We've discussed this before, but by-election upsets are much more important when they happen just before a general election than when they happen at any other time. That's why Govan 1973 is more historically significant than Govan 1988 - it generated momentum that just a few weeks later carried the SNP to an unprecedented general election breakthrough.
Probably the most important by-election in modern history was Darlington in 1983, because it took place on the eve of a general election that would decide whether Labour lost their position as the main opposition to the Tories - and if they had, there might never have been a Labour government again. The SDP started the by-election as favourites, but their campaign was almost single-handedly destroyed by the BBC's Vincent Hanna (the Michael Crick of his day) who relentlessly undermined the candidate's credibility at press conferences. As a result, Labour held on for an unlikely victory, and a few weeks later fended off the SDP/Liberal Alliance on a national level by just 3%.
It's quite possible that Darlington changed the course of history - if the SDP had won, Labour might have slipped to third-party status, or else they might have quickly changed their leader, in which case Denis Healey could have mounted a credible challenge to Margaret Thatcher. Either way, things would have been very different.
Rochester and Strood tonight looks for all the world like a similar turning-point. If, as expected, there is a UKIP victory, it's likely there will be a bandwagon effect for Farage involving further defections, less than six months before a general election. But if there's a surprise Darlington-esque hold for the defending party, the UKIP bubble may be deflated somewhat, and next May could prove to be a massive anti-climax for them.
From a hard-headed tactical point of view, it's difficult to know what we should be hoping for - a UKIP surge could split the right-wing vote and bring about a majority Labour government, thus depriving the SNP of any leverage in a hung parliament. But on the other hand, a strong UKIP could hasten the crisis over Britain's EU membership that might lead to Scotland becoming independent fairly quickly. We'll just have to wait and see how it all plays out.
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So, who can resist a mysterious summons from Robin McAlpine? He asked if he could run something past me, so I ended up meeting him and Miriam Brett in a cafe in the west end of Glasgow this afternoon. I'm not sure I was able to help them as much as they'd hoped, but from my own perspective it was fascinating to hear about their future plans, which are almost mind-boggling in scope and ambition.
One thing that became clear is that Robin could urgently do with a good night's sleep, or a full day off now and again. So if anyone is trying to think of an 'alternative' Christmas present to give him...