Voters will go to the polls in by-elections in Wakefield and in Tiverton and Honiton, both triggered after the previous Conservative MPs resigned in disgrace.
The polls are seen as a key test for Boris Johnson’s leadership two weeks after 148 of his own MPs voted in favour of his removal in a confidence vote.
In Wakefield, a former industrial area in West Yorkshire, ex-Conservative MP Imran Ahmad Khan stood down after being found guilty of sexually assaulting a 15-year-old boy and jailed for 18 months.
Wakefield was one of the so-called Red Wall seats won by the Tories in the 2019 general election after being a Labour stronghold since the 1930s, but Labour is now hoping to take it back.
In Devon’s Tiverton and Honiton, Neil Parish, the Tory MP since 2010, resigned after admitting he had watched pornography on his phone in the House of Commons.
The Liberal Democrats are the main challenger in this rural south-western constituency, where they are hoping to recreate by-election wins in North Shropshire in December and Chesham and Amersham a year ago.
Victory for the Lib Dems would require overturning a Conservative majority of 24,239, but party leader Sir Ed Davey was confident they were “neck and neck” with the Tories on the eve of the vote.
Residents are “fed up of Boris Johnson’s lies and neglect,” he said, as he called on them to back his party’s candidate, former soldier Richard Foord.
“Farmers have been sold out by foreign trade deals, people are struggling to access GPs and dentists, and local roads and schools have been left to crumble.
“Families are facing soaring petrol bills and food prices and this government’s only answer is to hammer them with constant tax rises”.
The Prime Minister urged voters to “stick up for British food and farming” by supporting Tory Helen Hurford, a former head teacher.
In a video shared on social media on Wednesday, Mr Johnson appeared alongside the candidate on a farm, saying the party was working to protect “our fantastic food and farming industry and “amazing agricultural sector”.
Losing the Tory stronghold would be seen as a sign of Mr Johnson’s diminishing electoral appeal after partygate and amid the cost-of-living crisis, and could spark a further backlash against his authority.
The Tories face a steeper challenge in retaining the Wakefield seat, with Labour now odds-on favourites to overturn the 2019 Conservative majority of 3,358.
Tory candidate Nadeem Ahmed raised eyebrows by arguing last week that voters should still trust the party after Mr Khan’s sexual assault conviction, just as they still trust GPs despite the crimes of mass murderer Harold Shipman.
Labour leader Sir Keir Starmer has said a victory in the northern constituency “could be the birthplace of the next Labour government”.
On the eve of polls opening, he said: “The public is fed up of Boris Johnson and a Conservative Party that promises big but never delivers.
“Labour’s candidate Simon Lightwood has run a positive campaign – with a plan to attract well-paid jobs and investment to Wakefield, put more police on the streets to tackle anti-social behaviour, and to fight for scrapping the unfair National Insurance rise.
“But a vote for anyone other than Labour will let Boris Johnson off the hook. The people of Wakefield have a chance to send a message to the Tories that enough is enough”.
If the Conservatives were to lose both by-elections, it would be only the seventh time a government has suffered such a double defeat since the Second World War.
Former Lib Dem leader Sir Vince Cable said a loss for the Tories “probably doesn’t matter a great deal” for Mr Johnson’s position as the Prime Minister would say “this result’s been priced in”.
Sir Vince told the PA news agency: “He’s very good at escaping from tight corners.
“I think the real problem for the Tory Government is later this year when the economy gets worse and worse and we’ve got high inflation, potential recession, a sliding Pound, Government budget under more and more pressure.”
Polling station for both by-elections will be open from 7am to 10pm on Thursday, with the results expected between 4am and 6am on Friday.