The Conservatives could lose once safe constituencies such as Theresa May’s former seat in the coming general election, with nearly 40% of people willing to vote tactically to oust the Tories, a campaign group has said. Setting out its recommendations for how people could maximise their chances of not electing a Conservative, Best for Britain said particularly efficient tactical voting could even unseat Liz Truss and Suella Braverman. While May has stepped down, her former seat of Maidenhead, held by the Tories since it was created in 1997, is now seen as a three-way marginal, with Best for Britain saying the Liberal Democrats are best placed to defeat the Tories. But the group said there were more likely “Portillo moments” – election night defeats of Tory big beasts – through tactical voting. These could include the cabinet ministers Penny Mordaunt, Jeremy Hunt, Robert Jenrick, Grant Shapps and James Cleverly. The group’s recommendations, based on multilevel regression and poststratification (MRP) polling, which gives seat by seat projections, include voting Labour in 370 seats, Lib Dem in 69, Green in three, SNP in seven, Plaid Cymru in two, and to “vote with your heart” in 181. The seat by seat recommendations, with the reasons behind them, are available at GetVoting.org, run by Best for Britain, along with data on how many constituents could vote tactically. The data is based on a poll of 22,000 people conducted for the group, which found that 39.6% of people said they would consider voting tactically to change the current government. Another 13.6% said they would do so to keep the Tories in power. The polling showed that a fifth of people who said they were planning to vote Labour on 4 July would be doing so tactically, and a third of Lib Dem voters. As well as the constituency by constituency website, Best for Britain is launching a parallel advertising campaign based on an entreaty to voters to do what they can to get rid of the government. At a launch event in central London, Naomi Smith, the group’s chief executive, said the UK was “now a nation of tactical voters”, a factor that could worsen the Conservatives’ electoral fortunes. Lewis Baston, a psephologist who also spoke at the event, said MRP polling showed that even where Conservative candidates remained ahead, for many this involved securing little more than a third of votes, making them vulnerable to opponents joining forces. “The Conservatives are under siege in a way that they really haven’t been before,” he said. “There’s no heartlands left, they’ve got serious competition on the right. The polling shows that anti-Tory voters are mobilised and in a harsh mood.” While the seat by seat recommendations are largely based on the sole factor of how to stop a Conservative winner, some take in more complex ideas, including Best for Britain’s view of other parties and even of specific MPs. For example, the group does not recommend voting Reform UK even if this could remove a Tory MP. The guide also makes recommendations in some seats where there is no prospect of a Conservative winning. These areas include Brighton Pavilion and Bristol Central, both Labour-Green battles. In the former it recommends voting Green, and in the latter Labour. Asked how such recommendations were made, Smith said it was in part a trade-off between the parties, but also because Thangam Debbonaire, the Labour frontbencher at risk from the Greens in Bristol, “has been an excellent MP on the issues that we care about, including Europe”.