Background
Hefin David had been the Welsh Labour member of the Senedd (MS) for the Senedd constituency of Caerphilly since the 2016 National Assembly for Wales election. [2] He served as a backbench MS for the duration of his career on the legislature before his sudden death on 12 August 2025. [2] [3] His death left the constituency's seat vacant and therefore triggered a by-election under the Government of Wales Act 2006 and Senedd Cymru (Members and Elections) Act 2024, which will be held to elect his successor as MS for Caerphilly. [4] By-elections must be held within three months of a vacancy, so this by-election must be held no later than 11 November 2025. [1] [5] It is expected that this will be the last by-election held before the Senedd's electoral system changes to a closed list system of proportional representation in 2026, and therefore the last by-election to the Senedd for the foreseeable future, as constituency vacancies under this system will be filled using the party list rather than through a public by-election. [4] [1]
The by-election was triggered on a background of favourable opinion polling for Reform UK, who are therefore considered a potential favourite heading into the by-election. A Reform victory may also give it momentum heading into the next Senedd election, which is due to be held in 2026. The other main contenders include Welsh Labour, who have held the seat since devolution began in 1999, and Plaid Cymru, who have led Caerphilly County Borough Council twice in the same period. [4] David's death has left the incumbent Welsh Labour government of Eluned Morgan with a minority of 29 of 59 seats in the Senedd, alongside this singular vacancy. A victory for Reform or Plaid may make it more difficult for Morgan's government to negotiate the next Welsh budget for 2026 to 2027. [4]
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