Plaid Cymru captured Caerphilly in the Senedd by-election held on 23 October 2025, with Lindsay Whittle polling 15,961 votes (47.4%). Reform UK’s Llŷr Powell finished second on 12,113 (36.0%), while Labour’s Richard Tunnicliffe placed third on 3,713 (11.0%). Turnout reached 50.43%, the highest recorded for a Senedd by-election.
The result ends Labour’s long dominance of the constituency at both Cardiff Bay and Westminster level. Labour took 46% here in 2021; falling to 11% in 2025 underlines the scale of change in voter behaviour in this part of south Wales.
The vote was called after the death of Hefin David, Labour’s Member of the Senedd for Caerphilly, who died on 12 August 2025. Tributes from across parties followed, and the by-election was scheduled within the required three-month window.
On the ground, Plaid Cymru and Reform UK presented the contest as a straight choice. Reform’s national leadership visited repeatedly and said it would “throw everything” at the seat, while Plaid campaigned on local delivery and change. The framing contributed to a two-party squeeze that sidelined Labour on the night.
The immediate parliamentary effect is clear: Welsh Labour remains the largest party but now holds 29 of 60 seats, increasing the pressure on ministers to assemble support for legislation and the budget in the months before the 2026 election.
Reform UK’s advance is notable in its own right. The party rose from a negligible base in Caerphilly in 2021 to 36% this week, reflecting intensive organisation and a strong national profile. Expect Reform to prioritise resources accordingly in neighbouring Valleys seats.
Attention now moves to the full Senedd election on 7 May 2026, which will be the first under the reformed electoral system. The Senedd Cymru (Members and Elections) Act 2024 increases the chamber to 96 members elected by closed-list proportional representation across 16 six-member constituencies, with a candidate residency requirement in Wales.
The change in rules matters for strategy. With proportional voting and multi-member constituencies, single‑party majorities are less likely, and campaigning is less about tactical squeezing and more about vote maximisation across lists. It is also expected that this Caerphilly contest will be the last Senedd by-election before the system change.
Plaid Cymru has recalibrated its constitutional offer under leader Rhun ap Iorwerth. The party has promised to publish a plan for independence but not to pursue a referendum in the first term of any Plaid‑led government, and has emphasised day‑to‑day priorities in the run‑up to May.
Reform UK, meanwhile, has set Wales as a priority for 2026 and has said it is competing to be the largest party, though Nigel Farage has ruled out standing as a candidate. The party’s performance in Caerphilly suggests it can translate national attention into votes under the new system.
Turnout of 50.43%-unusually high for a devolved by‑election-points to heightened engagement when voters perceive a competitive race. Reporting from the count indicated significant tactical voting to block Reform, which, combined with Plaid’s local profile, shaped the outcome.
Over the next six months, policy professionals should track three practical issues: how Welsh Labour manages confidence and supply to pass core legislation; how parties recruit and place list candidates under the residency rule; and whether polling continues to show a Plaid–Reform contest with no outright majority projected under PR.