Labour’s leadership rivals will significantly increase their campaigning efforts this weekend as they get access to party data allowing them to call thousands of members in an attempt to secure their votes.

As the race to succeed Jeremy Corbyn enters its sixth week, candidates who already have their names on the ballot – Rebecca Long-Bailey, Keir Starmer and Lisa Nandy – are assembling an army of volunteers to help them reach the party’s grassroots.

The shadow foreign secretary, Emily Thornberry, was making a final push for nominations from constituency Labour parties on Friday to try to make it through to the final round.

Labour leadership hopefuls clash over key issues and policies Read more

Nandy won a major boost when members of the Labour affiliate Jewish Labour Movement gave her their backing after a hustings, saying she understood the need to change the party’s culture.

A source for Starmer’s campaign said: “Keir has been building a grassroots campaign with thousands of supporters across the country. Over the next few weeks the campaign will be taking the argument to the Labour movement about why another future is possible for our party and country.”

Labour’s central office will hand over names and phone numbers of members to all of the final-round candidates between midday and 5pm on Saturday.

Labour’s grassroots group Momentum has been using its own data to run phone banks on behalf of Long-Bailey. Despite social media being an area of expertise for the leftwing group, it is said to be anxious about rivals ploughing significant resources into online campaigning. A source said they were expecting Starmer and Nandy to “unleash big Facebook advertising operations”.

Nandy’s team includes the MPs Stella Creasy, Louise Haigh and Jonathan Ashworth, who will be helping to run phone contact sessions.

To make it on to the final ballot, the leadership hopefuls need nominations from either 5% of constituency Labour parties, or affiliated unions and socialist societies.

Starmer won the backing of five trade unions, including a last-minute endorsement from the TSSA transport union. He also has Unison, Usdaw, Community and the Musicians’ Union. Long-Bailey won the backing of Unite, with the general secretary, Len McCluskey, stating she had the “brains and the brilliance” to lead the party. She also won support from the CWU, FBU, Bakers Union and Aslef.

Nandy won backing from the GMB and the National Union of Mineworkers.

Thornberry was scrambling to get the final nominations by midnight on Friday to get through to the members’ ballot. She sent a last minute tweet saying: “Your [constituency Labour party] can make all the difference! Nominate me to get the widest range of voices, skills and experience on the ballot. Labour members deserve to have the widest and best possible choice when deciding our next leader.”

She was three CLPs short of going through just hours before the deadline. In what her team billed as a major intervention in the campaign, Long-Bailey set out her four-point plan to rebuild Labour in Salford on Friday morning.

She said she had been behind at the start of the leadership race because her focus had been on winning the general election.

Play Video Rebecca Long-Bailey: 'I'm ready to take up the mantle of socialist leadership' – video

Long-Bailey only had 154 CLP nominations to Starmer’s 346 at the start of her speech, despite being the most leftwing of the candidates and having the backing of senior figures such as the shadow chancellor, John McDonnell.

Speaking at the Lowry Centre in her constituency she said: “[The leadership] is a long campaign and I’ll be honest, I didn’t have a campaign ready to go as soon as we lost the election because I spent my time trying to win the general election. So I was behind the other candidates, but we can see that movement growing.”

The venue, on the docks where her father once worked, was only two-thirds full by the end of her speech, with rows of empty chairs at the back of the room.

Local supporters said the turnout reflected a Friday morning: most of Long-Bailey’s supporters were workers who did not have the luxury of being able to take time off to attend campaign events.

She said her plan to rebuild Labour should be based on aspirational socialism, empowering members, a democratic revolution that boosted trade union membership, scrapping the House of Lords, and her own flagship policy, a green industrial revolution.

She thanked Corbyn, McDonnell and the shadow home secretary, Diane Abbott, for doing “their bit” but said it was now right for a new generation to take the movement forward and she was ready to take up the mantle of socialist leadership.