Labour's Angela Rayner has announced her bid to be the party's deputy, adding how she would back Rebecca Long-Bailey for leader if she runs.
Ms Rayner, who spoke about her working-class roots and lack of university education, told supporters in her home town of Stockport she would support her "friend" and flatmate Ms Long-Bailey - who is expected to run.
She said the deputy leadership election is a "chance to debate what went wrong, and that a core role of the next deputy leader will be to put it right".
"It is why I want the leadership of our party to be a team effort," she said. "I will be quite straightforward: I will be voting for my friend Rebecca Long-Bailey if she stands for the leadership."
Ms Rayner, setting out the high stakes for the party, she said Labour faced “the fight of our lives” over the next five years after the heavy defeat in the general election saw parts of its former heartlands turn to the Conservatives.
She added: "Our collective leadership must go far wider than simply who is elected to these positions. It is why I want us to have an honest, but friendly, conversation with each other.
"And at the end of it, a united party that starts winning elections for us all."
The announcement came as Labour's National Executive Committee (NEC) met to debate the rules for the forthcoming leadership election.
Issues set to be discussed include the cut-off date for the eligibility of new members to vote in the contest and the timetable for the election.
Ms Rayner, who joins fellow shadow cabinet member Richard Burgon in the race to be deputy, is likely now a favourite to succeed Tom Watson, who resigned from the role ahead of the general election.
But Mr Watson, who has not yet decided who to back, said he's concerned about the person Ms Rayner wants to lead the Labour Party.
"The one that I worry about - but I don't know what she stands for - I mean, when I look at Rebecca Long-Bailey, she's really the continuity candidate," he told Sky News on Monday morning.
"She sort of stands for Corbynism in its purest sense and that's perfectly legitimate but we have lost two elections with that play.
"But she hasn't said anything yet; as far as I know she has not formally announced and it might be that she chimes a different note in her opening bid and that she wants to take the party in a different direction and she's very candid about what went wrong."
If Ms Long-Bailey does join the leadership contest, she will face competition from Birmingham backbencher Jess Phillips, shadow Brexit secretary Sir Keir Starmer, Wigan MP Lisa Nandy, shadow treasury minister Clive Lewis and shadow foreign secretary Emily Thornberry.
Other Labour MPs have said they are considering a bid for the leadership, including party chairman Ian Lavery.
Mr Lavery, on his way to the NEC meeting, told ITV News Political Correspondent Paul Brand he is "going to wait and see what happens" before he decides whether to join the race.
"We’ll take everything as it goes," he added.
Sir Keir, the favourite to succeed Jeremy Corbyn, said "the next Labour leader needs to unite the Labour Party, provide really effective opposition to Boris Johnson and needs to be pulling together a strategy so that we can win in 2024".
Speaking on Sky’s Ridge On Sunday, he said: “What Jeremy Corbyn brought to the Labour Party in 2015 was the start of saying we should be anti-austerity and pro public services. That is right. We don’t want to throw that away.
“I’m not pretending we need to keep everything as it is, I’m not pretending there was anything good about that general election result, it was devastating, but we shouldn’t retreat from the radical… fundamental change is needed in this country and we must deliver it,” he said.
Ms Rayner highlighted seats that had gone from “heartlands to battlegrounds” and where people have felt Labour had “lost touch with them”.
Setting out what she believed was needed, Ms Rayner, who represents Ashton-under-Lyne, said: “I will be voting for my friend Rebecca Long-Bailey if she stands for the leadership.
“But our collective leadership must go far wider than simply who is elected to these positions. It is why I want us to have an honest, but friendly, conversation with each other.