Question: would Jeremy Corbyn now be “20 points ahead” in the polls if he came out against Brexit?

Answer: no.

The belief that he would is one that is held by a large number of New Statesman readers and does incredibly well whenever it is aired on Twitter. But I don’t see how it can possibly be true.

For one thing, the numbers don’t add up. At the general election, Labour got 40 per cent of the vote, two points behind the Conservatives. At time of writing, UK Polling Report’s rolling average gives the Conservatives a three-point lead, on 42 per cent to Labour’s 39 per cent.

At the general election, three-quarters of 2017 Conservative voters backed a Leave vote in 2016, but crucially, a quarter backed a Remain vote.

So there is just about a mathematical basis for the “20 points ahead” idea, in that if the remaining chunk of Conservative Remain-voters switched as one to Labour, the Labour vote would be up by ten points and the Tory vote down by ten – a 20-point difference.

But there are a couple of problems here. The first is that while three-quarters of 2017 Labour voters backed a Remain vote, a quarter of 2017 Labour voters backed a Leave vote. You can see the same pattern in Ian Warren’s seat-by-seat analysis of voters – in most seats, the amount of Remain voters backing the Conservatives and the number of Leave voters backing Labour is essentially equal.

So the “20 points ahead” thesis is making a series of big and, to my eyes, unsupportable claims: the first being that voters who backed a Remain vote in 2016 and a Conservative one in 2017 are sufficiently motivated by Brexit considerations that they will overcome any other objections they might have to Jeremy Corbyn’s Labour party, but aren’t sufficiently motivated by Brexit to have avoided voting for an explicitly pro-Brexit party.

The second is that this movement of voters would be one way – that there would not be an equal and opposite movement of Labour voters who backed a Leave vote.

Added to that problem, both parties ran on policies that were not only pro-Brexit but – thanks to a series of different red lines – committed them to fairly hard flavours of Brexit, too. The parties running on explicitly anti-Brexit platforms (the SNP, Plaid Cymru, the Greens and the Liberal Democrats) largely did worse in 2017 than in 2015, and where they did better it was at one another’s expense. (There were a handful of exceptions for the Liberal Democrats in London, Bath and Oxford.) They also lost Leave voters even in the cases where they gained Remain-backing ones.

So is there no evidence that there is an untapped pool of Remain voters waiting to boost Labour among the 2017 electorate? Well, there were some people who voted to Remain who did not vote in 2017, but a bigger group voted to Leave and did not vote in 2017. Once again, a big claim is being made: that you can win Remain voters without there being any reaction from Leave voters.

It is true that Labour Leave voters say they are less motivated by their referendum vote than Conservative Leave voters do. There is a regrettable dearth of detailed polling of Conservative Remain voters however, and this is what should chiefly occupy us. It seems likely, given that they voted for a party that is committed to Brexit, that they aren’t that motivated by their referendum vote – or at the least that they are no more motivated than Labour Leave voters.

What about the large chunk of Liberal Democrats who voted to Remain? Well, once again you have the same problem: there aren’t enough of them to outweigh losses of Leave voters for Labour, and crucially they mostly don’t live in electorally useful places.

None of which is to say that pro-Europeans shouldn’t oppose Brexit, or that they shouldn’t seek to persuade Labour to do so. But that opposition needs to be rooted in an acknowledgement that an anti-Brexit force of any real significance isn’t merely waiting to be called into force by a repentant Labour party. A pro-European majority in the United Kingdom and in the House of Commons can’t be sued, bullied, sneered at, hashtagged or shouted into existence. It has to be persuaded. And at the moment, it doesn’t exist.