One of the great dramatic rituals of British democracy will play out tonight when, at the exact moment Big Ben strikes 10, the main broadcasters simultaneously reveal the results of their jointly-funded exit poll. (In days gone by, the BBC and ITN each used to have their own exclusive exit poll, but now they put their combined eggs in one basket.) If past form is anything to go by, the results of the poll will probably not be leaked in advance - although we can be sure that the Neil Lovatt brigade will be interpreting every little movement in the betting markets from the afternoon onwards as a sign that somebody has found out something.
There are two key points that need to be borne in mind about the exit poll -
1) There is a significant chance that it will throw up a major shock. That's happened in all of the last three general elections. The 2005 exit poll forecast Tony Blair's majority to be "only" 66 seats, which was significantly lower than most people expected. In 2010, it was predicted that the Cleggasm had melted away and left the Liberal Democrats with fewer seats than they started with - something that hardly anyone had entertained as a serious possibility. And in 2015, expectations were completely turned on their heads as the Conservatives were predicted to have achieved a net gain, Labour were predicted to have suffered a net loss, and the SNP were forecast to have taken all but one seat in Scotland.
2) If a shock is predicted, it's highly likely to be reflected in the actual result. In 2005, Labour ended up with exactly the 66 majority that was suggested. And I'm sure we all remember Paddy Ashdown simply refusing to accept the exit poll's pessimistic estimates for the Lib Dems in both 2010 and 2015, and how he was later forced to eat his words.
So this is my worry. We're all a little bit too cosy in our assumptions about the minimum number of seats we think the SNP are practically guaranteed to win. Based on previous shocks, we should really be braced for almost any number to flash up on the screen - it could be below 40, in theory it could even be below 30. And if the prediction is much worse than expected, we'll almost immediately have to reconcile ourselves with the likelihood that we're not looking at a 1992-style dud poll, but rather at a reasonably accurate prediction.
It's not too hard to construct a case for why the SNP might underperform expectations. The polls we saw last night basically put the SNP in the same position - or perhaps a marginally worse position - than they were in at the time of the local elections. None of us need any reminders that the SNP's lead over the Tories in those elections was only 7%, rather than the 10-15% that the polls would have led us to anticipate. If you factor in the impact of independent candidates standing in areas where the Tories are strong, it's arguable that the SNP's de facto lead on the day was even lower than 7%. Now, we've tended to assume that the SNP only suffered in those elections because of particularly extreme differential turnout - Tory supporters were whipped up into a frenzy over the issue of an independence referendum, while the SNP fought a rather bland campaign that didn't motivate their own core support. I still think that's an extremely plausible theory...but what if it's wrong? What if, plain and simply, the polls have been systemically and significantly overstating the SNP's true lead over the Tories? In that case, all bets would be off for tonight.
That's the pessimistic side of the coin, but here's the optimistic side. A couple of threads ago, Calum identified some of the SNP-to-Labour switchers among his own peer group - young people who support independence, but who wrongly believe that a vote for Labour is the most effective way of getting the Tories out. I have a sneaking suspicion that if those people lived in Moray or in Perth & North Perthshire, they would be well aware by now, either from leaflets or through word of mouth, of the folly of that way of thinking in the context of the local battle. It's just possible that the polarisation of this campaign along Tory v anti-Tory lines is actually working in the SNP's favour in Tory-SNP battleground seats, with natural Corbyn supporters swinging heavily behind the SNP. If that's happening, the swing to Labour must be disproportionately taking place in ex-Labour heartlands - but even if that's true, the numbers might still be insufficient for Labour to take back very many seats from the SNP. They're starting from such a long way back almost everywhere.
So that's the scenario in which it's feasible that the SNP might just about get the strong mid-40s result that everyone seems to be expecting - but if it does happen, it looks like it could be a bit of a tightrope-walk.
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As long-term readers know, I have a great regard for the predictive powers of Stephen Bush (in spite of the brief slanging-match I had with him after the Brexit referendum). So I've been waiting with bated breath to hear his verdict on the suggestions that English Labour are doing much worse on the ground than the opinion polls suggest. Intriguingly, he departs from the conventional wisdom in saying that the Labour surge is real, and that he's found evidence of it from speaking to local organisers. However, he also thinks the extra votes are very inefficiently distributed and that Labour still face a net loss of seats even if their vote share increases markedly.
In a perverse way, I take some heart from that assessment. If there's to be even the remotest chance of a hung parliament, the number one prerequisite is that the change in public opinion detected by the polls at least has to be real.