In general, by-elections are the hardest political markets to predict. Opinion polls are even less reliable than usual, asking a mid-term question that only a small minority of engaged voters have spent more than a few minutes considering.
Recent UK by-elections have seen some highly unpredictable results, particularly in supposedly safe Labour seats.
In 2012, George Galloway's Respect pulled off an enormous upset in Labour-held Bradford West on a whopping 53% swing. In 2008, the SNP took Glasgow East from Labour on a 22% swing. Labour were rated in excess of 90% likely to win on the Betfair market on both occasions.
In 2006, the Lib Dems won Dunfermline and West Fife, after Labour had been matched at the minimum odds of 1-100. In all three cases, the seats returned to their natural owners at the next General Election.
Galloway aside, the big by-election story of the last parliament was the rise of UKIP. Nigel Farage's insurgents took two Tory seats by wide margins, and finished second in a spate of contests in safe Labour seats. In Eastleigh, they came within a whisker of taking this high-profile Lib/Con marginal.
Thursday's contest in Oldham West & Royton would, in normal times, be very low-key. Labour would expect to hold it, with UKIP almost certainly finishing a respectable, if distant, second. But these are not normal times.
As I wrote on Sunday, this contest is pivotal for the future of the Labour Party and, with it, UK politics in general. Jeremy Corbyn has come in for relentless abuse from the media since becoming leader, very much against the wishes of the majority of Labour MPs and grandees. If national approval ratings are a guide, he is already a damaged figure.
The party is split, particularly over Syria but also on a wide range of issues from tone, strategy, personalities and policies such as nuclear weapons. This is an absolute must-win contest for Corbyn.
After peaking around 1.49 (67%), Labour's odds have bounced back since the weekend to around 1.29 (78%), but still worse than when Neil Monnery shrewdly advised against them last month at [1.22] (82%). Let alone the initial 1.05 (95%) quote.
However unlike bigger national elections, where the wisdom of crowds theory is in play, betting markets are a dubious guide to by-elections, as those dramatic past upsets illustrate.
They usually revolve around turnout, as around a third of the usual electorate won't bother. It takes quite some level of commitment to go out and vote on a freezing cold December night in Greater Manchester.
That usually spells very bad news for Labour. The principal reason they lost May's General Election so badly was an inability to get their supporters out at anything like the same ratio as the Conservatives. It was certainly the reason behind those previous, unimaginable by-election losses.
They must hope the Corbyn revolution lives up to it's billing. Whatever his wider unpopularity, nobody should doubt the significance of his successful leadership campaign. He engaged hundreds of thousands of people that were previously detached from politics, selling out venues every night, even in places where the party had no chance.
Labour now have more members than all other parties combined, and it is time to deliver. Pro-Corbyn activists have been piling into Oldham in recent days to help out. This is also a test of the effectiveness of the trade union movement that backed Corbyn.
The numbers in Oldham certainly favour Labour. They hold a 15,000 majority from May, although some of that is probably due to a personal vote for longstanding MP Michael Meacher, whose death triggered the contest.
However UKIP have proven to be brilliant at getting their vote out in by-elections, particularly when it comes to squeezing the Tories. They only lost Eastleigh due to postal votes, earning more votes on the day than anyone else.
An even more worrying signal for Labour came in the 2014 Heywood & Middleton by-election (also in Greater Manchester). Because the party's attention and limited resources were distracted by the more winnable Rochester & Strood contest on the same day, UKIP made very little effort on the ground.
Though Labour held on, the result was shocking, with John Bickley coming within 700 votes of an enormous upset. Bickley is now the UKIP candidate for this week's contest, and the party are hyping up their chances. Rather than starting from scratch, they are already in second place and there is a substantial 8,000 strong Tory vote to squeeze.
Labour are facing an existential threat. They remain very strong in inner-cities and university towns, where their core support of progressive younger voters, public sector workers and ethnic minorities tend to live. Everywhere else, they are disintegrating. In Scotland, they collapsed in the face of a left-wing, yet fervently nationalist movement.
Across their North of England heartlands, UKIP are generally in second place and closing fast. Their brand of right-wing nationalism appeals to older, working-class, once core Labour voters. The Tory vote is transferring to UKIP en masse in these places. UKIP came from nowhere to grab second in Oldham last May, almost certainly without a meaningful local campaign.
This was a problem long before Corbyn took over. But his pacifist, anti-monarchy, socialist positioning is the direct opposite of right-wing nationalism and toxic to the type of voters tempted by UKIP.
Oldham is a mixture of both. There is a sizeable Muslim population here. It has been the scene of race riots and was targeted by the BNP for years. It remains to be seen how opinion has moved in the town since, but it is by no means certain that Labour and left-wing sentiment will be the beneficiaries.
My instinct says Labour hang on with a vastly reduced majority, but they make no betting appeal whatsoever. I wouldn't be in the least bit surprised were UKIP to edge it, and that must make them the value pick at [4.5]. Their chance of victory is better than 22%.
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