Rebecca Long-Bailey is widely thought to be the frontrunner in Labour’s leadership election. The contest may not have officially started yet, and the MP for Salford and Eccles has barely put a word on the record since the election, but she is understood to have the backing of those currently in charge of the party. Because Labour members are largely on the left of the party and make up most of those who get to vote for the next leader, it is often assumed that this race will be a walk in the park for Long-Bailey. But the reality is more complicated – and there is a significant risk that the Labour left could lose control of the leadership.

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There are a number of factors that will benefit Long-Bailey. She is described as the “continuity Corbyn” candidate, which frustrates those allies who think she would be her own person as leader. But it also means she doesn’t have to worry about pitching carefully to the left in the way that others must do. Keir Starmer, who is expected to formally declare his bid, needs to convince members above all that he does not represent a reincarnation of New Labour. Emily Thornberry and Lisa Nandy will also have to prove that they are not invested in factional politics that could drive away Corbyn supporters.

Long-Bailey has apparently struck a deal with Angela Rayner, the shadow education secretary, which will see the former run for leader and the latter for deputy. Rayner is known for her powerful backstory and barnstorming speeches, and she would have been well placed to split the Corbynite first preference vote in the main contest, despite being seen as soft left. The Greater Manchester duo now make for a highly promising joint ticket, with both politicians admired particularly in terms of policymaking. Both Long-Bailey’s green industrial revolution and Rayner’s national education service are popular among members.

But this contest is no foregone conclusion. LabourList conducted a self-selecting survey of our members after the election, and the results showed that almost identical proportions of our readers who answered the survey favoured Long-Bailey and Starmer. Although the poll is not a representative survey of Labour members, similar LabourList surveys in the past predicted Corbyn-mania in 2015, and Sadiq Khan’s selection as Labour’s candidate for London mayor in the same year. It would suggest that there is all to play for, particularly when those with lower profiles become better-known.

One of those people is Clive Lewis, who has formally declared his intention to stand for the leadership. His pitch has prioritised electoral reform, party democratisation and an end to political triangulation, particularly on immigration. All signs point to him trying to outflank other candidates from the left.

But Lewis, and other potential contenders such as Nandy, face a procedural problem. Members and registered supporters will only get to choose between candidates who get nominated by sufficient numbers of MPs and MEPs. Only a limited number of candidates can secure the minimum requirement of nominations from 10% of the parliamentary party plus either 5% of local parties or 5% of affiliated members, including two trade unions. Although technically a total of nine candidates could get on the ballot paper, in practice it’s likely to be fewer: MPs are unlikely to be lending nominations to “broaden the debate” as some did in 2015.

This explains why Thornberry, who would likely be competing for nominations from the same pool of MPs as Starmer, has set out pledges that tap into the prime concerns of her colleagues. MPs are interested in the staffing of the leader’s office, the internal culture of the party and whether a successor would step down if their poll ratings were dismal – all of which she has touched on. The nominations hurdle also explains why those with less chance of getting the 22 MPs and MEPs they need to nominate them have declared sooner: they need more time to build support, whereas Long-Bailey can already be confident of acquiring nominations.

Starmer has perceived downsides: Labour has never yet chosen a woman to lead the party, some believe Labour needs a leader from beyond north London, and the architect of Labour’s Brexit position could be vulnerable on that issue following Labour’s losses in leave-voting areas. But he could win this race. The membership is based mainly in big cities, and the grassroots pushed for the party to endorse a fresh referendum. It also helps that video clips of Starmer being interviewed about his pro bono support in the McLibel case, as a young barrister with a less polished accent, were widely shared on social media. Labour members will be looking for candidates with a history of backing causes important to the labour movement. And Starmer is popular among MPs.

Long-Bailey is expected to be gaining at least the endorsements of Unite and Momentum, which can both offer huge financial and organisational support. Every other candidate will need to spend time convincing MPs, unions and members that they don’t want to tear up the Corbyn project. Someone like Jess Phillips would need a serious membership drive to get anywhere. But no single candidate can be assured of winning yet – Labour’s leadership race remains wide open.

• Sienna Rodgers is editor of LabourList