Momentum is considering opening up its membership to people outside the Labour party in an attempt to energise a broader leftwing movement in the lead-up to the general election. Insiders say a shift in the group’s membership model will make it a “powerful force” and will ensure its financial sustainability and relevance if Keir Starmer becomes prime minister. Momentum currently requires supporters to be members of Labour – a rule that is having a knock-on effect on its funding as thousands of leftwingers have deserted the party under Starmer. If the motion is passed Momentum would hold the same membership criteria as the leftwing thinktank the Fabian Society, which is still deemed closely associated with the Labour party and has influenced Labour policy. While there is internal support for the motion, some have voiced concern that unions may veto the vote. The Guardian understands the Communication Workers Union which is affiliated to Momentum, has more than 100,000 members and as a result will get twice the weighted delegation size within the grassroots group. James Schneider, a co-founder of Momentum, said: “We want organisational inheritance from Corbyn that is able to be a force for our politics in British society. It needs to be open to the vast majority of people who could play a major part in that and that means people that are Labour members or people that are not Labour members.” A leftwing Labour source said the membership rule change may embolden MPs who are “still fearful of being sidelined” by the current leadership. Schneider said the membership change could create a “positive knock-on effect” within the Labour left. “Across the broader left there’s energy and activity. The Labour left is in a defensive crouch. There can be purpose in strategic retreat. But if the defensive crouch is stopping you from doing anything great it’s illustrative of some of the unnecessary cowed fear that we need to break out of on the Labour left … you should argue for your politics and then argue why they’re legitimate when the leadership tries to come for you.” The timing of the motion and Momentum’s convention comes as a number of independent leftwing candidates are preparing their general election campaigns, some directly challenging Starmer’s Labour party. A prominent figure on Labour’s left hinted there will be “quite a lot of excitement” about supporting specific seats where there is a “credible left of Labour challenge”. In some areas, the source said: “There will be a lot of leftwing independent candidates of various stripes possibly in the same seat, which could be a bit of a distraction.” A Labour membership rule change passed at last year’s conference means Labour members will be automatically expelled from the party if they campaign for independent candidates. Hilary Schan, a Momentum co-chair, said: “This weekend’s convention will be a testament to the enduring strength of our grassroots movement. Amidst the rubble of the Tories’ broken Britain, the need for bold and transformative solutions has never been clearer. “As a member-led force, we will keep campaigning for a fairer, more equal Britain for months and years to come, powered by thousands of ordinary people up and down the country.”