Britain will Brexit if young voters fail to turn out. It’s a real possibility.

Opinium was the most accurate online pollster at the 2015 General Election. They have just released their final poll of the referendum campaign: Leave 45%, Remain 44%, and 11% undecided.

There was a swing in the polls last week, which began before the Jo Cox assassination and seems to have primarily been driven by worries about a post-Brexit economy, which spiked sharply at the time of George Osborne’s ‘punishment budget’. That has stalled and may have even gone into gentle reverse.

There was an assumption that there would be a late Remain surge. It has not materialised. There is also an assumption that those making up their minds on polling day will break for more towards Remain. This is on the basis that late deciders allegedly ‘always’ break for the status quo in referenda. Well ‘always’ is too strong a word, and past history is not a crystal ball for future performance.

John Curtice, the Glasgow-based academic whose exit poll almost perfectly called last year’s General Election result, has warned repeatedly this week that the betting markets are greatly underestimating the possibility of a Leave vote and said this afternoon the vote sits on a 50:50 knife edge.

YouGov have warned that the huge age skew of the electorate makes things much harder for Remain. A look at their interactive model is sobering for Remainers. Older people are much more likely to vote than younger people – and many older people have already voted by post.

Britain Stronger In Europe are aware of that fact, and that they probably need epic youth turnout to counter it.

Hence the flood of celebrity endorsements this week, not all of them vapid, some of them well argued and personal: Rio Ferdinand’s and John Barnes’ are particularly good, and cockney darts legend Bobby George’s video is a masterpiece of profane viral marketing. The Clarkson-May endorsement video has now been seen by well over 2 million people between YouTube and Facebook, more than buy The Sun these days. I imagine a few promises of knighthoods were made in the process.

The Leave online effort late in the campaign has been pedestrian in comparison, mostly clips from TV debates. But then, it didn’t need to be, because their voters are more reliable, and are much more likely to read newspapers – heavily anti-Europe – and much less likely to get news largely from social media.

Both campaigns have fought relatively shambolic ground wars over much of the country. I know for a fact – I was involved in one and live in a core target area for the other. Neither was impressive.

So, what will decide this knife edge election? Three things, I think: obviously, what way the undecideds break; secondly, the intensity of the youth vote; and, finally, will the anti-politics angry people who haven’t voted in years turn out?

I don’t think there’s much doubt about the last one. Turnout will be high. Postal vote returns are above General Election levels. I reckon turnout will be in the low to mid 70s %. That’s sticking my neck out, but there is a quiet intensity here in small town England, and not just on the Leave side. All of a sudden, after the Jo Cox murder, people started walking around town with ‘In’ badges and posters appeared in windows, especially in the better off parts of town.

Will the young turn out? Almost certainly not as well as other people – they never do, anywhere, even in Obama’s 2008 wunderjahr in America. There is, however, hope for Remainers in that ORB’s tracker of certainty to vote by age has seen a real spike among under 35s over the past 10 days and also a slide in certainty to vote among over 65s. For Stronger In, the social media viral bombardment needs to keep going until 9.50 pm tomorrow, not just to get young people to vote, but to get them to nag their friends to vote.

So the final question is how will the undecideds break? My head tells me more towards Remain, and not just because of ‘conventional wisdom’. Those for whom Brexit could be sold on immigration and sovereignty are already voting to Leave. A lot of the undecideds are very conflicted – they like the idea of a European Union, but they hate what the actual EU looks like in practice. Mostly they really dislike the campaign Leavers have fought, but didn’t have much time for Project Fear either.

The decisive factor, to my mind? The economy, stupid.

I don’t think Leave has sold the deal on the economy. In unguarded moments, Leave leaders have repeatedly acknowledged there will be a post-Brexit economic dip, and we all know politicians spin, so that means they think it’s going to be worse than that. Particularly among the 25-45s, where debts are higher, especially on mortgages, job security is lower, and the emotional pull of nationalism less, reasoning tells me they’ll break for Remain, maybe quite heavily.

I can smell a very definite shift of Tory undecideds and even soft Leavers to Remain here in the southern English shires, and I have today seen three posts on Facebook, two from paid up Tory Party members, explaining why they had moved from conflicted Leavers to sceptical Remainers. All cited the lack of a credible post-Brexit gameplan as being crucial.

I’ve also long wondered would there be a ‘silent majority’ factor in play. People keep their heads down when they know they’re in a minority surrounded by people who passionately disagree with them. Leavers have been more vocal and passionate throughout the campaign. It takes self-confidence and a serious command of political facts to argue with an enthusiastic Leaver in full flight. In working-class areas, in particular, I think those with doubts on the wisdom of leaving have just kept quiet.

In the secrecy of the polling booth, people can do things they never thought they’d do when they had to justify it to others. The Left discovered this to their cost in many Western countries in the ‘80s and early ‘90s. Could the nationalist Right be about to discover that now?

Either way, there will be at least a very significant minority of last minute undecideds breaking for Leave.

Given that, the biggest single factor affecting Remain’s chances is how much higher than normal turnout among the under 35s is. The young, most strongly in favour of the EU and most likely to be negatively affected by any post-Brexit economic crash, must take the responsibility of keeping the UK a member into their own hands.

And now the fate of the nation rests on crosses marked in wooden booths on little bits of paper, and the soft rustling of ballot papers being counted.