Hopefully my radar isn't faulty on too many occasions, but it's probably fair to say my headline one year ago today of 'Is Rochester and Strood the most important by-election since Darlington 1983?' looks a tad silly in retrospect. I thought that a UKIP victory might produce a bandwagon effect leading to more defections, and ultimately to a telling breakthrough for Farage at the general election, but...well, it didn't. The outcome wasn't unexpected, and thus didn't produce any kind of shock factor - in fact the media reaction was extraordinarily muted.
But now that Jeremy Corbyn is Labour leader, we've moved into territory where we might expect to encounter a genuine Darlington-style contest sooner or later. One of the reasons that famous by-election 32 years ago was so crucial is that if Labour had lost to the SDP, as they were initially expected to do, Michael Foot's position as leader might have looked untenable and he might have been replaced by Denis Healey in time for the general election a few weeks later - in much the same way that Bob Hawke had just replaced Bill Hayden as Australian Labor leader after defeat in the Flinders by-election. The 1983 general election was probably unwinnable for Labour under any leader, but it seems plausible that Healey might have saved a good few dozen seats, perhaps paving the way for a return to power several years earlier than 1997, and under a much less divisive leader than Tony Blair. So Darlington may be a classic example of 'a good election to lose' - and unfortunately for Labour, that was the election they actually won in 1983.
For anyone who thinks Labour's decline into irrelevance across the UK serves the best interests of the pro-independence movement, it's therefore hard to judge what would be a good result in Oldham West and Royton, where UKIP are rumoured to be running Labour close. I'll say straight away that this isn't Corbyn's Darlington - he would survive a defeat, but such an early electoral wounding would clearly be a landmark moment that would further darken the mood within the PLP. From a strategic point of view, what we don't want to see is Corbyn eventually being replaced with a charismatic leader who can transcend the divisions within the party (I'm not really sure if such a person exists, but one or two names have been mentioned as possibilities). So if you think a UKIP win on 3rd December has the potential to help set in train a sequence of events leading to that outcome, it might just be better if Labour cling on.
There's also a more immediate arithmetical reason why the SNP might prefer a Labour victory. We've already seen one crucial vote in this parliament where the SNP, the DUP, Plaid Cymru and the Greens lined up together in an attempt to defeat the government - but Douglas Carswell of UKIP went into the Tory lobby. So one more UKIP seat could make it harder for the SNP to hold the balance of power when there is a modest Tory rebellion. (Having said that, if Labour carry on abstaining as often as they have, that'll largely be an academic point.)