Jeremy Corbyn's two main rivals for the Labour leadership are engaged in a bitter fight over who is best-placed to halt the left-winger's charge.
While the surprise frontrunner was addressing another packed public meeting, Yvette Cooper's campaign team called on Andy Burnham to "step back" in her favour.
They claimed polling showed she was now best placed to defeat Mr Corbyn - ahead of Mr Burnham in most parts of the country and attracting a significant majority of potentially-vital second preferences.
"If he isn't prepared to offer an alternative to Jeremy, he needs to step back and leave it to Yvette," a spokeswoman 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."
But 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.
"The stakes are desperately high with the future electability of the Labour Party now in mortal danger," shadow transport secretary Michael Dugher told the Guardian.
"History will not be kind to those who appear to cling on out of pride when the situation really is quite hopeless. 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."
The dominance of Mr Corbyn - who left a hall in Ealing, West London, to address crowds of supporters unable to get in to hear him - has left his mainstream rivals scrabbling for position.
As the 600,000-strong electorate started to cast their votes, Mr Burnham sought to reach out to Corbyn supporters by offering to "involve" him in his team and saying there was a "good deal of common ground" between them.
It drew a similar commitment from the Corbyn campaign.
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.
It came as the fourth candidate, Blairite Liz Kendall, was endorsed by former foreign secretary David Miliband.
The former MP - the latest in a string of party big beasts to speak out against Mr Corbyn - said the party needed "passionate reform not angry defiance".
Ms Kendall insisted neither of her rivals had asked her to drop out of the race to boost their chances of winning.