The new Senedd will be elected using a form of proportional representation (PR), meaning that the number of seats a party wins broadly reflects its share of the vote. But because the allocations under Wales’s new version of PR take place in each constituency, each of which elects six MSs, the system does have a small advantage towards parties that win a greater share of the vote.
This can be particularly seen with Plaid, whose central projection of 45% of the seats stems from a median vote share estimate in our model of 33%, which is nonetheless a gain of twelve points on their regional vote in 2021.
Reform UK are projected to win 27% of the vote, up from only 1% in 2021, while the 13% share projected for Labour is down a full 23 points on their previous vote, with the fall in the Conservative vote to only 7% a similarly seismic loss of 18 points.
While the Greens and Lib Dems both won 4% of the regional vote in 2021, our model shows clear divergence today, with the Greens now on 12%, up eight points, in contrast to the Lib Dems’ near-unchanged 5% in our central estimate.
Of course, the model in full projects a range of outcomes, and it is not definite that Plaid Cymru will win the most votes: their lower-most estimate (28%) is below the upper-most bound for Reform UK (31%), meaning that it remains a possibility that the latter wins the most votes, even if this is only the case in a small minority of our MRP’s simulations.