"SEISMIC" seems to be the superlative of choice today after Labour delivered an absolute thrashing of the SNP in the Rutherglen and Hamilton West by-election.

However you describe it, Rutherglen and Hamilton West will undoubtedly take its place in the pantheon of Scottish by-elections. The swing from the SNP to Labour was the fourth largest of any Scottish parliamentary by-election since the Second World War, in which both parties had stood at the previous election.

It trails only the SNP victories in Glasgow East in 2008 and Glasgow Govan in 1988 and 1973. Other colossal by-election gains include Roy Jenkins’ capture of Glasgow Hillhead for the Social Democratic Party in 1982 and Winnie Ewing’s (below) iconic victory in the 1967 Hamilton by-election. But these are not like-for-like comparisons.

The National:

READ MORE: LIVE: Reaction as Labour win Rutherglen and Hamilton West by-election

Going into the by-election, I and others pointed to a 15-point Labour margin of victory as a strong signal that they are on course for a major comeback in Scotland – winning, perhaps, 20 seats in the upcoming General Election.

In the event, Michael Shanks took 58.4% and Katy Loudon 27.5% - a margin of 30.9 points. Labour’s majority of 9446 votes was larger than the SNP’s 8399 total votes.

Even accounting for the depressed turnout, this was a massive win for a party that had been reduced to a single MP and third-party status in Scotland for most of the past decade.

A General Election in which Labour wins Rutherglen and Hamilton West by a 30-point margin would be catastrophic for the SNP – they would be reduced to low single figures in terms of seats, and Labour would return as the dominant party of Scottish Westminster politics.

Of course, the circumstances surrounding Margaret Ferrier’s departure as the MP for Rutherglen and Hamilton West will undoubtedly have fed existing anti-SNP sentiment and disaffection among the party’s voters, many of whom will have taken the traditional opportunity to give the governing party a kicking, even if they would ultimately vote for them in the higher-stakes setting of a General Election.

The National:

And while turnout was better than feared yesterday – and, in fact, pretty much bang on average for a by-election in this parliament – it was still significantly lower than the 2019 General Election. More than 20,000 voters who had voted in 2019 stayed home, more than enough to dramatically change the outcome.

But these are, and I cannot emphasise this enough, the dimmest of silver linings for the SNP. Yes, many of those voters who didn’t turn out would likely turn out and vote SNP in a General Election. But they have signalled in no uncertain terms that they are unhappy with Humza Yousaf’s party.

In other words, they are to be won by either the SNP or Labour.

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Moreover, tactical voting now clearly disadvantages the SNP in these contests, which will probably hold at the next General Election. The Conservative vote collapsed, and while many of their voters will have stayed home, it points to many of them tactically voting Labour to keep the SNP out – a dynamic that would see disproportionately large swings against the SNP in SNP-Labour contests.

The era of SNP hegemony is ending, and Scottish Labour are back as a credible electoral force posing a serious challenge to the SNP. So, what now?

The kinds of voters that might swing between the SNP and Labour tend to be left-leaning (at least on the economy) and pro-independence. Opinion polling shows that they are dissatisfied with the Scottish Government and are split on Humza Yousaf’s leadership.

But the same polls suggest they are unconvinced that Labour would do any better in government. Yes, more Scots like Sir Keir and Anas Sarwar than dislike them (very narrowly), but a significant chunk have no solid opinion of them.

The SNP should use their upcoming conference to reset, setting out a renewed vision and policy agenda for a growing, prosperous Scotland within the United Kingdom. More self-indulgent navel-gazing over independence process will not be good enough.

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For their part, Labour need to start laying out what they will do rather than what they won’t do. There is no approach more likely to let the SNP back into these contests than one that fails to give those left-leaning, independence-supporting swing voters something concrete and ambitious to vote for.

Scottish politics has become suddenly much more competitive, and the risks and opportunities for the SNP and Labour are much sharper. Change is coming; what change depends on how the parties respond now