Leader of the Green Party Zack Polanski speaks during the Wales Green Party campaign launch at Jacob's Market on March 31, 2026 When Keir Starmer dramatically resigned as the MP for Holborn and St Pancras this week, all eyes quickly swivelled to Zack Polanski. He was elected as Green Party leader just a year ago but he has quickly made a name for himself. Polanski quadrupled the party’s membership numbers within just six months in the post, led the Greens to victory in the Gorton and Denton by-election and oversaw their best-ever performance in the local elections in May. A YouGov poll even put them at second place, on 21%, behind Nigel Farage’s Reform UK for a brief stint in March. But what goes up must come down, as the Greens soon found out. Polanski attracted intense scrutiny after it emerged he had failed to vote in the local elections and that he had not paid council tax while living on a narrowboat for three years – though he has since been cleared by an ethics inquiry over the latter. The London Assembly member was then slammed for questioning the police response to the Golders Green attack online and for sharing an Instagram post of a t-shirt featuring a picture of a guillotine and the words: “We’re only making plans for Nigel.” Some progressives began drifting back to Labour after Keir Starmer was replaced by Andy Burnham in July, too. The Greens are now down in fourth place on 13% according to YouGov’s latest poll, while Polanski himself has the lowest favourability ratings of all party leaders on 19%. So it’s worth asking: why is he putting himself forward for a by-election now? Polanski is still the most high-profile person within the left-wing party and it would a historic victory if the Greens took the seat, making it their first ever London constituency win. The Green leader has long said he would consider running for parliament if a London seat became available, too. He wasted no time signalling his desire to be selected by local Green members to be the party candidate for Starmer’s former constituency in the capital this week. While the decision is not yet set in stone, Polanski is widely expected to be the party’s choice for the crunch vote in a head-to-head with Labour. As Sky News elections analyst Robert Ford wrote on X, the stakes are high for both parties. He said: “Greens have chance to turn the tide on recent decline and show Polanski is a winner. Labour have a chance to show that the Burnham bounce is real, consolidate the left in a London heartland seat, and deal a potential knockout blow to Green insurgency.” He added that Polanski’s decision to join the contest meant it is “no longer just Green vs Lab”. Ford claimed voters are now being asked: “Would you like the national leader of the Green party or a generic Lab backbencher as your local representative?” Green MP Hannah Spencer with Polanski after she won the Gorton and Denton by-election in February It’s rather precarious, though, especially as the seat is traditionally Labour. The Greens themselves are trying to put their best foot forward, with one party official telling HuffPost UK they feel “confident” about Polanski’s chances. Another senior insider shrugged off the idea that it was risky to put the Green leader forward, insisting he would be a “brilliant candidate”. A third pointed out that the Greens won “lots of traditionally Labour seats” in London back in May during the local elections. If he is chosen for the seat, he may have also have a slight advantage when it comes to winning over the left-wing votes. Former independent candidate Andrew Feinstein announced he would not be standing in the by-election, and would instead back the Green leader. Feinstein secured 19% of the vote in the constituency back in 2024, coming in second place behind Starmer. If his 7,312 votes and the Greens’ 4,030 votes from the general election had been combined at the time, it would have significantly reduced Labour’s majority – though the party would still win by some margin on 18,884 votes. Still, as Luke Tryl, the UK director for More in Common, told HuffPost UK if Feinstein’s voters “switch en masse to the Green Party, Polanski will need a much smaller swing to win”. He said: “The seat’s demographics could help the Greens too: young, diverse, Remain-voting, with lower concern about migration than recent by-elections, it’s strong territory for Polanski, whose approval among 18-24s is almost nine times what it is among the over-75s. “We know that high-profile candidates – whether it be Burnham in Makerfield or Farage in Clacton – can motivate voters and boost turnout in by-elections.” This is why Holborn and St Pancras beyond Polanski running matters - because of what it means for consolidation of the left bloc. Since Burnham came to power Labour have consolidated their hold on the left bloc, at the same time as Reform’s share of the right bloc has slipped. pic.twitter.com/BDHL8YJV6m — Luke Tryl (@LukeTryl) September 3, 2026 Labour is yet to choose its own candidate for the crunch vote. HuffPost UK was told by one source that it is “a slam dunk” that Camden Council leader and Starmer ally, Sagal Abdi-Wali, will be Labour’s candidate. The local insider said: “It will be a straight Lab-Green fight, so Polanski has a chance. Sagal will be a strong candidate with cast-iron local credentials and a personal story that will resonate with huge chunks of the electorate. “Her being Muslim and Somali will limit Polanski’s ability to attract people angry about Gaza.” However, other party sources insist the candidacy is still up for grabs. Tryl told HuffPost UK this by-election “will be a fascinating test of what the electoral landscape looks like on the left of British politics”. He noted Burnham started to unite the left since replacing Starmer, he has “halved the share of 2024 Labour voters defecting to the Greens since the spring”. A separate YouGov tracker shows 50% of respondents think Burnham is a better PM than Polanski would be, compared to 8% who believe it is vice versa. Tryl added: “Polanski’s own popularity has tumbled, leaving him the least popular party leader in the country. “On the raw numbers, that makes Labour look like a safe seat: Holborn and St Pancras isn’t even a top-100 target for the Greens, who’d need a 19-point swing to take it outright.” But the pollster also observed that “support for Labour has eroded in the seat in recent years”. “For Labour, this is one of the first real tests of whether Burnham has really managed to unite the left behind Labour again,” Tryl said. “For the Greens, it’s a test of whether Polanski can regain his mojo after a difficult few months.” Listen to Commons People, the podcast that makes politics easy. Every week, Kevin Schofield and Kate Nicholson unpack the week’s biggest stories to keep you informed. Join us for straightforward analysis of what’s going on at Westminster. Related... Exclusive Interview With A Green Peer: Has Zack Polanski’s Party Blown It? 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