Labour is to wage a new campaign to win over Tory-supporting pensioners in an attempt to neutralise one of the government’s last remaining electoral strengths, amid evidence the Conservatives are now performing as badly among the age group as they did under Liz Truss’s leadership.

With less than a fortnight to go until local elections in England, which some Tories fear could trigger an attempt to topple Rishi Sunak, the Observer understands that Keir Starmer’s top officials are reorienting their campaign after detecting alarm among pensioners over the impact a Conservative tax-cutting pledge could have on pensions and the NHS.

The shift, which will include a national media and targeted digital advertising blitz from this weekend, follows the decision by the chancellor, Jeremy Hunt, to signal the eventual abolition of employee national insurance contributions, a move Labour claims would cost about £46bn a year.

A private focus group run by Labour in the past week convinced its most senior officials that the announcement by Hunt is a huge blunder. Insiders said that “pensioner hero voters” – those who backed the Tories last time but who may switch to Labour – compared the move to Truss’s doomed plan for £45bn in unfunded tax cuts during her brief prime ministership.

Senior Labour figures said it had become an “unexpected bonus” with which to neutralise one of the few Tory strengths. “Their primary concern is that it is a huge unfunded spending commitment,” states an internal memo composed by Labour’s strategy chief, Deborah Mattinson.

“The unfunded commitment raises alarm bells and leads voters to spontaneously make comparisons to Truss’s mini-budget. Beyond this, they recognise the high risk to the future of the state pension – with some worrying it won’t be around for their children/grandchildren. This gives Labour its biggest opportunity with pensioners for some time.”

The news comes as the latest Opinium poll for the Observer reveals that the over-65s is the only group in which the Tories lead Labour, but by a narrow six points. However, the 35% support the Conservatives now enjoy among that group is lower than the 39% it recorded in the last Opinium poll published before Truss’s resignation.

There is also evidence that pensioners are now heading to Reform UK. Support for the successor to the Brexit party among the over-65s stands at 18% in the latest poll, up from 13% just a fortnight ago. Overall, Labour has maintained a 16-point lead over the Tories.

In the wake of more criticisms of the party by Truss, disunity has emerged as a handbrake on the Tory recovery, with 61% of voters seeing the party as disunited and only 15% disagreeing. About 60% of voters who backed the Tories at the last election now see them as disunited.

Hunt has already been accused by Tory backbenchers of failing to help pensioners in his last budget. However, the chancellor has backed the “triple lock” that ensures the state pension rises in line with either earnings, inflation or 2.5% – whichever is highest.

Starmer has not confirmed Labour will back the triple lock in its manifesto, but has said he is “committed” to the policy. Backing it will cause some disquiet within the party, with many MPs desperate for him to dedicate more funds to public services.

Older voters in Labour’s focus groups were also said to be worried that removing national insurance would prevent funds being poured into the struggling health service.

At a campaign meeting on Wednesday, it was agreed to focus the last weeks of the local election campaign on targeting the group over the issue. As well as national media and digital ads launched from Sunday, shadow cabinet ministers have been ordered to focus attention on the national insurance abolition pledge. “This is a crucial moment for our local and general election campaigns,” states a memo to the shadow cabinet by Pat McFadden, the veteran MP preparing Labour’s election campaign. “We know from our strategy and insights team that voters share this concern. Scrutinising the Conservatives’ £46bn plan will be a central line of our attack.”

A Conservative party spokesperson said: “We are cutting national insurance contributions again for 29 million working people to end the unfairness of double taxation on work and making sure being in work pays. Our long-term ambition is to abolish it entirely, which we cannot do overnight and will only do in a fiscally responsible way, without compromising high quality public services.

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