Whether the UK votes to remain in the EU depends on Labour supporters. As many as two-thirds of Tories and almost all Ukippers oppose membership. To win, the In side needs the 9.3 million who voted Labour (and who mostly support the EU) to turn out.

The fear, recently expressed by David Cameron, is that they won’t. No.10 recognises that a campaign defined by Tory infighting, with Labour marginalised, could hand victory to Out. Without the opposition pulling its weight, the referendum risks becoming an opportunity to punish Cameron and George Osborne.

It is this danger that motivated Ed Miliband to give his first major speech since the general election. Speaking at Coin Street community centre in south London, Miliband was quick to note that his address would be followed by one from “our leader Jeremy Corbyn” after Easter. Many have charged Corbyn, who was agnostic about EU membership as recently as last year, with insufficient enthusiasm for the cause.

Miliband went on to confront the most popular arguments made by the EU’s left-wing opponents (“the lexiters”). Some, he noted, “see quite a lot they don’t like: austerity, the remoteness of some EU institutions, the response to the migration crisis, the proposed trade agreement with the US [TTIP].” In response, he made a vigorous social democratic case for remaining.

“Let’s not take the flaws in the implementation of a great principle and conclude that cooperation between countries is somehow the problem,” he said. “The idea that we could confront the great causes of the 21st century outside the European Union is simply a fantasy. We can’t end centre-right austerity across Europe on our own. We can’t tackle climate change on our own. We can’t make companies pay their taxes on our own. We can’t solve the refugee crisis on our own. We can’t confront any of the great injustices on our own. Nothing in our values, our history, our beliefs tells us otherwise.”

He continued: “We know the areas where we need change. We must champion the opening up of EU institutions. We must make the EU the powerhouse for tackling corporate tax avoidance. We should be persuaders for the EU stepping up on the environment and not shrinking back. And on the trade agreement with America, say ‘yes’ to trade across borders, but say ‘no’ to undemocratic, corporate dominated decision-making. This kind of reform agenda is not only necessary but is in my view, also possible.”

If it is TTIP that troubles Labour’s middle class members, it is immigration that troubles its working class supporters. Addressing the latter, Miliband warned that workers would lose more than they would gain from Brexit and that free movement might endure in any case (as in Norway and Switzerland). “The real answer is to do a far better job of tackling exploitation here at home,” he said.

While acknowledging that “there are honourable Labour colleagues who have been consistent advocates of Leave”, Miliband suggested that left-wingers should judge the Brexit campaign by its friends. “They believe low tax, low regulation is the way we succeed. You can hear it in their speeches and see it in their agenda and even read it in their articles in the Daily Telegraph. Think of the vision of Boris Johnson, Nigel Farage, Iain Duncan-Smith for the country. It is not my vision, it is not your vision and it is not the vision of nine million Labour voters either. If Britain left the European Union, it would not serve a progressive, optimistic agenda. It would serve a reactionary, pessimistic agenda. Tax avoiders want to divide country from country to drive down tax rates. Polluters want to turn country against country in a race to the bottom on standards. Russia and those who disagree with us want to divide Europe. Outside the EU that is what we would be exposed to.”

During the Q&A, I asked Miliband and Alan Johnson (the head of Labour’s In campaign), whether they were confident that the eurosceptic Corbyn would demonstrate the enthusiasm required. “Yes, I’ve talked to him about this ... All of us recognise our responsibility, I know Jeremy does too, in going out and making this argument,” Miliband told me (later praising Corbyn’s “top class” response to the Budget).

Johnson replied that while Corbyn was indeed "making his mind up" during the Labour leadership campaign, he was now an an unambiguous EU supporter. "After all the interventions, the words that are on the record, the video clips you can look at of Jeremy, who is not one, incidentally, for saying things he doesn’t believe in just for the sake of it"

It is characteristic of the surreal state of British politics that Cameron’s survival may yet depend on Corbyn persuading left-wingers that they too should set aside their concerns and vote to Remain on 23 June.