The Conservative party’s faltering general election campaign suffered a potentially damaging blow when Nigel Farage announced he intended to stand as an MP and lead the Reform party for the next five years. The former Ukip and Brexit party leader said he would stand in Clacton, Essex, after changing his mind while spending time on the campaign trail. He claimed that he did not want to let his supporters down. Farage will also take over as leader of Reform UK from Richard Tice, pledging to stay in post for a full parliamentary term. While his announcement poses an immediate threat to the Tory candidate in Clacton, it may also energise his party’s national campaign, splitting the rightwing vote in other constituencies. It also raises the spectre of Farage antagonising the Tories as they descend into a post-election battle for the soul of their party. Farage’s bid to win in Clacton, which was the first to elect a Ukip MP in 2014 and has a Tory majority of 24,702, will be his eighth attempt to enter parliament. He has failed on each of the previous seven occasions. In a further blow to Sunak, YouGov’s first MRP constituency projection, before Farage’s announcement, showed Keir Starmer could win a 194 majority, bigger even than Tony Blair’s 179 majority in 1997. It put Labour on 422 seats (+222 from the 2019 election, based on new constituency boundaries), the Tories on 140 (-232), the Lib Dems on 48 (+40) and the SNP on 17 (-31). One senior Tory described Farage’s return as an “existential” risk. At a news conference in central London, the rightwinger said that he had witnessed “a rejection of the political class” across the country in a way that had not been seen in modern times and that he wanted to lead a “political revolt”. He also sought to portray Reform UK’s surge as part of what he described as “a new phenomenon” ahead of elections this weekend to the European parliament, when parties of the far right are expected to do well. “I promise you something is happening out there,” he said. His decision will mean the debate about immigration will become even more prominent during the campaign. The Tories will on Tuesday announce an annual cap on the number of visas issued to reduce overall migration. The figure would be recommended by the migration advisory committee before being put to MPs for a vote. Starmer, meanwhile, will continue his focus on national security by pledging that Labour will “close the door on Putin” by reducing Britain’s reliance on fossil fuel from overseas. On Tuesday, he will say the party’s plan to set up GB Energy – a publicly owned clean energy company – will help to protect the UK from rises in the price of fuel like those that followed Russia’s invasion of Ukraine. Farage had been expected to choose to run either in Clacton or in Thanet East in Kent, which includes much of the old constituency where he made two failed bids to become an MP in 2015 and 2005. But Clacton would now appear to be a better bet for Reform UK, even though the Tory candidate Giles Watling, who backed remain, is defending a majority of 24,702. Farage fsuggested that Reform UK could take more than the 3.9m votes that Ukip won at the 2015 general election, before the Brexit referendum, and could even win some seats, although he acknowledged that it would be harder without proportional representation. Reform UK is aiming to hive off votes from the Conservatives’ right flank, potentially splitting the vote in some marginal seats and allowing Labour through the middle. “They are split down the middle on policy, and frankly right now they don’t stand for a damn thing,” Farage said of the Tory party. “So our aim in this election is to get many, many millions of votes. I’m talking far more votes than Ukip got back in 2015.” However, the return of the rightwing firebrand, who is close to the US presidential candidate Donald Trump, to frontline politics will alarm even moderate Conservatives. In an indication of the type of campaign he will run, Farage described the UK contest as the “immigration election”. Tory MPs warned that a Reform UK surge could result in the party pitching further to the right to try to win back support, with more tough rhetoric on the party’s Rwanda plan, and even a pledge to leave the European convention on human rights. “It also possibly makes it more likely that we have a daft, full-blown ‘leave the ECHR policy’ in the manifesto,” said one candidate. “That’s attractive to one end of the market and really unattractive to the other. We needs to remain attractive to both.” They were downbeat about Reform’s impact on the Tories’ electoral chances. “It probably just locks in the worst-case scenario with Reform and means it’s very hard for the Tories to make as much of a dent as we need to do to be in better territory.” Another said Farage’s decision was a “big blow” to Sunak. “He will undoubtedly have an impact on both campaigns – but probably more of an impact on the Conservative one. Reform were always going to nibble around the ages of the Conservative vote but their inroad will be bigger now.” It also reinforced the view among some Tories that Sunak’s decision to call the election for 4 July, rather than the autumn, when Farage was likely to be in the US supporting the Trump campaign, was a mistake. A Tory candidate said: “The PM’s decision was really stupid – and obviously really stupid. Your whole thing is the Tories are about stability and the economy is going to get better, and then you call an election before anyone bloody notices. “I just find it inexplicable. It was [deputy prime minister Oliver] Dowden’s argument that won, which was that it’s going to get worse. Well, show me how.” The return of the former Brexit party leader now also opens up the possibility of unhappy former Tory donors opening their wallets, with Farage saying the party had started to get some “serious money” since last week. Those gathered at the Reform UK press conference included the property developer Nick Candy, who has previously donated hundreds of thousands of pounds to the Tories, and the Pimlico Plumbers founder, Charlie Mullins. The new Reform UK leader deflected questions about whether his party’s members had been ignored by a backroom deal that has led to Tice being replaced. “Sometimes tough things have to be done,” he said.