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.