Sir Ed Davey will kick off the Liberal Democrat campaign in Mid-Bedfordshire as his party tries to secure a shock victory in the seat of Nadine Dorries after her resignation.
The Lib Dems see the constituency as a winnable target after making massive changes to take the so-called blue wall seats in the south of England where Tory support has traditionally been strong.
Former cabinet minister Sir Ed will use his visit on Monday to declare the constituency “a tight race” between his outfit and an “out of touch” Tory party.
Ms Dorries, who won 60% of the vote in the 2019 general election to secure a majority of 24,000, resigned as an MP on Friday before Boris Johnson’s resignation honors were published.
The former culture secretary has blamed her second workload as a broadcaster and newspaper columnist for wanting to leave the Commons immediately.
She had already announced that she was not planning to run in the next elections.
Ms Dorries was reportedly one of several Tory MPs, including Nigel Adams and Sir Alok Sharma, who were nominated for a peerage by Mr Johnson – who also quit Westminster – but were not on the published list.
Sir Ed’s visit to his constituency comes after a host of Tory MPs spent the weekend campaigning in Mid-Bedfordshire as Prime Minister Rishi Sunak’s party seeks to retain the seat.
Conservative Speaker Greg Hands posted photos of himself campaigning in the region on Sunday ahead of what he expected to be a “hotly contested” by-election.
Labor campaign co-ordinator Shabana Mahmood also visited over the weekend, posting on Saturday that “only” her party could “turn the page on the Tories and their 13 years of failure”.
The Lib Dems came third in the seat in 2019 and would need a 23.6% swing to win in a by-election.
In June 2022, the party won the Tiverton & Honiton seat from the Conservatives with a 29.9% swing.
Sir Ed said: “The British public is fed up with the sordidness and endless scandal of the Conservative Party.
“Next month the people of Bedfordshire will have the chance to finally shake off this chaotic sham of a government.
“Mid-Bedfordshire will be a tight race between the Liberal Democrats and an out of touch Tory party.
“This Conservative government takes people for granted; from soaring mortgage rates, to save NHS waiting lists and sewage in rivers.
As well as speaking to voters, Sir Ed will inspect a river which his party says is at risk of being rejected by Anglian Water.
A date for the by-election has not yet been set, with a “writ”, a motion giving between 21 and 27 working days for a vote to be held, which has yet to be moved.
The Tories face an upcoming three by-elections after Ms Dorries, Mr Johnson and Mr Adams, MP for Selby and Ainsty, resigned less than 24 hours apart.
Campaigning in Mr Johnson’s headquarters in Uxbridge and South Ruislip, west London, has already begun, with Ms Mahmood and Shadow Work and Pensions Secretary Jonathan Ashworth in the area on Saturday to support candidate Danny Beales .
Even before the former British leader’s resignation, Labor had marked his seat as one of its main targets ahead of the next general election, which is due to be held next year.
The Tories are defending a majority of 7,000 in Uxbridge and a cushion of 20,000 in Mr Adams’ constituency.
Mr Adams has pledged to help sack a Tory MP in the by-election he called in his North Yorkshire seat.