Bookmakers last week made Liberal Democrat candidate Helen Morgan the favourite to win the seat after a disastrous period for Boris Johnson which included accusations of lockdown rule-breaking in Downing Street last year.
But Neil Shastri-Hurst has now returned to the position of odds-on favourite to hold the seat for the Conservatives, who won with a majority of 23,000 in 2019.
Tory Party chairman Oliver Dowden was out campaigning with Dr Shastri-Hurst on Wednesday as activists and politicians from across the country descended on the constituency's market towns.
Mr Dowden was pictured outside the Fox and Hounds pub in Shawbury, on the edge of the constituency boundary, while Shadow Cabinet ministers Preet Kaur Gill and Lucy Powell joined Labour's Ben Wood in Oswestry.
Polling stations open at 7am with results expected in the early hours from the count at Shrewsbury Sports Village.
Apart from two years in the 1900s, North Shropshire has always been Conservative but Thursday's vote is expected to be far closer than normal after the resignation of Owen Paterson who was found to have broken paid lobbying rules.
Boris Johnson U-turned and withdrew his support for Mr Paterson - who earned more than £100,000 a year from second jobs - and the Tories have been damaged further by the revelations over Downing Street parties in the run-up to last Christmas.
North Shropshire by-election odds
According to SkyBet, as of 8pm on December 15.
Liberal Democrats 10/11
Reform UK 150/1
Freedom Alliance, Heritage Party, Monster Raving Loony Party, Party Party, Rejoin EU, UKIP, 2 x independents, all 500/1
Dr Shastri-Hurst, who is from Birmingham, has also been criticised for being the only major party candidate who does not live in Shropshire - with one Tory councillor defecting to Reform UK in protest.
Despite having no Shropshire councillors in North Shropshire and Labour coming ahead of them in all but one of the last five parliamentary elections, the Lib Dems have claimed to be the closest rivals to the Conservatives throughout the campaign.
And there have been suggestions that Labour, who are now priced at 50/1 or longer with all major bookies, have deliberately not committed resources to winning the by-election under an agreement to give the Lib Dems a better chance of winning.
Although deputy leader Angela Rayner visited Whitchurch this week, Sir Keir Starmer has not joined the campaign trail unlike fellow party leaders Boris Johnson who dropped into Oswestry and Sir Ed Davey who has visited five times.
A Labour spokesman denied this on Wednesday, saying: "You had Angela Rayner there yesterday, you've got two members of the shadow cabinet there, at least, today that I've already seen on social media are there, they're campaigning.
"We've got a brilliant local candidate in Ben Wood and we've run a focused campaign there, and I think what's clear is that Boris Johnson's incompetence and broken promises is what's coming up repeatedly on the doorstep."
Asked if he was able to deny there had been a "gentleman's agreement" between Labour and the Lib Dems, he said: "Yes, I am."
And asked how confident the party was of its chance, he added: "There's only 24 hours to wait, so let's wait and see what the result is."
A loss for the Conservatives would top a miserable week for the Prime Minister which has seen nearly 100 backbenchers defy the party whip to vote against the Government's Plan B coronavirus restrictions.
Asked if Mr Johnson would quit if North Shropshire was lost, the Prime Minister's press secretary said: "We are fighting for every vote."
In total 14 candidates are standing in Thursday's vote, and you can read Q&As with all but one of them on our dedicated by-election page.
Aside from the three major parties the candidates expected to poll the highest are:
Kirsty Walmsley, the Reform UK candidate who is the daughter of former Shropshire Council leader Keith Barrow
Duncan Kerr, the first ever Green Party councillor in Shropshire and former Oswestry mayor
Martin Daubney of Reclaim, the party led by anti-lockdown campaigner Laurence Fox
You can follow the election voting, count and results live, from Thursday morning until the early hours of Friday, from our reporting team in North Shropshire.