Momentum’s future is “hanging in the balance” after the leftwing grassroots group’s co-chair resigned and quit Labour to campaign for the Green party and independent candidates. Hilary Schan said she had begun contemplating her role within Labour in October when councillors first expressed their frustrations over the leadership’s “unwillingness to show value to the humanity of Palestinian lives”. Schan took over from Andrew Scattergood and Gaya Sriskanthan with co-chair Kate Dove in 2022. Her departure from the group after two years to many Labour insiders marks “the beginning of the end of hardline stubborn leftwingers” within Labour, but also the start of a new coalition of leftwing voices outside Labour. “Momentum’s future looks pretty bleak without Hilary and will be hanging by a thread,” a leftwing Labour source said. “We should’ve campaigned for a Fabian-style membership model. The energy is not in the parliamentary sphere of the party. Momentum will be less combative to the leadership and have less of a public presence.” Schan said she had waited until the end of the local elections campaign as she was supporting the Worthing Labour council leader, Beccy Cooper, and did not want to disrupt efforts to get the public voting. “There’s no doubt there’s been a purge of left voices in Labour. They’ve felt the impact in Oldham, for example. Keir Starmer stood on a pledge of uniting the party, by doing this he’s alienating a large element of the party who are considering looking elsewhere ahead of the general election.” Schan is joining the We Deserve Better campaign, which she believes will help build an alternative by electing candidates who, along with socialist Labour MPs, can “pressure Starmer to finally listen to progressive voters he has taken for granted”. We Deserve Better, launched by the columnist Owen Jones, is seeking to mobilise the more than 200,000 people who have left Labour to campaign for socialist and pro-Palestine Green and independent candidates. Keir Starmer on Saturday said he was determined to win back the trust of those who had rejected his party in the local elections as a result of his stance on Gaza. “I have heard you. I have listened. And I am determined to meet your concerns and to gain your respect and trust again in the future,” he said. The party failed to regain control of Oxford after a string of prominent defections over its messaging on the Middle East crisis, and, in a similar blow, lost control of Oldham council in Greater Manchester to independents. Labour also lost council seats to independents in Blackburn with Darwen and Bradford, while the Workers’ party gained from it in Rochdale. In Manchester, the Labour deputy leader of the council, Luthfur Rahman, lost his seat to Shahbaz Sarwar of George Galloway’s Workers’ party. Momentum insiders believe at least 50 councillors linked to the group were elected during the local elections, and there remains a “significant minority” of leftwingers who are also Labour members “who can still get their voices heard” within the party. The Guardian understands Momentum has faced internal battles on how much the group publicly criticises the party, its policies and its disciplinary and complaints process. Voicing support for Jeremy Corbyn after he was blocked from standing as a Labour MP last year became a huge source of conflict but a number of leftwingers have expressed their shock at Momentum’s initial hesitance to express support for Diane Abbott and the length of her suspension, noting even Labour MPs who aren’t leftwingers have voiced their support for her. A Momentum spokesperson said they’re proud to have “led the resistance to Starmer’s purge on and crackdown on party democracy including standing with Diane and Jeremy from the very beginning”. “As Hilary leaves Momentum, we’d like to place on record our thanks for her leadership and dedicated service to our movement. “Momentum remains focused on organising for a democratic Labour party which views its members and core voters as an asset, not an inconvenience. We will keep campaigning for real Labour policies which deliver the country the transformative change it is crying out for, instead of constant U-turns and corporate-friendly policies.” The Labour party declined to comment. • This article was amended on 5 May and 8 May 2024. Hilary Schan took over as co-chair in 2022, not 2020; and she succeeded Andrew Scattergood and Gaya Sriskanthan, not Jon Lansman as an earlier version said.