Labour MPs have a few hours left to decide who to back to be their leader ahead of the close of nominations.
Four candidates, Sir Keir Starmer, Jess Phillips, Lisa Nandy and Rebecca Long Bailey, have already met the threshold needed to get on the ballot paper.
But Clive Lewis and Emily Thornberry are struggling to get the necessary support from 22 MPs and MEPs.
Mr Lewis told the BBC he had “faith” that enough colleagues would back him by the 14.30 GMT deadline.
As it stands, Mr Lewis has only four nominations, one of them from himself.
The Norwich South MP told Radio 4’s Today programme Labour had to “transform or die”, saying it was not enough for the party to “change leader, trim policy or have a sharper message” if it did not also overhaul its structures and work better with other progressive forces in British politics.
Jeremy Corbyn’s successor as Labour leader will be announced on 4 April. He is standing down after Labour lost its fourth general election in a row last month.
Shadow Brexit secretary Sir Keir Starmer currently leads the way with 68 nominations, well ahead of his nearest rival, shadow business secretary Mrs Long Bailey on 26.
Wigan MP Lisa Nandy has 24 nominations while Birmingham Yardley MP Jess Phillips has 22.
The deputy leader post is also vacant after Tom Watson stood down from Parliament.
Angela Rayner and Ian Murray have secured enough nominations to get onto the ballot paper, but Richard Burgon, Dawn Butler and Rosena Allin-Khan are still short of the threshold.
In a visit to a school later, Ms Phillips will call for a universal childcare service, based on provision in some Scandinavian countries.
In the Mirror, she said Labour must focus on a few key areas rather than offering a “shopping list” of pledges.
“I will prioritise childcare to support mums, dads, nans and grandads, in the knowledge that their kids and grandkids are in good hands,” she said. “In the years ahead I want people to look back and ask: how did we manage before National Universal Childcare?”
More than 50 Labour MPs have yet to state who they are backing, although Mr Corbyn has said he will not endorse anyone. All the candidates have so far nominated themselves, except Sir Keir and Ms Thornberry.
Asked whether he could pick up another 18 nominations in the short time left, Mr Lewis joked that there were “hours left in the day and empires have risen and fallen in that time”.
He added: “I have got faith in my colleagues. I understand it is difficult because I am talking about some things which are hard to hear.”
Ms Thornberry, the shadow foreign secretary, has said she is “fairly confident” of getting through to the next stage. She told the BBC on Sunday her campaign had been a “slow starter”, but added: “As long as I don’t get any slippage I’ll be fine, I’ll get across the line.”
Monday marks the close of the first stage of the nominations process.
Candidates also have to win the support of 5% of constituency Labour parties and three affiliate organisations, two of which must be trade unions, by 14 February. Unison, the UK’s largest union, has said it is supporting Sir Keir.