When Labour members voted at the party’s conference in Brighton last autumn to back their leader’s Brexit stance, one close ally exulted: “This is Jeremy’s party now!”

Faced with the choice of pressing for the remain position many passionately believed in and showing loyalty to Jeremy Corbyn, members opted for the latter.

Last month’s crushing election defeat, however, has reopened the question of where members will put their faith. Early polling, which gave Keir Starmer a clear lead this week, suggests they are not ready to back the most leftwing candidate in the field en masse.

Brexit is likely to be part of the explanation. The party, whose members are disproportionately in London and the south, is overwhelmingly pro-remain, even while many of its heartland constituencies – a swath of which were lost last month – voted to leave.

Starmer’s patient and deliberate efforts to push the leadership toward a remain position were welcomed by many grassroots members, though his critics will use the issue to raise questions about his political judgment.

Team Corbyn blame the shift toward backing a second referendum for the party’s electoral defeat and ridicule the fears of Starmer and his fellow leadership contender Emily Thornberry, expressed around the shadow cabinet table, that the Lib Dems could threaten them in their London seats.

This and the prospect of yet another male leader – “we could be on our second Keir and we haven’t had our first woman yet,” said one exasperated frontbencher - are likely to be used against Starmer in the weeks and months ahead.

Rival campaign teams also claim MPs and members scarred by the election result are clutching at the prospect of a Starmer leadership as a safe option, a “comfort blanket” as one candidate put it.

The other contenders, including the high-profile backbencher Jess Phillips and the Wigan MP, Lisa Nandy, will hope to shake up the race and fight back by pointing to the scale of Labour’s defeat and the need for a change of direction. This would be in contrast to Starmer’s insistence that he would not “oversteer”.

They face a scramble, however, to secure the necessary nominations from MPs and MEPs, and depending on the timetable the national executive committee (NEC) sets on Monday that race could be over in a week or less. Nandy and the anti-Brexit frontbencher Clive Lewis may struggle to convince the requisite 21 MPs, according to some colleagues.

The leftwing frontrunner, Rebecca Long-Bailey, appears to have made a faltering start despite being Corbyn’s heir apparent. The Labour leader spoke warmly about her when the pair appeared alongside each other at a string of general election events.

The Guardian understands he has told friends he would like her to succeed him as the torchbearer of the left, but even some of his longtime allies harbour niggling doubts about the shadow business secretary.

They feel she has not been tested enough, and wonder whether she might be tempted to tack toward the political centre ground at the behest of restive MPs. “A lot of us on the left are talking about this. She’s just never been under pressure,” said one frontbencher.

Some speak wistfully of Laura Pidcock, the outspoken young former North West Durham MP who was another Corbyn favourite, but who lost her seat to the Conservatives and so is out of the running.

The party chair Ian Lavery’s suggestion in recent days that he has not ruled out running for the leadership was read by many as a sign that key Corbyn lieutenants are not yet ready to throw their weight wholeheartedly behind Starmer, the shadow Brexit secretary.

Floated on the hyper-partisan blog Skwawkbox, Lavery’s candidacy is widely thought to be the brainchild of Corbyn’s former chief of staff Karie Murphy, who was a key figure in organising Labour’s election campaign and has close links with the Unite union.

Unite is expected to support Long-Bailey, though no formal decision has been made. Other major unions, wary of a stitch-up or of being accused of narrowing the choice for party members, may decide to opt for alternative candidates.

Union support will be crucial because of rule changes introduced since 2015 to balance the influence of MPs. Under the new and more complex system, candidates must not only win over 10% of their parliamentary colleagues, but also secure the support either of 5% of constituency parties, or of at least three trade unions or affiliated societies, which between them account for 5% or more of the affiliated membership.

Whatever the reservations of some Corbyn supporters, however, Long-Bailey remains the leftwing candidate with by far the best chance of getting on the ballot paper. As such, she is likely to have the weight of Momentum’s formidable campaigning machine behind her.

Other candidates will hope to attract thousands of new registered supporters as Corbyn did in 2015 to bolster their cause, provided the NEC, which has sweeping powers over the race, does not try to limit their ability to do so.

As the 2015 contest showed, Labour leadership campaigns, particularly when conducted in the aftermath of a jarring general election defeat, can be anything but predictable.