One of the leading candidates to run Labour’s most generous union donor has warned Keir Starmer against any attempt to abandon the leftwing pledges he endorsed at the start of his leadership. With thousands of Unite members set to cast their votes this week to decide its next general secretary, Steve Turner, the leading left candidate, said that Starmer must “keep his promise to the members” by resisting demands to shift the party to the right in the wake of its stalling performance. Starmer has already been accused of abandoning leadership pledges designed to show he was retaining many of Jeremy Corbyn’s key policies, covering issues such as nationalisation and higher taxes. Turner said that while he was relieved about Labour’s win in the Batley and Spen byelection, the party should not “kid ourselves” about its performance under Starmer. “Labour needs to find its voice, to sharpen its attacks on this appalling government – to let people see a real opposition and a party that speaks for and to them,” he said. “Keir also has to keep his promise to the members. He stood on a programme that saw trade union members give him their vote. He can’t just bin this because [former New Labour architect] Peter Mandelson doesn’t like it. To do so is to insult those who put their trust in him, and that’s not a great place for a leader to be.” His warning comes before a crucial week in the contest to replace Len McCluskey. It represents a major fork in the road for the union, with huge implications for Starmer and Labour. Unite has been firmly in the hands of its left wing under McCluskey, who played a key role in fighting off challenges to Corbyn. Unite has also reduced its funding for Labour under Starmer. Turner said the union, Labour’s most generous donor, would not simply remain silent and “only appear when the cheque book is needed” under his leadership. “It is up to Keir now to show that he has got the message, that he understands that the party is in deep trouble in our heartlands and our trade union communities. Batley gives the party some space but there can be no complacency about scraping a victory with a handful of votes.” Turner’s main rival is seen as Gerard Coyne, who narrowly missed out on winning the Unite leadership in 2017 and has been a major critic of the union’s heavy involvement with Labour and its spending projects. He describes himself as a moderate who wants to reduce interference in Labour. He said he wanted to change an “aggressive culture” within the union that clamped down on dissent. He said that he wanted to open up the union in the wake of concern over the cost of building a hotel and conference centre in Birmingham. “If you have a culture of openness and you throw the shutters open and let the light in, then you don’t have to be defending yourself in an aggressive way,” he said. “When we were involved in high-profile legal suits that ended up costing the union in excess of £2m, where’s the accountability?” A Unite spokesperson insisted Coyne had supported the hotel project. Coyne said he was not dismissing Unite’s involvement in Labour politics, but said he was concerned about how far the “pendulum has swung”. He said: “It’s been characterised that my message about not being the backseat driver of the Labour party is absolving us of any involvement in politics whatsoever. I’m not suggesting in any way shape or form that we are completely walking off the field here. It’s about where your focus is, as general secretary. “We will have our concerns and I’ll be as vocal on those concerns as any general secretary will be. But whether you do that in a constructive way, or you do it shouting from the rooftops and criticising the party, are two different things.” Both candidates said their main priority would be to increase union membership and influence in the wake of the pandemic. A third candidate in the race, senior official Sharon Graham, surprised many with the level of endorsement she has received from local branches. She has claimed to be winning support from all wings of the union, but is also seen as a rival leftwing candidate to Turner and has been endorsed by the Socialist party.