A seismic bye election - but what does it mean?

Few traditions in British politics has a longer or more overwrought history than over-interpreting bye election results. A combination of Britain's voting system, the culture of party politics and the behaviours of voters have created many mid-term dynamics which do not filter through to general elections.

Traditionally British voters punish incumbent parties in mid-term bye elections but return to the fold in sufficient numbers by the next election that what looked like something of a seismic change fizzles out before it changes anything significant. 'You have to consolidate votes to keep the other lot out' carries a lot of weight in a First Past the Post system.

And yet it is hard not to feel that the Gorton and Denton bye election result is somewhat different. For a start, it is the sixth worse electoral swing for Labour in history, and if we look at the five bye election swings that were worse there is an alarming pattern.

The Ashfield bye election of 1977 was a Winter of Discontent disaster for Labour (it's worst swing ever) and signalled clearly that there was a fundamental shift in politics. Thatcher took power two years later. Next to that is SNP win in Glasgow Govan in 1988, which was a not-inconsiderable milestone on the way to the devolutionary transformations that began ten years later.

Then comes Winnie Ewing's victory in Hamilton in 1968 which created political shockwaves. Next to that is Runcorn and Helsby from last year which really cemented awareness that Starmer Labour was floundering. And then after that was the 2008 loss of Glasgow East to the SNP which was a last step on the way to an overall SNP majority in 2011.

So there is clearly significance in the scale of this result, and also in the direction of the shift. Labour has lost to its left before, arguably to the SNP but more clearly in instances such as George Galloway's bye election victories. Those, however, at no point looked like they were likely to be sustained in a General Election. That doesn't look true this time.

The English Greens are not a protest party or a vehicle for a high-profile established politician, and it wasn't in Scotland. Everyone has noted that Labour may have positioned itself on the wrong side of the 'vote for us to stop them' equation this time in a number of constituencies.

The implications across the UK as a whole can be guessed at but prediction would be unwise. The most likely outcome is the removal of Starmer as leader, and a psychological barrier passed for many with the Greens. This may or may not be seismic, but it is not insignificant.

The implications in Scotland are harder still to read. The English Greens made a decision a few years ago to downplay its liberal, identity-based politics and play up its economic radicalism. It is to be noted that the Scottish Greens did not only not follow suit, they voted to sever all relations with the English Greens in a move that now appears incredibly short sighted, though they appear to be making more of that shift now.

The Scottish Greens have another problem; power. They were in government, and their track record was not strong. They are not a challenger party but in many ways an establishment party (though with more radical policy positioning). And while the SNP have a very similar problem of voter disillusionment, it is not of the scale of that faced by Starmer's SNP.

Scottish Labour might be hit hard by the perception it is 'finished', or it could actually benefit from the departure of Starmer, or it might end up consolidating more than expected 'not SNP, not Reform' vote in the way the SNP is doing the same from the pro-independence side.

One of the likely problems in Scotland is that there is some reason to believe that the Scottish Election will have a large regional element and that trends may not be consistent across the country. So national averages may not tell us everything about what is going to happen on a seat-by-seat basis.

Equally, perhaps one of the lessons of Gorton and Denton is that unpopular parties which gain big electoral victories by default and continue to disappoint are extremely vulnerable to a backlash. Perhaps the SNP could win big and then rapidly decline in support.

Yet for all this uncertainty and for all the likelihood that this bye election result turns out to be one of the consequential ones, it remains likely that Scotland is predictable in a way that England and Wales is not just now. It looks like the SNP will end up with a strong result but short of overall power in a divided parliament for which no-one seems to have any real vision or sense of mission.

The lesson for Scotland from today's seismic shock may be learned in the Holyrood Election, but it might be a better parallel for what comes afterwards. Or for Scotland it might turn out to be one of those flash-in-the-pan results that leads nowhere.

When people say our politics is more volatile than at any point in post-war history, this is what they mean. Something important happened – but what? And what will it mean? The answer to that will unfold shortly.