Liberal Democrat leader Sir Ed Davey has appealed to Labour and Green party supporters to vote tactically in Thursday’s crucial West Country byelection – to help deliver a “knockout” blow to Boris Johnson’s premiership. The Lib Dems believe they have a realistic chance of causing one of the biggest byelection shocks of recent times by coming from third place to win in the normally safe Conservative stronghold of Tiverton and Honiton. However, Lib Dem and Tory activists are reporting the race is too close to call, with four days of campaigning to go, despite widespread anger about Partygate and concern in the rural community that the Tories are not doing enough to help farmers or tackle the cost of living crisis. On the ground, Lib Dem organisers are saying it has become harder to win over Tory voters to their cause in the last week and a half, as the Conservatives have emphasised what are called “wedge issues” – immigration, opposition to rail strikes, and Brexit – in order to try to persuade their supporters not to desert. “There has definitely been a hardening among those Tory voters and it has become more difficult in the last few days,” said a Liberal Democrat source. “It is going to be very tight and the majority to overturn is very big.” The seat fell vacant after its Conservative MP for the past 12 years, Neil Parish, resigned having admitted to watching pornography twice on his mobile phone in the House of Commons chamber. The contest, along with a second byelection on Thursday in Wakefield, is seen as a key to Boris Johnson’s chances of surviving as prime minister in the aftermath of the Partygate scandal, which continues to rumble on. Last night, in reponse to a question from the Lib Dem chief whip, Wendy Chamberlain, the Metropolitan police announced that two people had failed to return questionnaires to police investigating lockdown parties, although the deputy assistant commissioner, Jane Connors, insisted that “certainly did not prevent” them receiving a fixed penalty notice. If Johnson were to lose the West Country seat, which the Tories won with a thumping majority of 24,239 at the 2019 general election, to the Lib Dems, and Wakefield to Labour on the same night, many Conservatives believe Johnson’s days in No 10 would be numbered. A double defeat will be seen as evidence of the Tories’ electoral unpopularity both in so-called “blue wall” seats in the south of England and “red wall” areas in the Midlands and north that the party took from Labour in 2019. On Thursday the Tory candidate Helen Hurford, the owner of a beauty salon, was jeered and heckled at a hustings as she tried to avoid giving direct answers to questions about the prime minister’s honesty and integrity. Increasingly, activists and canvassers on the ground believe the race in Tiverton and Honiton – and therefore potentially Johnson’s future as prime minister – could be decided by the number of Labour and Green supporters who vote tactically on Thursday. Speaking to the Observer, Davey urged people who regarded Johnson as a liar who was harming their country not to waste their votes. He said: “If we can beat the Conservatives here in one of their safest seats, overturning the biggest majority in a byelection in political history, it would send a shiver down the spine of hundreds of Tory MPs. “The future of the country is hanging in the balance. If Labour supporters vote tactically and back the Liberal Democrats in Tiverton and Honiton, they can help deliver the knockout blow to Boris Johnson.” Lib Dems say that the party was performing well when postal votes arrived, with Johnson in serious trouble as his MPs launched a confidence vote in his leadership. At the 2019 general election the Tories won 35,893 votes in Tiverton and Honiton, Labour 11,654, the Lib Dems 8,807 and the Green party 2,291. As soon as the byelections were called the Lib Dems made it clear they would focus their resources on Tiverton and Honiton, while Labour prioritised the Wakefield seat, where the Tory majority in 2019 was only 3,358. Campaigning in Wakefield on Saturday, Labour leader Sir Keir Starmer told supporters that a victory could mean the city becoming “the birthplace of the next Labour government”. He said: “It feels like we can reach out and touch this. It feels positive. We’ve just got to make sure we don’t take our foot off the pedal now, there’s no complacency.” Wakefield was won by the Tories in the 2019general election after being a Labour stronghold since the 1930s. But a byelection was called after the resignation of Conservative Imran Ahmad Khan, who won in 2019, after his conviction for sexually assaulting a boy.