IF by Friday morning it becomes clear that the long-running saga of the Rutherglen constituency, and its former MP Margaret Ferrier, has played a key role in a major Labour recovery across Scotland, the main lesson the SNP may take away is that pro-independence seats in Parliament are precious resources and that there are long-term consequences to throwing them away like confetti.

When Ferrier was found to have broken Covid rules, the SNP didn’t stop at just suspending her. They later joined Labour as one of only two officially registered participants in the campaign to persuade voters to sign a petition to recall Ferrier and trigger a by-election.

That by-election resulted in a massive 20% swing from SNP to Labour, generating some of the pro-Labour momentum that the SNP are having to resist this week all over the country. The strategic thinking was probably that the petition would succeed anyway and voters would give the SNP at least some credit for having been morally righteous. But fewer than 15% of the constituency’s electorate signed the petition, not far above the 10% threshold required for success.

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Therefore without the SNP’s active participation it’s just conceivable that Ferrier might have served out the rest of her term as an independent MP, and the SNP would now be trying to defend her seat with a new candidate in less unfavourable circumstances.

However, local SNP activists may feel there’s a glass-half-full way of looking at the situation, because although trying to overcome a Labour majority of 9446 is a tall order, they at least have a chance to achieve something that will probably be quite rare, if not unique, tomorrow night – a swing from Labour to the SNP, albeit measured from the by-election result.

The 24-percentage-point increase in support Labour enjoyed at the by-election was well in excess of the 15 to 19-point national boost for them that the opinion polls are currently implying, so in principle it should be possible to repair some of the damage with the right campaign.

However, the SNP are unlikely to be naive enough to buy into one of the most dangerous myths about the by-election outcome, namely that it was somehow a “bad result for Labour” because the absolute number of votes recorded for Labour dropped by 700 as compared to the 2019 General Election.

The argument is that SNP voters didn’t switch to Labour but simply stayed at home in their thousands, but in truth it doesn’t work that way. Differential turnout can happen to some extent, but a huge drop in turnout is bound to affect all parties. There’s no way Labour could have recorded such a mammoth swing without a big number of direct switchers from the SNP.

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The defeated SNP candidate from the by-election Katy Loudon is once again standing, and with far less of a burden of expectation on her this time she can set about the valuable task of winning some of those lost voters back.

The reality is that even without the Ferrier episode, Rutherglen would have been a tough constituency for the SNP to defend, because it was one of the seven Scottish seats Labour won under Jeremy Corbyn in 2017, and even in the SNP’s landslide year of 2019, Labour were less than 10 points behind.

That means Labour could have gained the seat on a uniform swing this time even if the SNP had a national lead that was well into double digits.

In Loudon’s favour is that boundary changes, which see the west of Hamilton removed from the constituency, have weakened Labour’s position just a touch.

Rutherglen also contains a disproportionately large number of independence supporters, with the overlapping Scottish Parliament constituency having recorded a 50% Yes vote in the 2014 referendum.

The one obvious advantage to having already hit rock bottom is that the only way is up, and there are certainly grounds for optimism that the SNP can at least start on the road to recovery in Rutherglen tomorrow.