Labour officials are expecting the party to be repeatedly attacked from the left by the SNP during a pivotal byelection regarded as a barometer of the party’s hopes of winning power. In the past week, the SNP has been widely distributing a leaflet targeting Keir Starmer’s decision not to scrap the two-child benefit cap as it attempts to hold on to the Rutherglen and Hamilton West seat. Its campaign literature also suggests there is no difference between Starmer and Rishi Sunak. A byelection in the seat is expected in the autumn after constituents voted to remove the serving MP, Margaret Ferrier, after she broke Covid rules. Ferrier had been an SNP MP but now sits as an independent. She was suspended from the Commons for 30 days after being convicted of breaking lockdown travel rules. Senior Labour figures now regard the contest for the seat, which delivered a 5,000 majority for the SNP at the last election, as a test of its success in repairing the party’s performance in Scotland. The ability to win seats from the SNP at the next general election is seen as a key factor in whether Starmer can secure a workable majority. While Starmer has deployed a cautious approach on spending and policy to attract former Tory voters in England, the SNP says it opens him up to attack from the left in Scotland. “The avenue of attack is that Labour is not progressive enough,” said a Labour source close to the campaign. “It’s a cynical message and it’s not working. But there’s a lot of apathy out there; they’re saying there’s no real difference between [Labour and the Tories].” Labour’s candidate, Michael Shanks, has vowed to oppose his own party if it tries to keep the two-child policy in place. Officials also concede that many voters remain uninspired by Labour. The party’s main message in the byelection is likely to be that a Labour win will help force the Tories out of office in Westminster. “In general, people think the SNP and the Tories deserve to lose,” said a Labour party source. “We’ve still got to demonstrate that we deserve to win. That’s how it feels on the doorstep. We’re in a good position but it’s not a hugely secure position. If the SNP were suddenly to get their act together, it might be quite difficult.” It comes as the latest Opinium poll for the Observer shows Labour has its smallest lead since Kwasi Kwarteng’s disastrous mini-budget last year. The fallout led to the swift disintegration of Liz Truss’s premiership. However, Labour still has a healthy 14-point lead, with 40% of the vote. The Tories are still on 26%. Sunak’s approval rating remains poor while Starmer has seen a small recovery in his personal ratings. A total of 24% approve of the job Sunak is doing, with 51% disapproving – a net score of -27%. Starmer’s net score stands at -7%. Starmer maintains a narrow lead in terms of who voters see as the best prime minister. “In the wake of a challenging few weeks for Labour, Keir Starmer will be concerned to see the party at its lowest share of the vote since the fallout from Liz Truss’s mini-budget last year,” said James Crouch, head of policy and public affairs at Opinium. “Labour’s saving grace remains that the Conservatives continue to be incredibly unpopular, with only 26% of the vote.”