When Britain’s former Prime Minister Keir Starmer stood down as the member of parliament for Holborn and St Pancras this week, he ended more than a decade representing a seat that has never elected anyone but a Labour MP since it was established in 1983.

The by-election it triggered which is expected later this year would ordinarily be a formality, but instead, it has become the most closely watched contest of the parliamentary term.

That is because Zack Polanski, the Green Party leader, the self-styled “eco-populist”, who has taken his party from the margins to double-digit polling in a year, announced on Thursday that he will seek nomination for the seat. He now needs to be chosen by local party members as the Green candidate for the seat.

Voting records offer him both encouragement and a warning.

Starmer won 48.9 per cent of the vote in 2024; the Greens finished third on 10.4 per cent, behind a pro-Palestinian independent, Andrew Feinstein.

But in the Camden council elections in May, the borough the constituency is in, the Greens won 28.8 percent across its wards to Labour’s 38.8, taking seats in Starmer’s own patch for the first time.

Al Jazeera spoke to political scientists and election analysts about whether Polanski has a realistic path to Westminster.

A Green win would be a ‘surprise’

Since Polanski became Green Party leader a year ago, it has achieved some success in taking support from Labour’s left.

That includes a surprise by-election victory in Greater Manchester, while the party is currently polling as high as 20 percent nationally.

Despite the Green Party’s recent results in Camden, experts say that Holborn and St Pancras is a strong Labour seat.

“A Green win would be a surprise,” Jonathan Tonge, professor of politics at the University of Liverpool, told Al Jazeera.

For that to change, he said, the current prime minister and Labour leader, Andy Burnham, would have to see his “honeymoon period” end quickly. A “disastrous budget” on October 28 could do just that, he added.

He also said the timing of the by-election is key.

“If Burnham has a good party conference in Liverpool (in late September), the honeymoon should continue until that budget at least; the later the by-election, the trickier for Labour. They will want a quick contest.”

Burnham came to power after nine years as mayor of Greater Manchester and arrived untainted by scandals.

To many Britons, it felt refreshing, after Starmer’s appointment of Peter Mandelson as ambassador to Washington – despite his ties to Jeffrey Epstein.

In an August YouGov survey, 46 percent of Britons had a favourable opinion of Burnham.

In contrast to Starmer’s lawyerly caution, Burnham brings a more down-to-earth, outsider style that seeks to cut through Westminster politics, an approach that Polanski has echoed.

Polanski’s history at play

Polanski’s critics have said he would look parachuted in, while he says he has lived and worked in the area for years.

Polanski told Camden New Journal, “I have lived and worked in this community over many years. I know it well, and I love it”.

“It represents so many of the things that make London so special. I am so excited by the prospect of representing this place in Parliament,” he is quoted as saying.

But Tonge said Polanski is seen “as too left-wing by some voters who prefer the safer Labour option”.

The so-called “Burnham bounce” Labour is revelling in may also hamper Polanski’s chances, he said.

The Green leader’s political history may also stand in his way, John McTernan, political secretary to former Prime Minister Tony Blair, told Al Jazeera, pointing out that Polanski first stood in the constituency in 2015 as a Liberal Democrat, winning 96 votes in a Camden council by-election.

He also had to apologise earlier this year over a council tax issue on his houseboat, with a Green Party spokesperson saying: “Zack apologises sincerely for the unintentional mistake”.

He later admitted he had not voted in May’s local election, saying he had been moving and “fell short of time” to register at his new address.

Greens may have ‘peaked’: Expert

The Greens have never been here before. From 2010 to 2024, they had just one MP.

That changed in February when the Greens won the Gorton and Denton by-election. Hannah Spencer, a local plumber and Green councillor, won 40.7 per cent of the vote in an area Labour had represented since 1931.

It marked the Greens’ first-ever parliamentary by-election win, their highest by-election vote share by a margin of 30 points. The win ushered in their first MP in the north of England.

In May, across London, they went from 18 council seats to 279, taking six boroughs and winning their first directly elected mayors in Hackney and Lewisham, while Labour recorded its worst London local result since 1968. Nationally, the party won 587 seats, its highest ever.

The Greens have momentum on their side but may have “peaked”, said Tonge.

McTernan said Polanski should be wary of conflating this year’s successes with evidence of his own popularity. He should instead see them as dissatisfaction with Starmer and a “message sent by progressive voters” that they didn’t want him.

The August YouGov survey also shows that just 19 percent of Britons now have a favourable opinion of Polanski, down seven points from his peak in April.

McTernan said putting Polanski forward may hamper the Greens as “by-elections often punish candidates perceived as focused on their own self-importance”.

He pointed to Clacton, where Nigel Farage, leader of the far-right, anti-immigration party Reform UK, resigned his seat in July amid scrutiny of his finances and then recontested it.

With the main parties boycotting that election, the satirical candidate Count Binface, a comedian who campaigns wearing a bin on his head, took 26.7 percent of the vote, the highest share ever won by a novelty candidate in a British parliamentary election.