Following an assertion on social media, I chose to look at two recent parliamentary by-elections for their consistency with national opinion polling.
The two latest by-elections in Peterborough and Brecon & Radnorshire both show consistency with timely opinion polls.
In Peterborough and Brecon
The Peterborough by-election was forced by the first successful recall of an MP under the Act of 2015. Fiona Onasanya was formerly a Labour MP — who had the whip removed prior to the successful recall — after being found guilty of perjury.
Ms Onasanya did not contest the seat, with Labour candidate Lisa Forbes becoming the new MP in the by-election on 6th June 2019.
We can compare Westminster opinion polls to Prof Hanretty’s constituency mapping of the 2019 European elections. Simplistically applying a uniform swing to the constituency projection essentially recovers the by-election result.
In the case of Brecon & Radnorshire, Chris Davies (then the Conservative MP) was recalled after making a false expenses claim. Mr Davies was then re-selected by the Conservatives to fight the by-election.
Additionally, neither Plaid Cymru nor the Greens put up candidates — directing their campaigners to support the Liberal Democrat candidate Jane Dodds. On a Butler swing from the Conservatives to Liberal Democrats of 12 points, the Liberal Democrats gained the seat.
Alternately, we can look at the 2017 General Election for the constituency result, and uniformly apply estimated change in national opinion since the previous General Election.
The polling average projection suggested a modest Liberal Democrat lead in the seat: the levels were somewhat understated for both the large parties.
Additionally, Prof Awan-Scully found YouGov’s Welsh poll in July 2019 (on behalf of ITV Cymru and the Welsh Governance Centre) also indicated the Liberal Democrats were likely to gain Brecon & Radnorshire.
The two recent by-elections appear consistent with national opinion polling. The accuracy of those opinion polls may become a concern in any future General Election campaign.