LABOUR will face a “marginally bigger” challenge than the SNP in the Rutherglen by-election, with Keir Starmer likely to be the most nervous of all the party leaders, a polling expert has said.

Mark Diffley, founder of research firm the Diffley Partnership, said while both parties have a lot to gain and lose, the expectations that Labour will win the seat means a failure to do so will be a “big loss” for the party ahead of the General Election.

SNP candidate Katy Loudon is set to battle it out with Labour’s Michael Shanks.

Speaking at an online event hosted by his research firm, Diffley said the contest would be a challenge for both parties.

READ MORE: Katy Loudon: Voters care about cost-of-living, not the SNP police probe

But he added: “I would argue it is marginally a bigger challenge for Labour than it is for the SNP for a couple of reasons.

“We have almost got this - it might be [a] bubble thing of course - but we are kind of primed now to think Labour are going to win this.

“A lot of this is about expectations, and the expectation now is that Labour will win.

“So I think if Labour don’t it will be – not just feel like – it will be quite a big loss for Labour given we are probably only a year out from the election.”

Diffley pointed to the example of the recent by-election in Boris Johnson’s former seat of Uxbridge and South Ruislip, where Labour suffered a defeat which was blamed on London Mayor Sadiq Khan's plans to expand the Ultra Low Emission Zone (Ulez).

He said the context is very different, but the level of expectation and failure to take the seat may put extra pressure on the party for the Rutherglen by-election.

The National: Michael Shanks, centre left, is Labour's candidate for the by-electionMichael Shanks, centre left, is Labour's candidate for the by-election (Image: Colin Mearns)

“When you look at this in the Scotland context or really importantly at the minute a UK-wide context, this is the sort of seat that if Labour don’t win it will tell us something about their chances of victory at next year’s General Election,” he said.

“This will be as much about the size of that victory – as I think Labour probably will win – but it will be as much about the size of that victory I think, the majority they get as anything else.”

He added: “The most nervous person will be Keir Starmer.

“He will be the most nervous of all the party leaders because of what this will mean, the next big electoral test is the General Election. I know everyone says it and it is true, if Labour can’t win this seat then it really does diminish their hopes next year.

READ MORE: Michael Shanks blasted for claim he would vote against Keir Starmer

“But I think in Scotland leader terms both Anas Sarwar and Humza Yousaf have a lot to gain and a lot to lose.”

Speaking at the same event, former Scottish Labour leader Kezia Dugdale, who is now director of Glasgow University’s John Smith Centre, said incentivising people to turn out to vote would be key.

She said the SNP’s key messages were likely to be around independence and attacking Labour over its stance on the two-child benefit cap policy.

She said: “If I were to sit within the SNP camp I think their core message is really obvious – they are going to play to their base, they know they have got 44% of the vote there from last time around, if they get 44% out on this occasion, they are onto a winner.”

The National: Dugdale said Labour need to 'pick their enemy'Dugdale said Labour need to 'pick their enemy'

But Dugdale said it was more difficult to see how Labour will run the campaign despite having a “very strong candidate”.

She went on: “Beyond putting him front and centre of elect a local voice type traditional byelection campaign, Labour need to work out who their enemy is here.

“They could say this a chance to send a message to the SNP and point to all the failing of the SNP and hope that drives up Labour voters to turn out and register an objection towards the SNP.”

But she added: “I wonder whether they would be smarter to make this as a by-election as a trial General Election and go after the Tories and say this is your chance to send a strong voice from Rutherglen to take on the Tory government and here’s all the things Labour would do in power and this is the first step to getting the Tories out of office.

“That might be a more motivational message for Labour to put forward, it’s not clear to me which of these two enemies they are going to pick – but I do think they need to pick one and stick with it.”