It is a truth universally acknowledged that each and every by-election which arises must be categorised as “crucial”.

Such is certainly the case with the pending contest in Rutherglen and Hamilton West.

Admittedly, the distinctive nature of its origin – an erring MP eventually ousted – lends the battle an intriguing air. However, I would also argue that it would be rash to draw too many firm conclusions from the outcome, given those distinctive circumstances.

Whatever happens, the First Minister must be wondering this weekend when he is going to catch a break.

As campaigning accelerates in Rutherglen, Humza Yousaf is awaiting the conclusion of the police investigation into SNP finances.

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The departing Chief Constable suggests that it might be better done relatively quickly, without truncating process. Either way, Mr Yousaf is powerless to act.

In running his devolved government, he has a multitude of problems. Very few seem calculated to endear the electorate to him or his party, at least in the interim.

And now along comes a by-election caused solely by misbehaviour on the part of one of his MPs.

A contest, what is more, in a Westminster seat which his party only regained at the last General Election – and where they used to weigh the Labour vote in times gone by.

It would be understandable if Mr Yousaf’s advisers, perched anxiously outside his office, discerned a plaintive cry from within: “Why me, what have I done to deserve this?”

Yet still he smiles, displaying his characteristic stoicism, while aware all the time that a heavy defeat in Rutherglen would be laid at his door, fairly or otherwise.

SNP supporters – including those who are already disenchanted – would be heard pondering whether Mr Yousaf is an ingrained loser, a debit line in the SNP’s carefully scrutinised accounts.

So, for the FM, there are very few reasons to be cheerful about Rutherglen. And yet. In the longer term, there may be causes to be sanguine.

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Firstly, it is likely that the origins of the by-election will dissolve into the political air. Yes, the actions of the ousted MP, Margaret Ferrier, are indefensible. She broke Covid rules which were observed by her constituents, often with difficulty.

When caught, she clung to her Commons seat, despite being suspended by the SNP and, eventually, the House. She only left when she lost a recall petition.

Yet this too will pass. The by-election may open with Ms Ferrier’s transgressions but it will soon shift to other issues, not least the condition of the economy, the grievous struggles endured by the citizenry.

That issue is likely to pose problems, in particular, for the Conservatives as they seek to defend the Chancellor’s record.

The Tories will not win Rutherglen but they had a decent vote here in the last two contests – between eight and ten thousand. Either they will retain that with stolid endeavour – or it is up for grabs by rivals in a by-election squeeze.

The Liberal Democrats have a presence on the ground in this urban constituency to the south-east of Glasgow. It is relatively thin but they will be keen to sustain it.

Which leaves us primarily contemplating the SNP and Labour. And the question. In a battle over the economy, are the SNP oppositional critics – or incumbents in devolved government and thus open to attack?

Labour will certainly pursue the attack route. They will triangulate the Tories and the SNP, describing them as comparably incompetent governments, each acting to the detriment of Rutherglen.

By-election victory would give Anas Sarwar clout with Keir Starmer – and an argument to divert further resources to Scotland in search of General Election seats. Defeat is unthinkable for the Scottish Labour leader.

But SNP campaigners are also sketching out a triangle of their own. They will seek to depict Labour as emulating Tory policies – from Brexit to benefits.

This week, for example, Mr Yousaf challenged Scottish Labour to condemn the two-child benefit cap and its attendant “rape clause”.

Labour’s UK leader, Sir Keir Starmer, has suggested that lack of public cash means it will not be possible to shift the cap on entering government.

Which left the estimable Dame Jackie Baillie in a somewhat tricky spot when she was challenged on the wireless about an apparent mismatch with Scottish Labour’s previously voluble rhetoric against the policy.

For Humza Yousaf, it is a palpable hit upon Labour. But will it be sufficient to counter negatives confronting the SNP? Will this and like attacks sway sufficient votes in this coming contest?

The SNP, of course, have a permanent weapon at their side. Independence. They will seek to persuade voters that the UK’s economic woes highlight the need to end the Union.

More precisely, they will urge those who back the concept of independence to stick with its principal advocates, the SNP. Labour will seek to label independence as a distraction.

So an interesting contest in prospect. But crucial? Perhaps not. For one thing, the distinctive nature of this by-election provides little in the way of a read-through to the UK General Election next year.

Remember that this is via a constituency petition. There have been three other attempted recalls. One failed. In the others, the incumbent party lost one seat but held the other. In short, no evident or long-term pattern.

Few by-elections change the course of history, despite the sound and fury which attends them at the time. Orpington in 1962? Or Glasgow Hillhead in 1982? The rise of third-party politics? Aye, but the UK political mould, while under strain, has yet to shatter entirely.

More can be made, perhaps, of Hamilton in 1967 when Winnie Ewing stopped the world to allow Scotland to embark. Or Govan in 1973 and 1988. Signs and signals.

Much more recently, the Tories held Uxbridge and South Ruislip, Boris Johnson’s seat, on a night when they lost two other constituencies to Labour. No decisive indicators either way for Rishi Sunak.

To be clear, this by-election matters. To Scottish and UK politics – and to the decent folk of the constituency. But while we rightly turn our attention to Rutherglen let us also elevate our eyes to the broader picture.