Allies of Jeremy Corbyn’s preferred candidate to run the Labour party are attempting to calm concerns that her arrival will see a “clearout” of staff deemed insufficiently supportive of the Labour leader.

Senior backers of Jennie Formby, the Unite union’s former political director and the frontrunner to become Labour’s new general secretary, are trying to reassure party staff that there are no planned overhauls should she secure the job.

It comes as senior party and union figures try to find a last-minute “compromise candidate” to take on Formby, with several sources warning she has had run-ins with some of the other major Labour-affiliated unions that have left them seriously concerned about her appointment.

Formby is on the verge of securing enough support from Labour’s national executive committee (NEC), which will vote later this month. The post gives the incumbent significant power in party disputes, selections and resources. While Formby is the clear frontrunner for the post made vacant by the sudden departure of Iain McNicol, the race burst into life earlier this month when Jon Lansman, the founder of the Momentum campaign group formed out of Corbyn’s first leadership bid, entered the race – against the wishes of Corbyn.

The contest has exposed a growing tension among Corbyn supporters, between those who want to preserve the power of the unions who helped deliver the leadership and those who prioritise turning Labour into a member-led movement. It has become seen as a power struggle between Unite and Momentum.

Formby is regarded as part of the inner circle of the leader’s office, which includes Unite leader Len McCluskey, shadow chancellor John McDonnell and Corbyn’s increasingly influential gatekeeper, Karie Murphy. Formby is being touted by supporters as Corbyn’s pick for the job, rather than a “Unite transplant” designed to give the union more power.

There are now attempts to ease fears among Labour staff about personnel shake-ups at Labour HQ, often regarded by the party’s left wing as insufficiently committed to Corbyn. Rumours have been swirling of a pot of money to help pay for redundancy packages should Formby win – a concern dismissed by her supporters.

“Those fears are misplaced,” said a source. “I don’t think there is any likelihood or, indeed, possibility of a clearout. The idea that Labour HQ is the centre of anti-Corbyn intrigue is wrong anyway. There is not the slightest requirement for a clearout, nor will there be one.If individuals want to leave, there is nothing to stop them, but there are people whipping up a fear that has no foundation at all.”

Formby’s fearsome reputation as Unite’s political director is causing concern within Labour HQ, with even some major supporters saying that she can be a “ruthless operator”. However, they say that such fears are outweighed by her organisational strengths and her strong links with the leader’s office. Other allies dismiss the fears, saying many who have worked with her remain fiercely loyal.

Lansman is said to have launched his bid to ensure a contest for the role. He has vowed to back turning the post into one elected by members – handing more power to the rank and file. He said he was running against “machine politics” and has urged other candidates to come forward.

The tensions between Momentum and Unite boiled over this week when Christine Shawcroft, an NEC member who backs Lansman, caused uproar by calling for the party to sever its trades union links. She later retracted her comments after being criticised by union leaders and Lansman.

A compromise candidate is still being sought to win support from the centre and right, as well as those on the left who are nervous of Formby. There is a belief that the next general secretary should be a woman. Liz Snape, a senior trade unionist, is one name being pushed, but there is no sign she wants the job. Laura Parker, a Momentum official, would also be a serious contender if she opted to run.

Formby remains the firm favourite, but insiders say Unite will “end up paying a heavy price” for its handling of the appointment.

The split between Lansman and Formby supporters has also spilled into an online battle, with tensions among a group of leftwing blogs and news sites that emerged to support Corbyn’s leadership. The Skwawkbox, which is seen as having strong links with the leader’s office, has been pushing for Formby’s appointment and has questioned Lansman’s decision to run, while Novara Media, another Corbyn-supporting outlet, has backed both the opening of the contest and a member-elected general secretary. The internal tension has also seen the arrival of the Red Roar, a more centrist blog that details the fights raging within the ranks.

Some moderate Labour MPs now believe the forces that brought Corbyn to power are dividing. The split has even been criticised by the Labour Party Marxists group, which said it was “at best, ludicrous and, at worst, irresponsible”.

Skwawkbox

Early in Corbyn’s leadership, party insiders believed the site was receiving detailed briefings from figures close to the Labour and Unite leaders. It would run lengthy articles against figures such as deputy leader Tom Watson and Gerard Coyne, the Unite official who ran against Len McCluskey in the union’s leadership race. In the general secretary fight, it has been making the case for Jennie Formby’s candidacy and has questioned Jon Lansman’s decision to run.

Novara Media

Set up by passionate Corbyn backer Aaron Bastani, the site has championed Momentum’s attempts to hand more power to members - with MPs usually losing out. While it has not backed calls to disaffiliate Labour from the unions, it has welcomed Lansman’s decision to run, and supports his idea that the general secretary post should eventually be fully elected.

The Red Roar

As the party’s left gains strength, another blog has popped up, written by the left’s critics. It opened its Twitter account with a story on how London mayor Sadiq Khan had been blocked from speaking at last year’s conference (a decision later reversed) and has also run unflattering stories on McDonnell and Unite. It has not taken a view on the general secretary race, though a centrist candidate has not yet emerged. Still has fewer than 3,000 Twitter followers.