Zack Polanski is having fun. But can the Green leader become a serious player?

"I'm having the best time ever," Zack Polanski tells me.

It's not hard to see why the Green Party leader for England and Wales is enjoying himself, a lot.

The size of the party has ballooned to more than 100,000 members. It's within touching distance of Labour in some polls, even ahead in one survey by pollster Find Out Now. And the Gorton and Denton by-election in Greater Manchester to replace former Labour MP Andrew Gwynne has landed the Greens a prize opportunity to show what they can do.

"This will be the most throwing-the-kitchen-sink-at-it by-election that we have ever done," says one Green MP, boasting of 30,000 leaflets being delivered already, and hundreds of activists descending on the constituency this weekend. Why do the Greens seem to be on a roll?

A new party boss with a new style has certainly made a big difference. Colleagues speak warmly of the self-described eco-populist's "huge energy and passion" and his "immediacy and authenticity". Another says the public "don't see how kind a soul he is… he just really cares".

One staffer highlights his mastery of a skill that's always been a huge plus for politicians: remembering people's names and stories, whether he's having a cup of tea one on one (always with oat milk, if available, I'm told), throwing questionable shapes on the dancefloor at a nightclub fundraiser, or stopping to pat every dog when he's out on the doorsteps.

He and his team have supercharged his ability to connect too, whether that's with grabby videos on social media or chats on his own podcast. He also has a very healthy appetite for a huge range of interviews - for example, with us again this Sunday - and pops up in places like the comedy show The Last Leg or in music magazine NME.

Even some of his backers, though, point out the surge is not all down to him. As ever, in politics, timing can weave its own magic spell. Several Green sources acknowledge public disillusionment with Labour has been a gift for the party, with one saying it's "created the conditions for people to be incredibly disappointed". They liken the growing movement to the buzz around former Labour leader Jeremy Corbyn - a "huge progressive moment that loves to coalesce around something exciting" - but say that excitement "wasn't really about him".

So perhaps, it's not all about Zack. Jeremy Corbyn's own new vehicle, Your Party, spluttered rather than surged into life with his co-founder Zarah Sultana telling us that the setup process had been "really difficult".

One Green MP says Polanski was "fortunate that an opportunity presented itself just as he took over", thanks to Corbyn and Sultana's party "raising hopes, then dashing them", leaving many "looking for a new home, just after they thought they'd found it".

And Reform's rise has created an appetite too. On the left, there is rising anxiety about Nigel Farage's success. Whether in the Gorton by-election or beyond, one Green MP tells me their equation for winning votes is getting voters who feel "A) I hate Labour; B) I want to stop Reform - so that has to equal the Greens".

He might be lucky with the timing, but there is no doubt Polanski is playing a good game with the hand he's been dealt. One of the party's MPs enthuses: "We are a happy party!" - something you don't hear that much these days.

But you can hear a hint of nerves around the edges about Polanski's more vigorous, edgy style. In recent years, the Greens have managed to appeal to metropolitan voters, with progressive ideals, or - to use a term Polanski is happy to embrace - "woke" values. At the same time, they have found support in rural constituencies that might be more socially conservative but have concerns about housing developments, planning or the state of the countryside.

Does Polanski's bolder style work both ways? One party source tells me existing Green voters "need reassurance we have the same values …they want to see a grown-up approach".

One of the party's handful of MPs acknowledges some eyebrows have been raised, saying they have had a "tiny, tiny, tiny handful of emails saying I'm a bit concerned about your leader – but he is a huge overall positive – we might lose 1% from a previous demographic that might find Zack a little bit much, but I'm intensely relaxed".

Polanski might be new in post, but he's not new to the Greens, having been a member since 2017 and represented the party in the London Assembly. If you have a nerd-like political brain, you might remember he was a Liberal Democrat before that. A big backer of the coalition government, who even belted out Marvin Gaye on the Lib Dem conference stage and attempted to stand in the leafy south-west London constituency of Richmond Park.

