Westminster By-Elections
Results and model signal — 2024–present Parliament
2 by-elections held since July 2024. 2 Labour losses.
Makerfield
Lab–Reform marginal · Thursday 18 June 2026 — triggered by Josh Simons' resignation to clear Andy Burnham's path back to Westminster. Fourteen candidates; effectively a two-horse race.
The left is consolidated behind Burnham; the right is split. Whether Reform squeezes Restore’s vote decides the seat.
Aberdeen South
SNP–Con three-way marginal · Thursday 18 June 2026 — triggered by Stephen Flynn's resignation. A genuine three-way at the top: the SNP starts narrow favourite, but a by-election protest against the SNP government and unionist consolidation behind the Conservatives make this genuinely competitive — the Conservatives are a live threat.
The unionist vote is split between Conservative and Reform. Whether Reform's vote consolidates behind the Conservatives — strongest in the Deeside wards — decides whether the SNP holds.
Arbroath and Broughty Ferry
Safe SNP hold; Reform–Labour fight for second · Thursday 18 June 2026 — triggered by Stephen Gethins' resignation. A safe SNP hold on every read; the live questions are the size of the majority and whether Reform or a collapsed Labour finishes second.
The SNP holds comfortably on every read. The real contest is for second place: Reform (71.8%) versus a collapsed Labour (14.8%).
Probability of finishing second (the seat itself is not in doubt).
What the by-elections tell us
Both results since the 2024 election show the same pattern: Reform and Greens significantly up, Labour significantly down. Both had unusual triggers (MP misconduct / MP suspended before resignation) which reduces their weight in the model. The direction — Labour weaker than polls suggest, Reform and Greens stronger — is consistent with national polling but more extreme, likely amplified by by-election protest effects.
LOW confidence — n=2, abnormal triggers. Weight: 0.08
Gorton and Denton
26 February 2026 · North West · metropolitan
Green Party gain from Labour — majority 4,402 (12.0%)
Result
| Party | By-election | 2024 GE | Swing |
|---|---|---|---|
| Green | 40.7% | 13.2% | +27.5pp |
| Reform | 28.7% | 14.0% | +14.7pp |
| Labour | 25.4% | 50.8% | -25.4pp |
| Conservative | 1.9% | 7.9% | -6.0pp |
| Lib Dem | 1.8% | 3.9% | -2.1pp |
Context
Andrew Gwynne (sitting as Independent after Lab suspension for antisemitic WhatsApp messages) resigned on health grounds 22 January 2026
Green Party's first Westminster by-election win and first MP in the North. Labour collapsed -25pp. Three-way tactical voting: Forward Democracy, TacticalVote.co.uk all endorsed Greens to stop Reform. Reform strong second on 28.7% (+14.7pp). Con wiped out (7.9% to 1.9%). Highly abnormal seat: large Muslim community, Manchester urban, student areas -- not representative of Lab marginals. Trigger was Labour-suspended MP, further reducing signal quality. Polanski Green leadership (elected Sep 2025) appears to be driving membership and vote surge. Weight: 0.5.
Model Signal
Why discounted: Abnormal trigger (resignation). Result inflated by protest effects specific to the by-election context.
Signal will be reviewed after May 2026 locals. If locals confirm the direction, weight may increase.
Runcorn and Helsby
1 May 2025 · North West · unitary
Reform UK gain from Labour — majority 6 (0.0%)
Result
| Party | By-election | 2024 GE | Swing |
|---|---|---|---|
| Reform | 38.3% | 18.1% | +20.2pp |
| Labour | 38.3% | 53.0% | -14.7pp |
| Conservative | 7.1% | 16.0% | -8.9pp |
| Green | 7.0% | 6.4% | +0.6pp |
| Lib Dem | 2.9% | 5.1% | -2.2pp |
Context
Mike Amesbury (Lab) convicted of assault, sentenced to 10 weeks prison (suspended), resigned 17 March 2025
Reform's first Westminster by-election win. Six-vote majority. Con collapsed from 16% to 7%, split between Reform and potential Lab tactical voters. Lab vote held better than feared (~53% to 38%). Seat won by scandal effect (Labour MP convicted) + national anti-government swing. Abnormal trigger: weight reduced to 0.6.
Model Signal
Why discounted: Abnormal trigger (misconduct). Result inflated by protest effects specific to the by-election context.
Signal will be reviewed after May 2026 locals. If locals confirm the direction, weight may increase.
Historical Context
By-elections are an imperfect signal for general elections. Turnout is lower (46–48% here vs ~60% at GEs), protest voting is stronger, and tactical voting is often explicitly coordinated. The model applies a 0.08 weight to by-election signals — enough to register the direction of travel without letting two data points override hundreds of polls.
| Gorton | Runcorn | GE average | |
|---|---|---|---|
| Turnout | 47.5% | 46.2% | ~60% |
| Con vote share | 1.9% | 7.1% | ~17% |
| Lab-Reform gap | -3.3pp | 0.0pp | ~-3pp |
The Con vote share comparison is the starkest illustration of by-election squeeze effects vs what would happen at a GE.