Plaid Cymru's victory in the Caerphilly Senedd byelection marks a historic break from over a century of Labour dominance in the constituency. The result, with Plaid Cymru securing 47.4% of the vote, was a convincing win in a race that saw Labour and the Conservatives combined achieve just 13% of the vote.
The byelection, only the fifth for a Senedd constituency seat since 1999, drew a significantly higher turnout than the previous Senedd election in the seat. Observers noted that the electorate appeared motivated by a desire to prevent Reform UK from winning, as the party had led in the only constituency poll during the campaign.
Reform UK, which achieved 36% of the vote, will be disappointed despite their share being above current national polling. The party had launched its 2024 general election manifesto nearby in Merthyr Tydfil, raising expectations for a stronger showing.
The result reflects a broader trend of declining two-party dominance, with the Labour-Conservative combined vote share dropping below 60% in the 2024 general election. In Caerphilly, the two main parties were effectively sidelined, with voters turning to smaller parties.
For Labour, the loss underscores challenges ahead of the 2026 Senedd elections, which will be held under a new proportional representation system. While the party may hope to replicate the anti-Reform dynamic in future Westminster elections, the PR system makes it harder to consolidate votes against a single opponent.
