If the Green party leader wins the Holborn and St Pancras by-election on 8 October, the result will not overturn the government’s Commons majority. But victory in a former prime minister’s seat could transform the political contest on the left, intensify pressure on Labour and add another disruptive force to an already fragmented opposition.
By-elections are often described as verdicts on the government. The contest in Holborn and St Pancras could become something more consequential: a test of which party is best placed to channel discontent with Labour from the progressive left.
The constituency was represented by former prime minister Keir Starmer and should, on paper, remain Labour territory. But the Green party is treating the election as an opportunity to demonstrate that Labour can no longer take support in metropolitan constituencies for granted.
Its leader, Zack Polanski, has personally entered the contest against Labour candidate Sagal Abdi-Wali. He is campaigning on housing costs, rent controls, inequality, climate policy, public ownership and resistance to further North Sea oil and gas development.
The Greens remain the underdogs. Yet if Polanski were to win on 8 October, the political effect would extend far beyond one central London constituency.
It would not simply be another Green gain. It would place the party’s national leader inside the House of Commons and give him a parliamentary platform from which to challenge the government directly.
A symbolic defeat in Labour’s political heartland
Holborn and St Pancras carries particular symbolic weight because it is the former seat of the prime minister whom Andy Burnham replaced.
Labour losing it so soon after a change of national leadership would be interpreted as a rejection of more than one candidate or local campaign. It would raise questions about Labour’s relationship with progressive, younger, urban and environmentally concerned voters.
The constituency also reflects several pressures facing people across London: expensive housing, high private rents, inequality, homelessness, strained public services and concern about the cost of living.
These are precisely the issues on which the Greens are attempting to outflank Labour from the left.
Polanski has presented the by-election as a choice between another Labour representative and an independent Green voice capable of putting pressure on the government. His party argues that a Green victory would force Labour to respond more urgently on housing, public services, taxation, climate action and inequality.
If that argument succeeds in a seat long regarded as safely Labour, MPs representing Bristol, Brighton, inner London, university towns and other progressive urban areas will start examining their own majorities more nervously.
The result could influence Labour’s political positioning well before the next general election.
The opposition would develop two distinct fronts
The formal position in Parliament would not change dramatically.
Labour currently has more than 400 MPs, while the Conservatives remain the Official Opposition with 118. The Liberal Democrats have 71 MPs, Reform UK has eight and the Greens have five, according to the current House of Commons party figures.
A Polanski victory would take the Greens to six MPs. That would not give them the parliamentary resources, debating time or procedural privileges available to the Conservatives or Liberal Democrats.
But political influence is not measured by seat numbers alone.
The government is already being challenged from two different directions.
On the right, Reform UK is competing with the Conservatives over immigration, national identity, taxation, public spending and the future of the state.
On the progressive left, the Greens are seeking to become the principal alternative for voters who believe Labour has become too cautious, managerial or willing to compromise.
A Green victory in Holborn and St Pancras would reinforce this two-front dynamic:
- Reform would continue to apply pressure from the populist right.
- The Conservatives would struggle to retain ownership of the traditional opposition role while also responding to Reform.
- The Greens would challenge Labour from the left on climate, housing, public ownership and social justice.
- The Liberal Democrats would face stronger competition for progressive and anti-government voters.
- Labour backbenchers would become more sensitive to electoral threats from both the Greens and independent candidates.
The opposition would become less hierarchical and more competitive. The Conservatives might remain the Official Opposition, but they would no longer necessarily dominate every political argument taking place outside the government.
Polanski would gain a national parliamentary platform
Leading a political party from outside the House of Commons places obvious limits on visibility.
Media attention at Westminster is still strongly shaped by parliamentary events: Prime Minister’s Questions, urgent questions, major statements, select committees, opposition debates and votes on legislation.
If elected, Polanski would be able to intervene directly in those proceedings.
He would not receive a regular question at Prime Minister’s Questions simply by becoming an MP. However, his position as party leader would make his speeches, interventions and questions more likely to attract attention than those of an ordinary backbencher.
He could respond to major political developments from inside Parliament, challenge ministers across the dispatch box, introduce amendments and use parliamentary procedure to generate campaigns and media coverage.
The Green parliamentary group would also acquire a clearer centre of gravity. Its MPs could coordinate parliamentary activity around their national leader and connect Commons interventions more directly to the party’s wider campaign strategy.
That would allow the Greens to combine three forms of pressure:
- Electoral pressure in Labour-held urban constituencies;
- Parliamentary pressure through questions, debates and legislation; and
- External campaigning pressure through members, councillors, activists and social media.
For a smaller party, this combination can carry influence disproportionate to its number of MPs.
Labour’s argument for tactical loyalty would weaken
Labour has traditionally been able to warn progressive voters that supporting smaller parties risks helping the Conservatives.
That argument becomes less persuasive in constituencies where the principal competition is between Labour and the Greens.
If Polanski wins, the Greens will present the result as evidence that voting for them is not merely symbolic. They will argue that they can defeat Labour directly in urban seats.
That could change voter behaviour.
People who broadly identify with the left but are disappointed with the government may become more willing to support the Greens if they believe the party can win. Donors, campaigners and potential candidates may also take the party more seriously.
Political momentum matters because elections are not only contests between policies. They are also contests over credibility.
A victory would allow the Greens to say: we are no longer simply asking Labour to listen—we can take its seats.