His exit from the party didn't leave behind the best of memories for some colleagues. One Lib Dem staffer who dealt with Polanski told me: "He was fearfully ambitious and was after the best way to get elected – he appeared out of nowhere and tried to get really in with the hierarchy – but then he rubbed everybody up the wrong way - it was all about him not the team."

Polanski himself wrote about his disappointment when he was not selected to run in Richmond Park, saying he was told "a local candidate is a trump card for many parties". But the Lib Dem source remembers him as "incredibly angry and counterproductive" after he was not allowed to stand. Another senior Lib Dem figure, who saw him regularly when he was in the party, wonders whether Polanski is driven by principle or ambition, telling me he was "a strident supporter of coalition and now he has ended up as quite a left-wing party leader – my impression is that has been driven by what suited every circumstance".

Politics is not a kind business. Perhaps it's not surprising that former colleagues in a different party aren't particularly enthusiastic about the departed Polanski. But some Green Party sources, while delighted with the party's growth, do slightly hesitate to join in what's felt like a breathless chorus of praise.

One suggests those hailing Polanski as representing a radical new direction are getting it wrong and that, fundamentally, the party has the same plans, even though its new leader is communicating them in a fresher, more vigorous way. They tell me: "It's frustrating… the difference is strategy and style – he is not dramatically rewriting the policy book." Another source wonders if Polanski has shown yet that he can concentrate on the perhaps boring (but vital) bits of taking a political party from the fringes to becoming a truly serious player.

With politics in flux, the opportunity for the Greens could be enormous. But the source cautions that, to have a striking influence at national level and perhaps even a say in a potential hung parliament, "we would have to be very different in terms of how we operate, and I don't yet know if he is willing to do the leadership needed to build us up in that wider sense".

Creating the infrastructure to mount meaningful national campaigns means recruitment, fundraising, planning. Running big political parties means making big decisions and running big organisations.

For all the excitement that's running high in the Green ranks, their national polling success isn't necessarily being replicated in the council by-elections that take place all the time. Wild enthusiasm about their prospects in Gorton and Denton are scoffed at by Labour, whose insiders note the Greens don't have any councillors in the constituency. Quite deliberately, Labour has chosen a short campaign that will make it harder for the insurgent parties, whether that be the Greens or Reform, to get going.

The Greens have certainly been quick off the mark, sending in hundreds of activists. And there's no question they've been bolstered by the enthusiasm of their new ranks of members.

Reform has been working the seat since the last three months of 2025 and, ahead in the national polls, seems better placed to grab the seat from Labour. But just as the Greens and Reform are chucking everything at that contest, Labour also knows how much it matters. And not just for the sake of Sir Keir Starmer's leadership but, as one senior figure puts it, "it's a test of the risk we face – we have to show we are the leaders of the anti-Reform majority".

So stand by for the Greens' detractors to try to poke holes in their record - whether that's on their proposals to legalise drugs or to welcome immigration. They may also take aim at Polanski's belief that the UK should leave Nato and work more closely with its European allies (though this is not official party policy) or at the Greens' 2024 motion designating Israel an "apartheid" state. Only recently, the prime minister himself accused Polanski's party of being "high on drugs and soft on Putin". Spin back a while and it's hard to imagine the prime minister would have spent any time attacking the Greens at all.

That, in a sense, is the ultimate symbol of what's going on. That No 10 feels the need to criticise the Greens is a backhanded compliment that shows they matter. Under Zack Polanski, the party is making its presence felt in a way it simply hasn't before.

That does not mean the party's on course to win a by-election. That doesn't mean the Greens' ambition to become a decisive force in politics will come to pass. But Zack Polanski is having fun, and that matters in the here and now.

BBC InDepth is the home on the website and app for the best analysis, with fresh perspectives that challenge assumptions and deep reporting on the biggest issues of the day. Emma Barnett and John Simpson bring their pick of the most thought-provoking deep reads and analysis, every Saturday.Sign up for the newsletter here