Plaid Cymru has won the Caerphilly Senedd by-election today in a dramatic gain from Labour, capitalising on a collapse in support for the party in a seat it had held for decades.
Plaid’s Lindsay Whittle prevailed after a campaign that saw Labour slump to historically low levels in the constituency and Reform UK mount a strong challenge that ultimately fell short. Final polling prior to the vote showed Plaid and Reform running very close and Labour down into the low-teens, signalling a fracture of Labour’s once-dominant base in the Valleys. Plaid’s success reflects its capacity to attract centre-left ex-Labour voters uneasy with Reform but dissatisfied with the status quo.
Being from Caerphilly, and with my Facebook feed full of people urging others to vote one way or another, it was clear that the anti-Farage voice was both louder and more organised. Turnout reached just over 50% – higher than the 44% recorded in the seat in 2021, and above any national average ever seen in a Senedd vote. Plaid Cymru won decisively, securing a strong majority of 3,848. Reform UK, which scored just 495 votes four years ago, turned the contest into a two-horse race but ultimately finished second with 36%. The party had poured resources into the seat, with a string of senior figures, including Nigel Farage himself, visiting Caerphilly on polling day. Labour’s Richard Tunnicliffe came a distant third with just 11% of the vote, underscoring the scale of the party’s collapse in one of its former heartlands.
Plaid’s messaging of asking voters to lend them their vote to stop Reform clearly worked, but as we head into a new Senedd voting system in 2026, tactical voting will become less important.
While the result only shifts the Senedd arithmetic slightly, it is seismic for the Welsh Government. Labour’s fragile minority position at the Senedd, already tightly balanced, is made more precarious by the loss of a safe seat and dwindling public support.
Before the sad death of Hefin David, the Government held exactly half of the 60 Senedd seats and relied on a Liberal Democrat MS, Jane Dodds, to help pass its last Budget, with Plaid and the Tories voting against it. The loss of Caerphilly to Plaid means that Labour now needs two Senedd members to pass the upcoming Budget. If the Budget does not pass by the start of the financial year in April, then the Welsh Government and directly funded bodies could only spend up to 75% of the limits approved in the previous financial year.
If no Budget is passed by the end of July, that authorisation rises to 95% of the previous year’s spending limits, effectively amounting to a 5% cash reduction, with no adjustment for inflation.
Heading into the May 2026 Senedd Election which will take place under the new expanded PR system that will see the chamber grow to 96 members, Plaid stands to gain more reliably from an upsurge in its vote share than would have been the case under the old constituency-heavy model. That makes the by-election result strategically valuable for building regional lists and negotiating potential post-election coalitions.
Labour’s strategy and comms team will undoubtedly get their politicians to say the result is a ‘painful, alarming, wake up call’, blaming a combination of UK wide grievances over national stories that have dented trust. With by-election polling showing the party reduced to single-digit or low-teens support in Caerphilly, the leadership will face pressure to revamp its messaging again. It’s true to say that the Prime Minister’s UK Labour Government is unpopular, but as we approach Halloween, the polling is even scarier for Baroness Morgan’s Welsh Labour Government.
Plaid will use the victory to argue it is the principal vehicle for progressive change in Wales and that it beat Reform UK. The party’s win also complicates Reform’s calculations: although it had been polling strongly and offered an aggressive challenge, Plaid’s ability to reclaim disenchanted Labour support demonstrates that the anti-Labour vote is not monolithic. Reform can’t just rely on disgruntled Tory voters in Wales either, because the Conservatives have never been the largest party in the Senedd. Even if Reform takes every single 2021 vote from the Conservatives, they still wouldn’t win the election in Wales. Their team will know that they’ll have to broaden their appeal and take the fight to Plaid.
Whichever party’s candidate won, Caerphilly was the litmus test for Labour’s durability in Wales and for the ability of alternative parties to convert disaffection into seats. With the Senedd expanding and moving to a new electoral formula in May 2026, yesterday’s by-election was both a local event and an audition, and the result will reshape party messaging, candidate selection, and coalition maths in the months ahead.
Picture courtesy of @PeredurPlaidAS via X.
