And some bookmakers have installed the Liberal Democrats as favourites in next week's vote in the wake of the Downing Street Christmas party scandal.
After a video was leaked showing No 10 staff laughing days after the event is alleged to have taken place in December 2020, the odds of a Liberal Democrat victory next week have been slashed.
As a result there is little to choose between the Lib Dem candidate, Helen Morgan, and the Conservatives' Dr Neil Shastri-Hurst, with some even installing Mrs Morgan as favourite.
After days of Downing Street denials that a party took place last year, Mr Johnson was forced to announce an investigation as well as pledging to pass evidence to police.
Advisor Allegra Stratton has resigned after she and adviser Ed Oldfield, along with other aides, filmed laughing about a “fictional” Downing Street party in December 2020.
The issue has taken centre-stage in the North Shropshire by-election, with one bookmaker making the Liberal Democrats favourites to take the seat on December 16 – a success that would involve overturning a 23,000 Conservative majority.
Local Conservative sources have spoken of concern at the impact national issues are having on the race. One said they now expected a close race, which could go either by as few as 500 votes.
Ladbrokes tweeted on Wednesday afternoon to say the Liberal Democrats were now favourites, ahead of the Tories in second, Labour third, and the Greens, Reform and Reclaim tied in fourth place.
Betfair has also said Mrs Morgan is now favourite to replace Owen Paterson, who resigned last month amid a lobbying scandal, as do SkyBet, which has the Liberal Democrats at 5/6 and Conservatives at 10/11.
SkyBet also have Labour at 50/1, Reform UK at 150/1 and the Greens at 200/1.
As of Wednesday afternoon, bookmaking comparison site Oddschecker's best odds put the Tories ahead of the Lib Dems but not by much.
Dr Shastri-Hurt stands at 10/11 to win, with Mrs Morgan on 11/10.
No other party has been seen as a likely challenger to the seat, with the rest of the candidates coming in at:
Labour (Ben Wood) - 99/1
Greens (Duncan Kerr) - 200/1 (some sites offering special offers of 999/1)
Reform UK (Kirsty Walmsley) - 200/1
Reclaim (Martin Daubney) - 250/1
Ukip (Andrea Allen) - 500/1
Independent (Yolande Kenward) - 500/1
The Party Party (Russell Dean) - 500/1
Rejoin EU (Boris Been-Bunged) - 500/1
Independent (Suzie Akers Smith) - 500/1
Freedom Alliance (Earl Jesse) - 500/1
Heritage Party (James Elliot) - 500/1
Official Monster Raving Loony Party (Howling Laud Hope) - 999/1
There have been just two instances of non-Tory MPs in the North Shropshire area in the past 189 years. At the first election in 1832, when the constituency was much larger, and elected two MPs, Whig John Cotes was elected in second place, behind Tory Sir Rowland Hill.
At the 2019 General Election Owen Paterson was voted in with a majority of 22,949, having attracted 62.7 per cent of the vote.
Stay up-to-date with all the latest North Shropshire by-election news on our dedicated page here.