Meanwhile, the Liberal Democrats are seeking to overturn a Tory majority of 24,239 in Tiverton and Honiton.
That is more than 1,000 larger than the shock win that the party achieved in North Shropshire last year, and almost 10,000 more than its win in rural Chesham and Amersham.
That by-election followed the high-profile departure of Neil Parish, the rural Devon constituency’s previous Conservative MP, who resigned after admitting that he watched pornography on his phone in the House of Commons.
Since the seat of Tiverton and Honiton was created in 1997, it has always been held by the Tories by a significant margin.
The Telegraph revealed last week that Mr Johnson has been “airbrushed” from the Conservatives’ by-election campaign literature, with leaflets and online advertisements not mentioning him.
A six-page pamphlet distributed by Helen Hurford, the Tory candidate in Tiverton and Honiton, failed to mention the Conservative Party until the fourth page and made no references to Mr Johnson.
Another pamphlet distributed by Nadeem Ahmed, the party’s candidate in Wakefield, also included no references to the Prime Minister or photographs of him.