Jeremy Corbyn's two main rivals for the Labour leadership are engaged a bitter fight over who is best-placed to halt the left-winger's charge.
The competing camps of Andy Burnham and Yvette Cooper traded demands for each to clear the way for the other amid claims of sexism and desperation.
Surprise frontrunner Mr Corbyn, whose domination of the contest to succeed Ed Miliband has left mainstream competitors scrabbling for position, is unveiling his plans to renationalise the railways.
His call for a train network "run by the people for the people" comes after he insisted his policies could propel the party back to power in 2020, at the latest in a series of packed rallies.
The veteran MP faced opposition from the latest in a succession of Labour big beasts, former party leader Lord Kinnock warning "perpetual demonstration" would make the party unelectable.
But the fight to prevent him taking the top job descended into an angry war of words.
A spokeswoman for the Cooper camp openly urged Mr Burnham to "step back" in her favour, arguing the shadow home secretary stood the best chance of securing sufficient second preferences to win.
"If he isn't prepared to offer an alternative to Jeremy, he needs to step back and leave it to Yvette," she said.
"And he should do the right thing by the party and tell people who do still support him to put second preferences for Yvette - something he is still refusing to do."
Mr Burnham's campaign chief hit back with a suggestion the shadow home secretary was refusing to give way "out of pride" despite facing a "hopeless" situation.
"It's time now to rally behind the only person in this contest who can beat Jeremy Corbyn and that is very clearly Andy Burnham," Michael Dugher told The Guardian.
The two sides have adopted starkly different approaches, Mr Burnham reaching out to Corbyn supporters by stressing the "good deal of common ground" between them.
In contrast Ms Cooper has declared herself unwilling to work alongside the Islington North MP if he wins and has launched a series of attacks on his policy platform.
Her supporters gave a furious response to reports that someone in the Burnham camp had accused her of a "panicked, desperate stunt straight out of the Ed Balls playbook" - a reference to her husband and former cabinet colleague, who lost his Commons seat in May.
Feltham and Heston MP Seema Malhotra said: "Andy needs to stop his team resorting to sexist jibes. It doesn't help his campaign or the Labour Party."
Lord Kinnock told BBC2's Newsnight: "I can see why people are angry and want to protest.
"But then they have got to make a decision whether they want to be part of a Labour movement which produced a political party to seriously contest democratic power or they want to be in perpetual demonstration, which is fulfilling and noble but ultimately rarely effective."
It came after former foreign secretary David Miliband said the party needed "passionate reform not angry defiance" as he backed the fourth candidate, Blairite Liz Kendall.