AS I write, the polling stations in Rutherglen are still open. It is therefore somewhat perilous for me to offer what amounts to a pre-mortem of the result, however, with some trepidation, here I go.

If Labour wins, then what we are witnessing is, in some senses, precisely what we are supposed to witness. The SNP has been in power for a remarkable 16 years in Scotland, and the Tories for 13 years throughout the UK. Their opposition, Labour, has been enjoying a resurgence in the polls, and both governments are in the middle of their current terms of office. It is entirely normal, and indeed expected, for oppositions to win by-elections as voters in a single constituency are encouraged by the peer pressure of the rest of the country to give the government a bloody nose.

That level of expectation can cause problems. A Labour win in Boris Johnson’s constituency of Uxbridge and South Ruislip was "baked-in" when a by-election was held there earlier this year, and the Tories’ surprise victory, credited to opposition to London Mayor Sadiq Khan’s ultra-low emission zone proposal, has been directly responsible for a significant shift in the Government’s policies on net zero, as well as a degree of introspection within Labour.

Read more by Andy Maciver: Why can’t the Tories see they’re on a hiding to nothing?

In Rutherglen and Hamilton West, the "bake-in"’ is sufficient to deem that Anas Sarwar’s Labour should win with something around 50 per cent of the vote.

This is the SNP’s 11th most marginal seat in Scotland, with less than a five per cent swing required. Combine that with the fact that the incumbent MP, Margaret Ferrier, was arrested and charged by the police, and subsequently suspended from the House of Commons, for a gross breach of Covid regulations, and it becomes one of the clearest-cut by-election shoo-ins you will ever see.

In that sense, the by-election carries more risk for Mr Sarwar than for his SNP opposite number, Humza Yousaf. Mr Yousaf is expected to lose, and if that happens I would be amazed if he does anything other than blame the defeat on Ms Ferrier and attempts to move the narrative on.

Not so for Mr Sarwar. Polling has Labour anywhere from a few points behind, to neck-and-neck with the SNP going into next year’s General Election. With at least 10 of Scotland’s 57 seats expected to remain with either the Tories or the Liberal Democrats, Labour and the SNP are fighting it out over the other 47, give or take a couple.

Labour, currently on one, has a long, long way to climb, and failing to win a seat such as Rutherglen and Hamilton West, or the two constituencies into which it will be split at the General Election, following boundary changes, would lead us to question whether it could even reach double-figures next year, let alone the 20-plus seats pollsters and commentators and currently predicting will turn red.

So if Labour loses, expect mild panic in its ranks. If, on the other hand, it wins, expect business as usual very swiftly.

Read more by Andy Maciver: What Scotland needs now is a Labour/SNP alliance 

However, this is not to downplay the significance of the shift that is happening in Scottish politics, of which this expected victory is part. There is a new business as usual, and it involves the SNP’s vice-like grip on power slipping away. It involves an SNP which has peaked and which, to steal a phrase from Rory McIlroy after last weekend’s Ryder Cup, is on the back nine of its time in power.

If you are watching, listening to, or reading about a Labour win tomorrow morning, you are witness to the first piece of real electoral evidence of this new era. Scotland’s 24-year-old devolution era can be split in two. There was the pre-2011 part, during which independence was off the table, and there was the post-2011 part, where it has been very firmly on the table.

Tomorrow, after a Labour win, we have confirmation of what, realistically, most of us already knew, which is that independence is now in the longest of the long grass. If Labour wins, it is confirmation of a trend which is very likely to lead to a Scottish Parliament election, in 2026, which will return a majority of Scotland’s 129 MSPs who wish Scotland to remain in the UK, and a minority who wish it to leave.

It is a trend which is highly likely to lead to a Scottish Parliament more similar to the one existing from 2007 to 2011, with a relatively equal SNP and Labour, and one of them forced into some form of collaborative government with the help of minorities, perhaps across the constitutional divide. And a Scottish Parliament which does not have the votes to ask Westminster for an independence referendum.

This by-election, as a single event, will be forgotten quickly. But, if the pollsters and pundits were right about the outcome, we have all witnessed the event that political historians will reference when they talk about the time we knew, for sure, that our world had changed.