The Liberal Democrats would face a strategic problem
A Green gain would also complicate the position of the Liberal Democrats.
With 71 MPs, the Liberal Democrats possess far greater parliamentary representation, staffing and institutional experience than the Greens. They remain well placed in constituencies where opposition to the Conservatives provides the main route to victory.
However, the two parties increasingly compete for environmentally concerned, socially liberal and anti-Conservative voters.
The Liberal Democrats have often relied on local campaigning, constituency service and tactical voting. The Greens under Polanski are offering a more ideological and confrontational appeal, particularly on wealth, public ownership, climate action, housing and Labour’s record in government.
A Polanski victory could produce a clearer geographical division.
The Liberal Democrats may remain strongest in Conservative-facing suburban, rural and prosperous constituencies. The Greens could increasingly target Labour-facing cities, university areas and younger urban electorates.
There may be calls for cooperation where both parties risk dividing the anti-Conservative vote. Yet in Labour-held areas, competition between them could intensify.
Reform and the Conservatives would respond in different ways
A Green victory would not only affect parties on the left.
Reform UK would probably use it to support its argument that the traditional two-party system is breaking down. It could claim that voters on both the left and right are abandoning the established parties for alternatives offering sharper political choices.
The Conservatives, meanwhile, would face an awkward strategic calculation.
They might welcome a result that embarrasses Labour, but the growth of another insurgent party would further weaken the idea that British politics is naturally returning to a Conservative-versus-Labour contest.
The Conservatives could also use a Green victory to warn about a Labour–Green electoral threat, particularly on taxation, energy and environmental regulation.
This may help them mobilise some voters, but it risks reinforcing the Greens’ national relevance.
The Conservatives would therefore remain the largest opposition party in Parliament while confronting challenges to their authority from two sides: Reform competing for voters on the right and the Greens attracting attention as Labour’s most energetic opponent on the left.
The Budget would become an immediate test
Polling day is 8 October. The Autumn Budget follows on 28 October.
If Polanski enters Parliament, he will have less than three weeks before one of the government’s most important political and economic events.
The Budget would provide an immediate opportunity to test how effectively he can convert an electoral victory into parliamentary influence.
The Greens would be likely to scrutinise:
- taxes on wealth and high incomes;
- spending on local government and public services;
- housing and council-house investment;
- household energy costs;
- climate and infrastructure spending;
- public transport;
- welfare and poverty measures; and
- support for further oil and gas development.
Labour MPs in vulnerable urban constituencies would be watching closely. If the Budget disappoints progressive voters, a new Green MP could become a highly visible focal point for opposition.
The government would face a difficult balance. Moving towards Green demands might reduce the threat on its left but expose it to Conservative and Reform attacks over taxation, spending and economic credibility.
Rejecting those demands outright could strengthen the Greens’ case that Labour no longer represents progressive voters.
One victory would not guarantee a national breakthrough
A by-election provides unusual conditions.
Turnout is often lower than at a general election. Campaigners can be concentrated in one constituency. Protest voting becomes more attractive, and governments frequently perform worse than they do nationally.
Holborn and St Pancras also has a distinctive electorate. A Green victory there would not automatically mean that the same campaign could succeed in every Labour seat.
The party would still face major tests.
It would need to demonstrate credible positions on the economy, defence, foreign affairs, immigration, public-service delivery and the practical costs of its programme. Greater attention would bring more scrutiny of its candidates, internal organisation and policy detail.
Polanski would also have to balance national leadership with the responsibilities of representing a demanding inner-London constituency.
Winning a by-election is one achievement. Building a disciplined national parliamentary party is another.
The deepest impact could be psychological
The greatest consequence of a Polanski victory may not be the addition of a sixth Green MP. It may be the collapse of assumptions that have guided the main parties.
Labour could no longer assume that progressive voters will remain loyal because the alternatives cannot win.
The Liberal Democrats could no longer assume that they are the natural home for every socially liberal protest vote.
The Conservatives could no longer claim exclusive ownership of parliamentary opposition.
Reform could no longer present itself as the only insurgent party capable of disrupting the established system.
And the Greens would have to adjust to being judged not only as campaigners, but as a party with growing electoral and parliamentary power.
The result would be a more fragmented and competitive opposition: the Conservatives formally leading it, Reform attacking from the right, the Liberal Democrats relying on constituency strength and the Greens applying increasingly direct pressure from the left.
What should public affairs professionals watch?
For public affairs teams, a Polanski victory would require more than adding another MP to a stakeholder list.
Organisations should consider:
- whether the Greens are becoming competitive in constituencies important to their organisation;
- which Labour MPs may respond to pressure from Green challengers;
- how Green amendments and parliamentary interventions could affect legislation;
- whether climate, housing, inequality and public-ownership arguments are gaining wider political traction;
- how the government adapts its language and policy before and after the Budget;
- whether Liberal Democrat and Green positions begin to converge or compete more sharply; and
- how parliamentary campaigns can be amplified by local government, elected mayors and external activist networks.
The formal balance of power at Westminster would barely move. The political balance of pressure could shift considerably.
A Polanski victory would not make the Greens the Official Opposition. It could, however, make them a more influential opposition—one capable of shaping Labour’s calculations, redirecting public debate and forcing every other opposition party to reconsider where it stands in Britain’s increasingly fractured political system.