In a stunning political upset that has sent shockwaves through Welsh and UK politics, Plaid Cymru has seized the Caerphilly Senedd seat from Labour, ending over a century of uninterrupted Labour dominance in the constituency. The by-election, held on October 23, 2025, was triggered by the death of long-serving Labour MS Hefin David. In an election marked by unusually high turnout and fierce competition, Plaid Cymru’s Lindsay Whittle emerged victorious with 15,960 votes, while Labour suffered a humiliating third-place finish with just 3,713 votes. Reform UK, the bookmakers’ favourite, finished second, but well behind Whittle’s commanding lead.
According to Caerphilly Observer, Plaid Cymru’s margin of victory over Reform was a striking 11.4%, a result that exceeded even the most optimistic forecasts for Whittle and his party. The shock was compounded by Labour’s collapse, which saw its vote share plummet in what had long been considered an impenetrable stronghold. As The Mirror noted, Labour had held the seat since the creation of the devolved Welsh parliament in 1999, and its loss signals a seismic shift in the political landscape of Wales.
By-elections are often seen as bellwethers for broader national trends, though analysts caution against drawing direct parallels. This particular contest is especially unrepresentative of the upcoming May 2026 Senedd election, as changes to the Welsh electoral system will eliminate the single-seat tier, reducing the impact of tactical voting. Still, the Caerphilly result offers a stark warning to Labour: the party is bleeding support on both its left and right flanks, and its traditional base appears to be in open revolt.
The turnout for the by-election was 50.43%, the first time a Senedd election has ever surpassed the 50% threshold. The high engagement reflected the intense interest in the contest and the sense among voters that something significant was at stake. The Conservative vote, meanwhile, collapsed from 17% in 2021 to a meager 2%, underscoring the extent to which right-leaning voters have migrated to Reform UK—or simply stayed home.
Reform UK, led by Nigel Farage, had poured resources into the Caerphilly campaign and was widely tipped to win. Yet, as Freebets.com reported, the party’s defeat has already had repercussions in the betting markets for the next general election. Reform’s odds of securing the most seats have slipped from 8/11 to 5/6, while their chances of winning an outright majority have dropped from 6/4 to 2/1. This shift suggests that the party’s momentum may have reached its ceiling, a view echoed in national polling where Reform’s popularity appears to have plateaued.
Andy Burnham, Mayor of Greater Manchester, seized on the result as evidence that Reform is not invincible. Speaking at an event reported by The Mirror, Burnham declared, “Isn’t the positive though from the Caerphilly by-election that Reform are beatable. That there is an appetite out there to stand up to them and stop them and challenge them.” His comments were echoed by Liverpool City Region Mayor Steve Rotheram, who noted that the Greens are also gaining ground at Labour’s expense: “We’re getting squeezed as well because the Greens are really picking up. We’re caught between things at the moment and we don’t know whether to go that way or to tact that way. And actually what we need to be is just Labour.”
Labour’s internal struggles have become increasingly visible. The party’s inability to distinguish itself from the previous Conservative administration has alienated both traditional supporters and left-leaning voters, particularly in Wales. The attempt by Welsh First Minister Eluned Morgan to promote a distinct “Red Welsh way” has failed to convince many that Welsh Labour operates differently from its Westminster counterpart. As a result, a “why not?” logic appears to have taken hold among disaffected voters, with some shifting to Reform on the right and others to Plaid Cymru or the Greens on the left.
Plaid Cymru’s leader, Rhun ap Iorwerth, was quick to frame the by-election as a mandate for change. “We are now the real choice for Wales, the only party able to stop billionaire-backed Reform and offering a better future that works for everyone. The message from Caerphilly is clear: Wales is ready for new leadership, and Plaid Cymru is leading the way,” he said, according to The Mirror. Upon his victory, Lindsay Whittle delivered a pointed message: “Listen now Cardiff, and listen Westminster.” The implication was unmistakable—both Welsh and UK Labour must heed the warning or risk further losses.
Labour’s path to recovery is complicated by the fact that its support is being eroded from both ideological extremes. On the right, Reform UK has capitalized on dissatisfaction with Keir Starmer’s government, while on the left, parties like Plaid Cymru and the Greens are attracting voters disillusioned with Labour’s centrist stance. According to polling cited by Caerphilly Observer, approximately 11% of Labour voters in the 2024 election would now consider voting for Reform, making it the chief beneficiary of disenchantment with Starmer’s leadership.
The by-election result also revealed the tactical calculations at play. Many voters appear to have rallied behind Plaid Cymru as the most viable “anti-Reform” option, a dynamic that could shift in next year’s proportional representation election, where the threat of a winner-takes-all outcome will be diminished. Nevertheless, winning back these voters will be no easy feat for Labour, especially given Starmer’s record-low approval ratings at this stage of his premiership.
For Reform UK, the second-place finish in Caerphilly is both a setback and a sign of progress. The party’s vote share surged by 34 points compared to 2021, when it managed just 2% in the same seat. This dramatic improvement suggests that Reform could be well positioned in areas currently held by Conservatives or where Labour’s majorities are slimmer. However, as Freebets.com noted, the party’s inability to convert polling leads into actual victories—especially in high-turnout contests—raises questions about its broader appeal.
Meanwhile, the Greens and other progressive parties are eyeing Labour’s decline with interest, sensing an opportunity to expand their influence. The national betting markets now favor a Hung Parliament at the next general election, with odds of 4/5 (55.6%), while the chances of a Labour majority stand at 5/2 (28.6%). Reform’s odds for a majority have softened, reflecting the uncertainty surrounding its future prospects.
The Caerphilly by-election has upended assumptions about Welsh politics and exposed deep fissures within Labour’s traditional base. As the dust settles, one thing is clear: the old certainties no longer hold, and the battle for Wales—and perhaps the UK—has entered a new and unpredictable phase.
