Four in five Labour members back Keir Starmer and believe he will win a majority at the next election, according to private polling that shows the transformation of the party’s grassroots. Two polls shared with the Guardian demonstrate how the composition of Labour’s membership has changed since Starmer was elected leader in April 2020. Since then his advisers have embarked on a mission to change the party, starting with the proscription of several far-left groups that had been supportive of Jeremy Corbyn. “There has absolutely been a deliberate strategy to change the membership,” one Labour official said. “The proscription of those groups was absolutely key because it sent a message that if you’re in any way affiliated with them, this is not the party for you.” Within the past four years, the grassroots has shrunk by about 150,000 and undergone considerable churn. Starmer became Labour leader after winning 56% of members’ votes in the April 2020 contest. His nearest rival, Rebecca Long-Bailey, got 28% of the vote and Lisa Nandy 16%. Since then, Labour Together – a group linked to Morgan McSweeney, arguably Starmer’s most influential aide and the party’s current campaign director – has been taking the temperature of party members. Two YouGov polls for the group, one conducted in April 2023 and the other in October 2023, demonstrate the change in the membership’s views and composition. In April 2023, 75% of those polled said they approved of Starmer’s leadership. In the most recent poll conducted in October 2023, this figure had increased to 81%. The growth in support for Starmer has come hand in hand with a belief that Labour will thrash the Conservatives in the election later this year. In April 2023, a few months into Rishi Sunak’s premiership, 56% of members polled said they thought Labour would win a majority. By October, that proportion had grown to 78% – a 22-point increase. Over the course of Starmer’s leadership, Labour’s membership has shrunk compared with its size under Corbyn. At the time of the 2020 leadership contest there were 552,835 members, according to official stats. Figures reported to the party’s ruling national executive committee (NEC), according to reports published by members Ann Black and Luke Akehurst, show Labour had 512,000 members at the start of 2021 – a fifth of whom had joined after Corbyn’s departure. This fell to 434,000 in 2022, 407,328 in 2023 and 390,000 in 2024. The Observer reported that Labour membership has since fallen to 366,604, according to figures presented to the NEC last week. In 2021, the NEC proscribed four far-left groups, banning their members from being part of the Labour party. In the spring of 2022, three more groups were proscribed. Labour insiders say this action led to the direct expulsion of a few hundred members, with thousands more leaving voluntarily as a result. At Labour conferences in autumn 2022 and 2023, the party introduced rule changes that strengthened Starmer’s control of the party. These included a ban on supporting independent candidates who stand against Labour in elections, which would prevent party members from campaigning for Corbyn or the North of Tyne mayor, Jamie Driscoll. The size of the party’s membership is still large compared with the bulk of the New Labour years. The number of paying supporters peaked at 405,000 in 1997 after Tony Blair’s landslide victory but by 2007 it had fallen to 176,891, thought to be its lowest level since Labour was founded. Josh Simons, the director of Labour Together, said: “Labour members can tell Starmer is relentlessly focused on delivering the next Labour government. They share his belief in the politics of power not of protest. “As they watch Sunak lose mayoral candidates and former party chiefs to fringe political parties, Labour members are rallying around Starmer to help him get to No 10.” Akehurst said: “The primary purpose of a political party is not to keep its members happy it’s to win elections so it can change the country.” Labour expects there will be a surge in new joiners this year as the election approaches. Mike Katz, chair of the Jewish Labour Movement, said: “This polling confirms what JLM members say – Keir Starmer has done a remarkable job in making Labour electable once again so quickly and completely. “Key to this has been rebuilding trust with the Jewish community. When we last surveyed them, nearly three-quarters of our members said Labour is a safe space for Jews under Keir’s leadership.” Guardian Newsroom: What is Labour’s plan? On Tuesday 16 April, 8pm–9.15pm BST, join Gaby Hinsliff, Tom Baldwin, Polly Toynbee and Kiran Stacey as they discuss the ideas and the events that might shape Labour in power.