How to understand the post-Gorton and Denton national poll that puts the Greens ahead of Labour

Polling by YouGov suggests a surge in support for the Green party across the country following the Gorton and Denton byelection. According to the poll, Zack Polanski’s party now has a national vote share of 21%, leapfrogging the Labour party. The Greens now sit within the margin of error behind Reform’s 23%.

In light of this result, some have claimed that the Green party’s byelection victory has boosted its national polling by making it look like a party that can win in an election. This is a version of what is known among political scientists as the bandwagon effect. The idea is that voters jump on the bandwagon of parties that are very popular with other voters.

But it’s not necessarily the case that voters are now thinking of going Green just because the Greens won in the byelection. Coverage of byelection results has a longstanding tendency to focus too much on which party wins and not enough on trends in vote shares.

Hannah Spencer won the seat for the Greens on a share of the vote fully 28 percentage points higher than her predecessor managed in 2024. Changes in vote shares like this matter because they can give some insight into the scale and direction of shifts in the standing of parties across the country as a whole.

This is true regardless of whether such trends tip a party over the threshold required to win the seat. In other words, had the Greens won Gorton and Denton by a narrow margin, as constituency polling late in the campaign suggested they might, that would not have been quite so seismic a result. What has shaken British politics is that the Greens did so much better than they did in 2024, that they did so much better than Labour – and so much better than expected.

The effects of these two related factors – the victory and the swing – cannot be teased apart. The win itself is not necessarily what has motivated voters to jump on the bandwagon in subsequent polling. It could just as plausibly be the Greens’ improving electoral performance that is doing the important work.

The striking and substantial surge in support in the byelection may be inspiring voters across the country to go Green. That surge just happened to also propel them to victory in the byelection.

Indeed, decades of research on the bandwagon effect has struggled to find any consistent evidence that voters flock to the most popular party or the “winner”. But some evidence suggests that when parties become more popular, regardless of whether that growth propels them into first place, this growth can become self-perpetuating as more voters jump on the bandwagon.

Why this matters

This distinction matters because it can shape the story we tell about why the effect is happening. If we focus on the Greens’ victory as the most important factor driving their subsequent poll boost, we will tend to tell a story about viability: the Greens won this election, so they could win others, and that makes people want to back them.

If instead we focus on the Greens’ growth, we uncover a story about momentum: the Greens have gained ground, so they could gain more ground, and that makes people want to get involved.

Of course, viability and momentum are related. For one thing, we know from primary campaigns in the US that smaller election victories can be seen as generating a kind of momentum that boosts perceptions of viability in bigger elections. For another, my research has shown that when a party’s vote share increases, even if it doesn’t move into first place, this momentum raises expectations of its chances of winning a future election.

These findings suggest something important. Even if voters are now flocking to the Greens because they see the party as viable, that does not mean the byelection victory alone is driving them to jump on the bandwagon. The party’s growth, separate from the victory that growth brought about, also probably matters for these perceptions of viability – and, therefore, helps explain why people are now jumping on the bandwagon in subsequent polling.

What also matters is the way the Green party’s victory is being covered. Voters do not learn about the election result in a vacuum; it is always presented with interpretation. And many of those interpretations are focusing on what the victory means for the Greens’ viability. This understandably superlative coverage of the byelection result contributes significantly to the perception that the party is becoming a viable electoral force.

Indeed, it has long been argued that numerical data representing parties’ performance alone is probably insufficient to produce a bandwagon effect, and that any such effect probably relies on the interpretation of what the results mean. If perceived viability drives people to vote Green, then it is largely the media’s insistence on that viability that is causing this movement.

Matthew Barnfield receives funding from the British Academy.