What now for the SNP after defeat in Rutherglen by-election?

“Our vote is there – but it chose to stay at home,” he said.

The victory by Labour may not have been unexpected, but the scale of the win – which saw a 20.4 percentage point swing from the SNP – was the big surprise of the night.

First Minister Humza Yousaf said it was a “disappointing night” for the SNP, but there were also “difficult contextual factors” such as the actions of former MP Margaret Ferrier and the police investigation into the party’s finances playing a part.

However, he pledged the party would “reflect, regroup, reorganise and come back stronger”.

In the wake of the vote, the prospect of Yousaf’s leadership coming under pressure has been raised by some, but polling expert Mark Diffley (below), founder of the Diffley Partnership, said he was “not convinced” that was a major issue.

“For me at least, looking at polling, I don’t think that is where the answer lies, at least not in the short term based on one by-election,” he said.

“Bearing in mind this was the perfect by-election for Labour at the perfect time – if they were going to choose any constituency to have a by-election in Scotland, this would be pretty much the one.

“It’s in the central belt, it’s swayed between Labour and SNP at every election since 2010, it’s gone backwards and forwards, it’s got a real Labour tradition in the seat.”

He said other factors such as tactical voting and the SNP and Conservatives both having to convince voters of their record after long periods in government also played a part in Labour’s win.

“All the stars were aligning for Labour, I think, in terms of where the by-election was and when it was,” he said.

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“I think the SNP need to bear that in mind – in other words, probably not rush to huge decisions that may not be in the best interest, and take a bit of time to digest what the voters are actually telling them.”

Diffley said even if SNP voters had turned out in Rutherglen and Hamilton West, it may not have saved the seat, with Labour candidate Michael Shanks taking 17,845 votes – more than double the number polled by the SNP’s Katy Loudon.

But he said it would have lessened the margin of victory for their opponents, and suggested the SNP had to look at what their offer is to voters and “sharpen messages” ahead of the General Election.

“The constitutional message needs to be clarified a little bit, there are different voices from within and from the independence movement about what that strategy should be, so that needs to be clearer,” he said.

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“There is a lot to digest – and as some people have said to me, better to get the bloody nose now a year out from the election where at least you have a little bit of time to fix things, rather than wander into the election not realising quite how parlous your position is.”Diffley said another major issue facing the SNP in the by-election was tactical voting, with a collapse in the Tories and LibDem vote appearing to be a result of this going to Labour instead.

But he said this may not happen to such a great extent in the General Election, with Tory supporters in particular facing a potential dilemma over whether to continue to back a rival party which is vying for the keys to Downing Street.

“Do they continue to vote Labour which – if you are a Tory voter – has the benefit of keeping the SNP out, but it hugely increases the chance of a Labour government at Westminster?” he said.

“Or do you vote Tory, knowing it’s likely to lessen the chances of a Labour government – but keep more SNP MPs in their seats?”

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Diffley said the Rutherglen and Hamilton West by-election had demonstrated the momentum is with Labour and the party could win in Scotland and probably across the central belt.

But he added: “It also raises questions about why is turnout so low, was it SNP voters that largely sat on their hands, will they come back next time, what will happen to Tory voters?

“So the turnout and the tactical voting are the really interesting things which come out of this in terms of pointers towards next year.

“It doesn’t predict what is going to happen, but it highlights the SNP will get blown away if they don’t bring their vote out, and the SNP will also get blown away if Tory voters in the central belt vote tactically en masse for Labour.

“They are the two big dangers for the SNP.”

Only 37.9% of the electorate turned out to vote in Rutherglen and Hamilton West on Thursday – although this was still within two percentage points of the average for by-elections.

Marta Miori, a PhD candidate at Manchester University who is studying Scottish independence, said while every party had lost numbers of voters, Labour appeared to hold their percentage of the electorate in the by-election.

“Labour went from holding 22% of the electorate in 2019 to 21%,” she said.

“So they basically retained all of their past voters or managed to get other voters to vote for them.

“Other parties had a big drop – the SNP only had 10% of voters.

“They lost quite a lot of voters – so if it’s a matter of turnout, it is not even among parties. Labour, even though it is a by-election and a lot of people don’t vote, managed to get quite a big vote share, compared to what they had before.”

She added: “The swing is much more likely to be just Labour managing to get their voters to come out, and the SNP not doing so.”

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When it come to the General Election, Miori said turnout would be expected to be higher for all parties.

“What you might find is all of these voters who couldn’t be bothered to go out this time will habitually vote anyway,” she said.

“People make a habit of it, so even if you are not excited, you go and do it.

“That holds a lot less for by-elections, but when it comes to it, probably a lot of less excited SNP voters will find themselves in the polling booth and vote SNP.”

She added: “The SNP losing votes to Labour because of people changing their minds, that will be more of an issue for the party.”

At the SNP’s annual conference this week, the by-election loss and issue of how to win back support will be dominating discussions, particularly around the strategy on independence.

Toni Giugliano, the party’s policy development convener, said while it was one by-election, there was a need to take heed of the result and look at a change of approach and how to remotivate SNP supporters.

“To me, we need to do two things now – we need to restore trust and we need to remotivate our support.

“It is clear our support stayed home and in terms of trust, we went into the by-election resulting from a disgraced MP but also with one hand behind our backs because of a live police investigation where we weren’t able to utter a word on the matter.”

He added: “We can’t achieve independence in an election if we don’t have strong support for the SNP – how do we enthuse independence supporters to back the SNP in this election? We have a lot of thinking to do on that.

“We need to realise we need a different approach here, this is not business as usual and we can’t go on as normal. We need to get tougher in some of our messaging and inspire.  “This is the kick we need to get our act together as a party and a movement to make ourselves relevant and put independence on the agenda.”