swinging in the wind on a self-made hook of his EU plebiscite.
If the vote is Yes to Europe, Labour can say the Tories have wasted two decades in the cul-de-sac of Euroscepticism and celebrate a defeat for the off-shore newspaper proprietors and their anti-EU obsessions. If Brexit is the result, Labour will demand Cameron's departure and a fresh consultation, via an election, to stop British isolationism and departure from the world stage as a major power.
But until then the referendum is Cameron's to lose and not Labour's to win for him.
Denis MacShane is a former Europe minister. A revised, updated version of his book, Brexit: How Britain Will Leave Europe, will be published by IB Tauris next month.