YouGov correctly predicted a historic shift in Welsh politics, with Labour’s dominance overturned by a Plaid Cymru and Reform UK surge

The 2026 Senedd election delivered a historic result in Welsh politics, driven by huge changes in a fragmented electorate voting a wholly new electoral system. Labour suffered a notable collapse, losing control of the Senedd for the first time and falling to just nine seats after heavy losses across the country – the extent of which YouGov led the pack in predicting. In contrast, Plaid Cymru surged to become the largest party with 43 seats, ending decades of Labour dominance and positioning Rhun ap Iorwerth to lead the next government.

Reform UK also achieved a major breakthrough, finishing second with 34 seats as part of a significant anti‑establishment movement across Wales from 2021 to 2026. The Conservatives fell well back on what was a record result for them the last time the Senedd was up for election, while the Greens improved on their prior performance and took advantage of the new, more proportional voting system to gain their first ever two Senedd seats.

Our MRP against the actual result

We were the most accurate pollster in the 2026 Welsh Senedd election by some distance. Our final call MRP exactly predicted the number of seats that Plaid, Reform, the Greens, Lib Dem and other parties would win. We also successfully predicted the marked decline in the Labour and Conservative seat tallies, which came well within our predicted bounds on both counts.

The proportional D’Hondt seat system, new for the 2026 election, means that just a few percentage points difference in vote share can have a huge impact in the seats allocated – something that made calling both elements of this election very tricky. The final vote shares for the leading parties were much closer than their seat counts, with Plaid Cymru securing 35% and Reform 29%. The two were in a tight race throughout the campaign, with YouGov illustrating Plaid enjoying a late swing (at the expense of the Green Party) in the final days and ending two days before the election. Our projections finished on 33% for Plaid and 29% for Reform.

All final vote shares for all projected parties fell well within our vote share prediction ranges, with an average error of just 1.3. This makes us one of the most accurate pollsters on this measure too. Throughout the campaign we were an outlier on the dramatic loss Labour were experiencing, predicting a substantially lower Labour vote share than competitors. Our final call of 12% was just 1 percentage point higher than the actual result, and the closest of all pollsters.

Our MRP model

In the face of a dynamic and changing political landscape in Wales, we built a complex multilevel modelling and post-stratification (MRP) model similar to those we have used before to successfully call elections in Spain in 2023, the UK in 2024 and Australia in 2025.

MRP projection models work by first estimating the relationship between a wide variety of characteristics about prospective voters and their opinions – in this case, which party they will vote for at the Senedd election – in a multilevel model. They then use data at the constituency level to predict the outcomes of seats based on the concentration of various different types of voters who live there, according to what the multilevel model says about their probability of voting for various parties (post-stratification).

We also built strong and detailed models of the likely Welsh electorate for 2026, considering how this might have changed since 2021 and using information from 2024 elections and the Caerphilly by-election in 2025 to properly calibrate expectations on who we thought would turn out in Wales (and, crucially, who they would be voting for).

Why our results are so accurate

The electorate has shifted notably since the previous election in 2021. Additionally, the 2026 Senedd election was fought following sweeping reforms to constituency boundaries, total seats, and indeed the entire electoral system. All of this, combined with the general difficulties in polling and projecting public opinion in geographically targeted areas, made political modelling for the election difficult and makes our success with the MRP all the more pleasing.

Three factors make a notable difference in YouGov delivering accurate results.

1. YouGov’s world-leading panel

The first is the quality of our data. Because we own our own panel, we have a unique relationship with the people taking our surveys and instant access to the views and opinions of hundreds of thousands of people in Wales. As a result, YouGov has a deep understanding of who they are, which helps us build and execute models at scale that are truly market-leading and help us remain the most accurate research company.

We were able to use this panel to conduct a stratified sampling approach, ensuring we derived high-quality, robust samples to feed into our MRP models. It also means we can trust our past voting behaviours as we capture and store it as close to an election as possible, minimising any false recall bias.

2. YouGov in-house experts

The second is the quality of our expertise across our Political and Social Analytics, Political Research, Data Journalism, and Panel teams, to name but a few. YouGov's experts in political polling, psephology, and advanced analytics worked hand-in-hand with experienced colleagues from our panel and data teams to deliver the most accurate results of any pollster in the 2026 Senedd election.

We are also incredibly grateful to Dr Jac Larner, Lecturer at University of Cardiff, for providing us his notional Senedd 2021 results, which were a significant help in the development of our modelling.

3. YouGov commitment to innovation

The third is the quality of our innovation. Our ground-breaking MRP relies not only on our gold-standard data and in-house expertise but also on our use of advanced technology and computing solutions that allow us to put these advantages to full use.

YouGov has been using MRP modelling for almost a decade, since its breakthrough at the 2017 UK general election. Since then, we have built up both a track record of success and – crucially – an approach based on a combination of the latest technology and considerable experience. As elections and electorates have evolved, we have updated and improved our MRP models to keep pace - a vital component in our continuing ability to get it right.

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