The Caerphilly by-election, held yesterday following the death of Labour MS Hefin David, was set against a backdrop of significant political dynamics and had been considered a key test of party strength in a seat which Labour has held for the last 100 years. James Bevan, Portland Director and former Labour Special Adviser, has worked for a number of Welsh MPs, below he explores the result’s impact on Welsh Labour and how the result could translate in next year’s Senedd election and, ultimately, the next General Election.
For anyone – like me – who has knocked doors in the Welsh Valleys for Labour, the result in the Caerphilly by-election is desperately disappointing. Losing a seat in a Labour heartland, with a 35% swing against the party and just 11% of the vote, confirms many of the deepest concerns among Labour figures.
By-elections are always an opportunity for voters to give the incumbent government a kick, but the scale of this defeat – little more than a year after Labour secured a huge majority at the General Election – shows just how volatile politics has become, and the scale of the challenge Labour faces in reconnecting with voters.
In Portland’s report Starmer’s Britain: The Battles Ahead, published back in July, we highlighted that Welsh Labour was set to face real challenges from Plaid Cymru and Reform in the May 2026 Senedd elections. Since then, the picture has worsened further. If these results were repeated in May – when the elections will be fought on a proportional representation basis – the party would suffer catastrophic losses. After a turbulent period, with changes of leadership and introspection, Welsh Labour will now urgently need to decide how they respond to this defeat, understand which voters they are targeting ahead of the Senedd elections and get a clear strategy in place for the 2026 campaign – including how they frame / attach Plaid Cymru and Reform.
At Portland, we are already working with clients to assess what this means next – and there are a few key things to look out for.
In Wales, we can expect to see a significant ramping up of scrutiny of Plaid Cymru and their plans for government. Observers should be careful not to assume this will mean a full-blown focus on Welsh independence. Plaid strategists know that overall support for independence remains low, especially among the kinds of voters they are trying to win over beyond their core base. However, we can expect to see more assertive calls for devolution of key powers, in areas like justice and fiscal policy.
More broadly, a Plaid-led Senedd would bring a wholesale change of Welsh Ministers – a new set of stakeholders likely to lead major departments, with big budgets and key decision-making powers over areas such as NHS commissioning, planning and the economy. Understanding this emerging Welsh political landscape will be crucial for those seeking to influence it.
While figures across the UK Labour Party will be humbled by the Caerphilly result, the small glimmer of comfort they will cling to is that Reform did not manage to storm to victory.
Campaigners in Caerphilly reported that many former Labour voters appeared to switch to Plaid Cymru in order to block Reform. Party organisers often talk about ‘squeezing the vote’, and they will hope that at a general election they can build a narrative around the threat of Reform and unite ‘progressive’ voters to keep Nigel Farage out of No.10.
However, nothing unsettles sitting government MPs like seeing huge electoral swings against their party. This spells real danger for Labour Ministers as they prepare for a tough period around the Budget. Polling just 11% in a Labour-held seat, while facing pressure to introduce further tax rises, is an uncomfortable position to be in and keeping MPs onside will be a huge challenge. There is a real political test ahead for Rachel Reeves – especially as MPs begin to feel the heat from an electorate restless for change.
