Labour is planning a campaigning blitz in order to take ownership of its new energy price cap policy in case the next Tory leader bows to pressure and cancels the 80% rise expected in October. Keir Starmer has vowed that his party “wouldn’t let people pay a penny more” on their gas and electricity bills this winter, proposing freezing the price cap at current levels and preventing the average household bill from reaching £3,600. Senior sources said the party had limited time to get out its message before the next Tory leader, presumed to be Liz Truss, enters No 10. The new prime minister will need to produce a comprehensive package on the cost of living, despite Truss’s reticence to spell out how she would help beyond tax cuts. Labour will increase efforts to promote its policy in the coming days, including with digital adverts, campaign tools for local parties and with direct mail for MPs to use. Plans for the summer offensive have been in the works since mid-July. Over the coming weeks, the party will set out more on its energy policy offer, including plans to upgrade 19m homes to make them more energy efficient, double onshore and offshore wind capacity and triple solar power. Starmer has said Labour’s plan, funded in part by an expanded windfall tax, is the radical approach needed to help households and reduce inflation, contrasting it with the inaction of a “lame duck” government. “We asked ourselves: do we want a plan that allows those prices to go up, causes that anxiety, and then rebates some people after the event, but doesn’t do anything about inflation, or do we want to be more radical, more bold, more ambitious?” he told BBC Radio 4’s Today programme. “One of the benefits of our proposal is that it brings inflation down, which benefits everybody, but particularly those who are most vulnerable, and those who are least well off.” Sources close to Starmer say plans for the party conference will need to be adapted on the basis of what the new prime minister announces in the first weeks in office, especially if there is an early emergency budget. Paul Johnson, of the Institute for Fiscal Studies, an economics thinktank, has queried whether Labour’s plan would help greatly with inflation, saying the rate would go up again once the energy subsidy ended. Starmer will visit marginal seats across the country almost every day of the coming fortnight, while the shadow chancellor, Rachel Reeves, will visit Scotland to outline how Labour’s plans compare with the SNP’s. “We are working with the assumption Liz Truss is going to do a big offer on energy – we are assuming that what she says now is fairly different to what she will do,” one senior Labour source said. “She will do what is politically salient. So we need to make sure we look like we are winning the argument now.” Senior Labour politicians will increase calls for the Tory candidates to outline their plans faster. Before the first Conservative hustings in Scotland, in Perth on Tuesday evening, Ed Miliband, the shadow climate change secretary, issued a call for the candidates to set out their cost of living packages before the price cap announcement on 26 August. “If this was a banking crisis, the government would act. If this was a war, the government would act,” he said. “Tonight, in their hustings, the candidates must immediately clarify whether they agree with Labour’s proposals. And if not, what their alternative proposals are. The British people deserve answers, not more delay from a distracted and out-of-touch Conservative government.” Momentum, the leftwing grassroots group that has often been at loggerheads with Starmer, issued a statement backing Labour’s plan on Monday night, though it said it should extend to full-scale nationalisation. Hilary Schan, a Momentum co-chair, said: “Labour should be arguing for public ownership, at a fraction of the price. It’s the commonsense solution, backed by most voters, and the best way to keep soaring bills down.” Reeves told TalkTV on Monday night that she remained sceptical of full-scale nationalisation, although she did not think the energy sector was “working today”. She added: “The whole point of the package we’ve put together today is that every single penny of it will go directly to lowering people’s bills.” Starmer has faced criticism for the time it has taken for Labour to put out its plan. A week ago the Liberal Democrats said the energy price cap rise should be stopped, and the former Labour prime minister Gordon Brown set out his own plan for the crisis. Starmer told BBC One’s Breakfast programme that his team had been working on the plan for more than six weeks. He added: “I’m not going to apologise for going on holiday with my kids. It’s the first time we’ve had a real holiday for about three years.”