1st October 2025 

Craig Lawton, Grasshopper’s Head of Public Affairs, takes a look at the upcoming by-election and the impact it could have on next year’s Senedd election 

 

This summer we received the heartbreaking news that Hefin David, MS for Caerphilly, had passed away at the age of just 47. 

Law requires that any Senedd vacancy must be filled within three months. With the Senedd election not taking place until May 2026, it was confirmed that the by-election will be held on Thursday 23rd October 2025. 

Labour has held the seat since the start of Welsh devolution. However, despite this record, Caerphilly has always been seen as a target for Plaid Cymru. Every Senedd election has seen them finish second to Labour, with Plaid candidate Lindsay Whittle coming within 1,500 votes of Labour back in 2016. 

Even at the 2024 General Election, where Reform UK won significant numbers of votes across the UK, Whittle and Plaid again came second in the constituency.* 

However, right-of-centre parties have still polled well over the years. 

UKIP finished second in the 2015 General Election, whilst the Conservatives – on strong Brexit platforms – were second in both 2017 and 2019. 2024 saw Reform UK gain 7,000 votes, mostly at the expense of the Conservatives, despite never having fielded a candidate before. 

Add to this the fact that Caerphilly voted strongly in favour of Brexit in 2016, by 57.6% to 42.4% – and arguably- the writing has been on the wall for some time. 

With Labour governments now at both ends of the M4, Labour’s current decisions as well as their record in Wales is coming under ever increasing levels of scrutiny. 

All this leads to the present, which sees Reform UK and Plaid Cymru practically level in the latest Senedd election poll. Whilst Plaid are ahead on 30% compared to Reform UK’s 29%, the biggest headline is the significant drop in support for Labour. 

Having once topped the polls at over 40% in Wales, Labour are presently polling at 14%, just four points ahead of the Conservatives. 

If there was a full Senedd election under the new electoral system using these figures, both Plaid and Reform UK would win 36 seats in the newly expanded 96 seat Senedd. 

Labour, having never been out of government in Wales, would win just 13. 

 

What does this mean for Caerphilly on 23rd October? 

If the polling is correct, we have a straight fight between Reform UK and Plaid Cymru.  

Reform UK are winning support from dissatisfied Labour and Conservative supporters – many of them Brexit voters. 

In comparison, Plaid Cymru support appears to be coming from dissatisfied liberal-minded Labour voters and a coalition of voters looking to oppose Reform UK at all costs, in addition to their already strong voter base in the constituency. 

 

Is the polling correct? 

The big question remains whether the polling is correct and whether, as we get nearer to decision time, voters will revert back to their previous voting habits. 

The other big question, and the one that could have the biggest impact on the result in Caerphilly, is whether those people that tend to not vote will in fact turn out. 

One of Reform UK’s biggest selling points to the electorate has been that they claim to be different to the other major parties, and that they stand up for values and policies that the other parties do not. 

This messaging is gaining significant media attention and building support from people who have for a long time been dissatisfied with national politics. However, these are also the most difficult voters to get to actually turn out and vote. 

Whilst pollsters will look to factor this in when making predictions, there is no greater way to judge the mood of the electorate that an actual election. 

The Caerphilly by-election provides us with a real snapshot of the electorate’s feeling and tests the polling we have had to date, all just six months out from a full blown Senedd election. 

 

What will it mean for the 2026 Senedd election 

The Caerphilly by-election result could play two significant roles in the run up to next year’s Senedd election. 

Firstly, the result will impact the mood and messaging of each party. 

Whilst it will certainly give a boost to those parties that poll well on 23rd October, we should keep an eye on the messaging and policies that come from those parties that do badly. 

Could this be the kick-start that they need to turn things around, or will it be the start of a difficult period in the political wilderness? 

Secondly, it gives us an opportunity to apply the new Senedd electoral system to actual election results. Up until now, all the projections for how many MSs each party will have in the new 96 seat Senedd has been based solely on opinion polls.  

Therefore, whilst Caerphilly’s by-election is an opportunity for local voters to elect a new Member of the Senedd, it could also have a significant impact on the national picture at next year’s Senedd election as well. 

 

*The UK Parliament constituency for Caerphilly has slightly different boundaries to the current Senedd constituency. 

 

Find out more information about Grasshopper’s 2026 Senedd election package here.