The Copeland by-election is being held on Thursday (23 February) after Jamie Reed quit the House of Commons in January to work at the Sellafield nuclear decommissioning plant, the largest employer in the West Cumbrian constituency.
Former doctor turned Labour Councillor Gillian Troughton has been selected by the party to defend Reed's majority of more than 2,500 votes from the 2015 general election, against Conservative candidate Trudy Harrison.
Beyond Sellafield and Corbyn's anti-nuclear stance, the NHS and the Cumberland Hospital in Whitehaven have dominated the campaign.
Prime Minister Theresa May was accused of dodging questions about maternity services at the hospital when she visited the seat, while the Conservative premier accused Labour of "scaremongering" about the NHS.
Brexit is another issue after 60% of Copeland residents voted Leave at the EU referendum. Labour are 18 points behind the Conservatives nationally, according to the last opinion poll from ICM.
But no governing party has gained an opposition seat with more than a 3% majority in any by-election since Worcester in 1878, according to Number Cruncher Politics founder Matt Singh.
The vote is being held on the same day as the Stoke-on-Trent Central by-election in Staffordshire, which was triggered after Tristram Hunt quit the Commons to become the director of London's Victoria and Albert Museum.
Storm Doris, with its 80mph winds, is also expected to fall on Thursday, the Met Office has warned.