“This is a battle for the soul of the nation,” Zack Polanski cried as he addressed a crowded room of Green Party campaigners in Gorton and Denton. “All eyes are on this by-election!”

The party leader is not wrong. While Keir Starmer’s authority over Labour is hanging on by a thread, the Greens and Reform are desperate to prove their sudden boom in support is not just a passing fad.

There’s a sense the Gorton and Denton by-election could be a turning point in British politics, especially if either of the up-and-coming parties – the Greens or Reform – manage to clinch the typically red constituency.

Pollsters believe there’s no clear winner yet, though bookies have slashed the odds for the Greens to win after £90,000 was wagered on the party’s candidate, Hannah Spencer, to win the crunch vote on February 26.

But, as tensions rise, there’s one clear issue which could be make or break for all of the candidates involved: the splitting of the left-wing vote.

While Labour is known for securing the centre-left ballots, the Greens’ growing popularity under Polanski means many disillusioned voters are flocking to their left-wing alternative.

Rob Ford, professor of political science at Manchester University, warned in a Substack post: “Both Labour leaning and Green leaning voters strongly prefer either party to Reform, and would very likely coalesce behind a left bloc front-runner if they knew for sure who that was. But they can’t because there isn’t one.”

He warned: “Both parties are therefore furiously posting leaflets into this information vacuum, but by doing so they only thicken the electoral fog of war that impedes their progress.”

Labour deputy leader Lucy Powell escalated tensions last week when she accused Polanski of trying to take support from her party to boost his profile nationally.

“I fear you are being played by Reform and have a different agenda,” she wrote in a scathing letter. “You know as well as I do, that the Green Party just doesn’t have the base or the breadth of support across the constituency to win the seat.”

She accused him of running a disingenuous campaign using misleading bar charts and misrepresenting political academics in their leaflets.

The Green Party leader said he had not replied, telling HuffPost UK: “I don’t think it’s worthy of a response, comparable to a “clear, desperate, scraping the barrel attack line”.

The Greens have also criticised Labour for using “bullshit” polls in their campaign.

When asked again if he had a response to Labour’s criticism, Polanski fired back: “I think the rebuttal is that from the moment the firing gun was started, this by-election is happening in the context of a Labour MP who made some deeply problematic comments.”

Andrew Gwynne was suspended from Labour a year ago after it emerged that he had made some offensive messages in a WhatsApp group.

He announced he was standing down in January, leading to widespread speculation about just who Labour would select as their candidate.

Polanski claimed Labour has taken people’s “votes for granted for years”, and alluded to the ongoing fallout around ex-Labour grandee Peter Mandelson’s ties to dead paedophile Jeffrey Epstein.

“They blew it before the contest even started,” Polanski alleged. “So it’s always been the Green Party versus Reform.

“If they think they’re in this race, then they clearly haven’t knocked on a single door.”

Labour sources deny this, insisting it was still all to play for and dismissing bookmakers’ predictions.

“It’s us versus Reform,” a party insider insisted.

Reform did not respond when repeatedly approached for comment about who they saw as their main rivals.

Polanski admitted in his Bold Politics podcast this week, that his “nightmare scenario” would be for Labour to “do disastrously” but to still take enough of the vote “so Reform get through”.

But, when asked if this means he is worried about the left-wing vote being split, Polanski told HuffPost: “The Labour Party couldn’t be any less a left-wing one than if they were trying not to be at the moment.

“In fact, I would consider them closer to Reform than they are to the Green Party,” referring to government efforts to reduce the welfare bill and its response to the Gaza war.

The London Assembly member – who decided not to run for the Manchester seat and save himself for constituency in the capital instead – went on to criticise Labour for not allowing regional mayor Andy Burnham to run for the seat.

Polanski added that he does not agree with the Greater Manchester mayor on “everything”.

However, he noted: “The fact that he’s apparently too left-wing or too progressive to even be their candidate in this constituency demonstrates how the Labour Party, under any measurable criteria, cannot be considered a left-wing vote.”

Might the Greens have been more open to a deal if Burnham was permitted to run as Labour’s candidate?

Polanski said definitely not, but added: “I do think it’s also true that the contest would have been friendly between the Green Party and the Labour Party had Andy Burnham run.”

A Labour campaign insider claimed this comment only proved it’s the Greens who have altered the tone of the contest with Labour, not the other way around.

Meanwhile, a Green activist suggested to HuffPost in passing that their party would not have had a chance at winning if Burnham had managed to thrown his hat into the ring.

Even so, it’s hard to get away from the speculation that the Greens are draining Labour’s support right now.

Polanski claimed one Labour parliamentarian had told him just the thought of him encouraged Labour figures to become more left-wing.

He said: “A Labour MP told me every time some of their colleagues think I’m going to run against them, they get a bit more left-wing and progressive.”

“Labour MPs keep worrying that I’m coming for them,” he added.

While the Greens have secured some Labour councillor defections, the party has not yet managed to persuade any serving MPs over to their side, despite their best efforts.

Polanski shrugged that concern off. He said: “Defections used to really be on my mind because I thought it was a way of increasing our poll rating, increasing our membership, and making those more on the national stage.

“But we’ve got that anyway without [defections].”

Meanwhile, Labour insiders firmly told HuffPost that they were confident their party still had a chance, even as the government in Westminster was in turmoil.

“Keir Starmer is only coming up a little on the doorstep,” a campaigner insisted, furiously downplaying any impact the chaos in Westminster – or Polanski – might have on their chances at retaining the seat.

Both the Greens and Labour have clearly singled out Reform as their main opponents.

But, with briefing rows like these, the biggest threat to both left-wing parties seems to be to one another – especially for this by-election.