JO Swinson, leader of the Liberal Democrats, will “never forgive” David Cameron for having called the referendum which resulted in the British people voting to leave the European Union. She’s so upset about it that her party’s official policy is now to reverse the popular vote on Brexit without consulting the public at all.

I wonder whether she could bring herself to forgive a politician who stood on a manifesto that promised a referendum on wide reform of the EU and joining the Euro, then on one that promised a straight In/Out referendum on the EU with a guarantee to implement the result, then on one that promised to introduce a soft Brexit, if that were the verdict after a second referendum that would offer remain as an option, then advocated a second referendum with a choice between no deal and remain, and later decided the best bet was a second referendum, to be ignored unless its answer was in favour of remaining in the EU, before get round to “Sod democracy; let’s just cancel the whole thing without any voting at all”. But since that politician is her, between 2005 and now, I imagine she can.

Even so, her party’s current position may, in political terms, be their best bet since, despite the previous range of opinion in both parties, the Conservatives under Boris Johnson are the party of Brexit come hell or high water while Labour under Jeremy Corbyn have become, well, frankly, anyone’s guess.

Indeed, it’s mystifying that the Lib Dems have not hoovered up more of the remain vote which was, after all, 48 per cent of the result. Yet polling has them at about 15 per cent (not much more than the Brexit Party). Being the Stop Brexit Party may be neither liberal nor democratic and, despite the impression given by Holyrood, Westminster, social and mainstream media, may even still be a minority view, but it’s certainly the one that ought to maximise the Lib Dem vote.

The principal argument against it is that it won’t happen, or as Niall Hodson, a brave dissenting delegate at the party conference, put it, lead to the Lib Dems “promising something and doing something else”.

The reason is that there is no prospect that the party will be the largest one at the next election, and Ms Swinson has already ruled out a coalition with either the Tories or Labour – something that this particular policy would, in any case, make extremely difficult.

It’s also an interesting inversion of the party’s position in relation to the SNP, which criticised the Lib Dems for arguing against a second independence referendum, even if the Nationalists get (as most people expect they will) a majority of Scottish seats.

Interesting because it seems reasonable to claim that, were the Lib Dems to win a majority of UK seats, that would look like a mandate to halt Brexit, while it’s highly contentious to claim that an SNP majority at either Holyrood or in Scottish Westminster seats automatically indicates a preference for independence, or even a second go at a referendum. That’s because the SNP are Scotland’s Lib Dems.

Of course, both parties have their true believers, who wholeheartedly support their respective policies. You can tell that’s true, because they do so even when the policies are the polar opposite of what they were a decade ago. But both also attract support from people who can’t bring themselves to vote Conservative or Labour at any given moment. Particularly because lots of people who formerly supported one of the two largest parties won’t, in any circumstances, vote for the other. Naturally, that goes double for defecting MPs, who almost all, Tory or Labour, discover they’ve secretly been Liberal Democrats all along.

But the electoral arithmetic translates the SNP vote (even if a little of it is just “not the other parties”) into majorities and real power. It doesn’t do the same for the Lib Dems – one reason why they are, reasonably enough, obsessed by the mechanics of the electoral system – except in by-elections, where they are famously dirty and successful campaigners.

In Scotland, the anti-Brexit (and the not-Tory, not-Labour) vote is SNP, unless the other Union is an even greater priority. But it’s not clear where the Lib Dems can make major gains in England and Wales, even though there should, one would think, be loads of people bursting to vote for them. Many key swing seats are straight Tory/Labour marginals, while the traditional Lib Dem heartland of the South West of England is, oddly enough, a Leave-voting area.

Voters who want above all to stop Brexit will, and indeed probably should, vote Lib Dem. In England, it may be their only option. If they believe in other Lib Dem policies, or at least prefer them to Tory and Labour ones, that’s a bonus. But if Brexit is halted or reversed, it won’t be by a Lib Dem government, because there isn’t going to be one.