Conservative leadership hopefuls are already lobbying for support to take over from Rishi Sunak amid widespread fears the party is heading for a disastrous defeat on 4 July, the Guardian has learned. With three weeks to go before the general election, candidates and advisers had begun lining up behind their preferred contenders, sources said, with some Tory campaigners complaining they were being inundated with messages from potential leaders. The manoeuvring comes as one poll put the Conservatives behind Reform UK for the first time, on 18%; a position that would lead to a historic wipeout for the Tories at next month’s election. The Reform UK leader, Nigel Farage, claimed on Friday the poll showed he was now in effect the leader of the opposition, though that job is likely to fall to one of up to a dozen senior Conservatives after the election. The early favourites for leader include former secretaries of state Priti Patel, Suella Braverman, Robert Jenrick, Kemi Badenoch, Penny Mordaunt and Grant Shapps. Several of those, however, are fighting to retain their seats, leaving their contention highly uncertain. One Tory adviser said: “There is quite a bit of manoeuvring going on already. Members of the cabinet are texting candidates regularly just to ‘check in’, while others are already lining up their leadership teams.” They added: “It can be quite annoying – sometimes you wish they would focus more on the general election campaign.” A senior party member said: “There is a sense now that a Labour victory is inevitable. We went into the campaign hoping for a hung parliament, but now the central assumption is we are trying to minimise their majority.” The focus on a possible leadership election after 4 July comes against the backdrop of another tough week for Sunak. He began the week by launching the Tory manifesto, but was soon having to deny accusations he was out of touch following the broadcast of his full interview with ITV in which he listed “Sky TV” as something he went without as a child. That interview was broadcast immediately before he and the Labour leader, Keir Starmer, took part in a televised Q&A on Sky News – a debate that two-thirds of viewers judged Starmer to have won. On Thursday night the Tories were rocked by the YouGov poll showing Reform had moved into second place in the polls – albeit within the margin of error. Farage told an impromptu press conference on Friday: “The election is over. Labour has won … But perhaps more importantly, who is going to be the opposition voice to Labour in the House of Commons and in the country? I’m putting it to you that I believe that I can be that voice of opposition.” John Curtice, a professor of politics at Strathclyde University, said Reform’s recent polling numbers spelled “utter disaster for the Conservatives”. Figures released by the Electoral Commission on Friday suggest donors were also deserting the Conservatives. The data shows Labour received nearly £1m in the second week of the campaign – 60% more than the Tories. Speaking from the G7 in Italy on Friday, Sunak insisted he would remain as an MP for the five-year term even if his party loses the election next month, in an attempt to scotch rumours he would quit soon afterwards for a job in the private sector. Tory party insiders said the prime minister’s authority had already begun to leach away, with contenders to replace him jostling for position. One adviser said several candidates had been receiving regular messages from Shapps checking in on their campaigns, in a move they thought indicated his desire to shore up support within the parliamentary party. An ally of Shapps insisted he was merely making sure candidates were on message with regards to defence policy, which was his department before the election was called. Several allies of Braverman said they were thinking of moving their support away from the former home secretary, either to her former Home Office colleague Robert Jenrick or to another unnamed candidate. “She is a busted flush, and we’re looking elsewhere,” one said. A Tory who said they were supporting Mordaunt had been singing her praises to colleagues. The person told the Guardian: “Badenoch is too soon. Braverman – an empty desk would be more useful. [James] Cleverly [the home secretary] – a man with the wrong name, although he’ll get some support. Jenrick – his problem is he keeps going on about immigration hoping we’ll all forget he was immigration minister. Shapps – he’ll be a runner if he survives. “My hunch is that Penny will emerge ahead of the pack. But it’s a treacherous thing to forecast.” Many of those with an interest in the leadership have admitted that much will depend on who keeps their seat. Though Shapps and Mordaunt – two of the leading centrist candidates – are involved in close fights to retain their seats, many others from that part of the party have large majorities, and could become the most powerful bloc after the election. “I think you’ll find the Conservatives will gradually shift more to the centre,” said one well-known candidate. “It won’t be completely the case – Badenoch and Braverman may still be around – but many of the people left will be the old centrist blue wall.” Meanwhile, Starmer has said he plans to go further in bolstering Britain’s public services than he set out in his party’s manifesto this week. He called Labour’s election pledges merely a “first step”. Starmer told the BBC he wanted to carry out bigger changes to areas such as health and education than suggested by his current pledges, which he said were focused on what he would do on “day one” of a Labour government. In a half-hour interview with the journalist Nick Robinson, Starmer defended himself against accusations that Labour’s manifesto promises were too small to deal with the problems the country faces. Talking about his pledge to recruit 6,500 extra teachers – which would amount to one teacher for every three schools – Starmer said: “That is set out in the manifesto very clearly as a first step, a down payment, one of the steps that I hope that we’ll be taking literally on day one in government starting down that recruitment … It’s a first step, it’s billed as a first step.” Asked separately about his pledge to create 2m more NHS appointments a year – less than a 2% increase – he again said: “It’s a first step, it’s a down payment.”