Whether or not Lord Randolph Churchill first declaimed that the “duty of the opposition is to oppose”, it has been an unwritten law of British politics since the 19th Century. Jeremy Corbyn has followed that advice – except on the one issue that matters most: Europe. Yes, he says that a Labour government would negotiate a better deal than a ‘Tory Brexit’. And Labour is preparing to vote against any likely deal in the Commons. But, despite his insistence that he (like Theresa May) voted to remain in the 2016 referendum Corbyn has never – including at the weekend – challenged the assumption that Brexit is a fait accompli.

After last night’s backing for a lowest common denominator resolution for Tuesday’s party conference debate on Brexit, Corbyn has less than 48 hours to reverse that acquiescence in his speech on Wednesday. Even though the polls suggest it would enhance his standing as a national leader, there is little sign of it; indeed his closest ally John McDonnell has now said that even if the referendum grudgingly offered as an option actually happened, it should not revisit the issue of EU membership. Yet Sunday night’s bloodless composite would still leave Corbyn the room – if he chose to take it – to lead a growing movement against leaving the EU, building support on the personal inclinations of most Commons MPs, much of Britain’s business, ”leave” voters increasingly nervous of their economic future and the 48% of the electorate who wanted to Remain and have been effectively disenfranchised by the failure of the Opposition to perform its historic function.

It would be a huge step of course. Corbyn would be finally shedding his own long held Euroscepticsm –and even more perhaps, that of his strategic advisers who harbour an old left wing antagonism, forged during the Cold War, to the EU as a capitalist club. But there are precedents. Corbyn might take comfort from an even more dramatic apostasy, by one of the few past politicians still revered on Labour’s left. In 1957, Aneurin Bevan astounded the Labour conference by vigorously opposing – and defeating – a call to halt testing and manufacture of Britain’s nuclear bomb. The Bevanites’ shock was all the greater because their hero had had the party whip removed only two years earlier for supporting that very call. The times, and the issues at the heart of politics were vastly different. But Bevan’s warning to the party against deciding that “all the international arrangements all the international facilities afforded to your friends and allies must immediately be destroyed” still resonates today as Britain threatens to turn its back on its European partners and go it alone when multilateralism has never been more important.

Unlike Corbyn, Bevan was not the party leader. But he was the undisputed leader of Labour’s left wing, as Corbyn is. And it would be much easier for Corbyn. Not only most Labour voters but many on the left, most party members, including the younger ones who joined in such impressive numbers because of Corbyn’s leadership, believe in the EU. He can also ignore the threadbare argument that “the British people have spoken” in 2016. Any self-respecting opposition, immediately after an election defeat, sets out to persuade the electorate to change its mind by voting it into government next time. There’s no constitutional reason why a referendum should be different. If there was, Corbyn would not himself have said in March 2017 it was “absolute fine” for the Scottish government to hold a second independence referendum. Nor is it treating the British public as ‘stupid’ to offer a rethink in the light of consequences never predicted by the Leave campaign in 2016. And finally Corbyn would be seeking the support of Tory pro-Europeans he will need to vote down whatever deal May achieves, and who would prefer a referendum to the risk of handing Corbyn the Downing Street keys in the election Corbyn would understandably favour.

And what a speech it could be. He could expose the delusions fostered in 2016 by the Leave campaign, from the mythical £350million Brexit would release for the National Health Service to the fantasy that it is possible to have all the benefits of the EU without its obligations. He could quote Ralf Speth, CEO of the flagship Jaguar Landrover, who this month warned of the “horrifying” cost of crashing out with a deal – the prospect again recklessly threatened by Mrs May’s ministers when she was rebuffed at last week’s Salzburg summit. But Seth also warned that already “many companies are being forced to make decisions about their businesses that will not be reversed, whatever the outcome, just to survive,” Corbyn could cite the indefensible risks the UK government is taking with the historic 1997 Belfast agreement. On immigration, dislike of which most fuelled the Brexit campaign, Corbyn could enumerate the several ways in which the UK could tighten controls inside the EU (and without violating its rules on free movement). He could argue that much of the NHS and social care for the elderly would collapse without immigrant labour, And above all that restoring the blighted “left behind” communities afflicted by de-industrialisation lies not primarily with immigration controls but in reversing a decade of Conservative austerity.

It is not treating the public as “stupid” to let it take the final decision in the light of what the negotiations have already revealed about what Brexit will actually mean. A referendum is not an easy option. Securing it is one thing. Winning one in which a mood among many voters of “let’s get Brexit over with” will take some beating, is quite another. But it is a crucial first step. One of Corbyn’s own would-be ministers Barry Gardiner said last month that offering a second referendum undermines the “the whole principle of democracy in this country”, In fact a second referendum would enhance exactly that principle. And by openly willing it, Labour would look like a serious opposition again.

Donald McIntyre is a political journalist and sketchwriter