Labour MPs have just two 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 five 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 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 75 declared nominations, at the last official count. He has been endorsed by former leader Ed Miliband who said he had “the best experience and vision to deliver 21st Century socialism for the country”.
Wigan MP Lisa Nandy is currently in second place with 28 nominations, ahead of shadow business secretary Rebecca Long Bailey on 26 and Birmingham Yardley MP Jess Phillips on 22.
The deputy leader post is also vacant after Tom Watson stood down from Parliament.
Angela Rayner, Ian Murray and Dawn Butler have secured enough nominations to get onto the ballot paper, but Richard Burgon and Rosena Allin-Khan are still short of the threshold.
As part of her pitch, Ms Phillips is calling for the creation of a universal childcare service, based on provision in some Scandinavian countries.
She told LBC the last Labour’s government offer of 15 hours free childcare a week had been a “lifeline” for her as a young mother at the time, but current level of provision was now “very limited and needed massive improvement”.
She said the state needed to think of affordable childcare as an economic catalyst for families, in the same way as transport and broadband connections, and not just focus on whether a women’s earnings made her eligible for support.
“Universal free childcare will not only provide thousands and thousands of jobs to groups of people who are desperately in need of good, secure income, it will also enable masses more people to properly go out to work and make their families’ lives better,” she said.
Asked who she would most like to serve under if she did not win, she chose Ms Nandy, arguing it was time for Labour to have its first female leader.
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.
Asked whether he could pick up another 17 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, who has 12 nominations, has said she is “fairly confident” of getting through to the next stage. The shadow foreign secretary told the BBC on Sunday her campaign had been a “slow starter” but she believed she could “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.
Meanwhile, Sir Keir has written to party officials calling for leadership hustings to take place in “each region and nation” of the UK.
He said the process of rebuilding trust in Labour must “happen in communities across the country, not just in those we currently represent”.