The Tories could be locked out of power for a generation if supporters of other parties vote tactically at the next general election, the Liberal Democrat leader, Ed Davey, says on Sunday, after his party chalked up another stunning West Country byelection victory. A massive 29% swing to the Lib Dems saw Sarah Dyke take the Somerton and Frome seat, overturning a Conservative majority of more than 19,200. Davey’s party now has a comfortable 11,000 majority of its own, the largest it has ever had over the Tories in the constituency. In June last year the Lib Dems won Tiverton and Honiton from the Tories, overturning a 24,000 Conservative majority as Boris Johnson’s reputation reached its nadir. Tactical voting clearly played a large part in the latest victory, with Labour’s share of the vote dropping sharply to just 2.6% compared with almost 13% at the general election in 2019, putting the party way back in fifth place. The Lib Dems were themselves squeezed in the other two byelections held on Thursday: in Selby and Ainsty in North Yorkshire, where Davey’s party won only 3.3% of the vote compared with 8.6% in 2019, and in Uxbridge and South Ruislip, where it won only 1.7%, down from 6.3% in 2019. As Davey hailed his party’s recovery in its traditionally strong territory of the south-west, he told the Observer that there was now a clear opportunity to remove the Tories from office at the next election and keep them out for the foreseeable future, if tactical voting occurred across the West Country and more widely. “It is no secret that some Labour and Green party voters lent us their votes in Somerset and we are very grateful for their support,” he said. “In many seats across the south-west, voters at the next election will be faced with a similar choice between the Liberal Democrats and the Conservatives. “My message to all voters who want to get rid of this out-of-touch Conservative government is clear: if the Liberal Democrats can win back our heartlands in the West Country, it could lock the Conservatives out of power for a generation.” The Lib Dems, who suffered a near wipeout at Westminster after forming a coalition government with the Tories in 2010, are now rebuilding at national and local government levels. Having returned only eight MPs to Westminster in 2015, they are now up to 15 after their victory in Somerton and Frome and have their eyes on 15 other Tory-held seats in the West Country alone, where the Conservative majorities are lower than in Somerton and Frome. They now run eight councils in the West Country, up from two in 2016. Naomi Smith, chief executive of Best for Britain, which campaigns for better democracy and favours parties working together to remove the Conservatives, said her organisation would be providing guidance to voters ahead of the next election. “In the absence of opposition parties standing aside for one another to defeat this failing government, progressive voters have little choice but to make tactical decisions come polling day,” she said. “But voters can’t take for granted who the main challenger to the Tories is, so Best for Britain will provide constituency-level guidance. For as long as we have an arcane first-past-the-post system that structurally favours the right, such tactics will be necessary.” Neal Lawson, the director of Compass, a centre-left pressure group, who is facing possible dismissal from the Labour party for encouraging people to vote tactically, said it had gained 250 members since his argument with Labour came to light last month, suggesting support for his argument. He added: “To translate big by election wins into a more widespread general election and deep political victory means Labour and the Lib Dems, and also the Greens, systematically squeezing the Tory vote and building an ideas agenda for lasting change.”