The by-election was caused by the sitting member, Ivor Guest, succeeding to a title that gave him a seat in the House of Lords.
The result at the last General election was: The local Conservative and Liberal parties could not agree on a joint candidate to succeed Guest as MP.
The local Labour party needed to find a new candidate, as their last, Leslie Haden Guest, had been elected in a 1937 by-election in the London constituency of Islington North.
This placed the Labour campaign at an advantage as Jackson was a farmer and Hanning Philipps admitted at his adoption meeting that he knew nothing about farming.
The absence of a Liberal candidate also allowed the Jackson campaign to appeal to those who favoured a Popular Front opposition to the Government that had been advocated by the likes of Sir Stafford Cripps.