The Liberal Democrats aim to use next month’s Chesham and Amersham byelection to test the extent of disillusionment with the Conservatives in southern commuter belt seats, Ed Davey has said – even if victory is still seen as a long shot. While there has been much focus about Tory gains in former Labour heartlands, this month’s local elections highlighted an apparent retreat for the party in more prosperous areas in the south of England and around London. On 17 June, voters in Chesham and Amersham will select a new MP to replace Cheryl Gillan, who died in April, in a Buckinghamshire constituency which has never seen the Conservatives win less than 50% support since it was created in 1974. Nonetheless there “definitely could be a shock”, the Lib Dem leader told the Guardian. Davey said: “It’s still quite a hill to climb, primarily because the Tories are doing quite well in the polls nationally. It’s absolutely clear it’s a two-horse race. But that doesn’t mean the anti-Tory vote will actually get behind us.” This could be a key factor in the Tories winning again – the lack of any “progressive alliance” with a Green party buoyed up by its local election results and, according to one opinion poll published on Sunday, now level with the Lib Dems for national support. In the 2019 general election, the Lib Dems and Greens, as well as Plaid Cymru, stood aside for each other in about 60 seats in England and Wales. However, more formal pacts have proved difficult, and has not happened for the upcoming byelection. Local sources from the Lib Dems and Greens said there was not even any discussions over a pact, amid generally tepid and sometimes suspicious relations between the two parties. The Lib Dems selected their candidate at the start of May. While there had been rumours the party could opt for a high-profile candidate, they picked Sarah Green, a longtime party member who runs a training company. The Lib Dems, and previously the Liberals, have almost always finished second in Chesham and Amersham – although Labour did this in 2017 – and took control of Amersham town council for the first time in the local elections. Davey said that while he accepted it would “need a big swing” to take the seat from the Conservatives, one likely factor was local disquiet about government proposals to change planning rules in a way expected to greatly increase housing targets in southern counties, which has met with opposition from many Tory MPs and councillors. “If the shock were to happen, it would be clear this would be a massive mandate for those of us who were campaigning against the planning reforms,” Davey said, adding his party was pushing for a less centralised, more locally-led system. “And I think it would make a lot of Conservative MPs around the south of England quite worried.” Even with the replacement of the popular and long-serving Gillan as Tory standard-bearer by the new candidate Peter Fleet, a former Ford executive who has spent most of the past 15 years based in Germany, Thailand and China, the odds remain heavily stacked in favour of the Conservatives. But Davey said it did seem as if the Tories had, in part, taken the seat for granted: “I’ve been canvassing there twice now, and for some people, they said no party had even knocked on their door before.” This article was amended on 24 May 2021. A previous version incorrectly stated that the Lib Dems had “always finished second” in Chesham and Amersham.