Enough Labour backers are still to make up their minds to swing the result of the party leadership race, Andy Burnham will insist as the campaign entered its final 10 days.
Left-winger Jeremy Corbyn remains the favourite to succeed Ed Miliband after being propelled from rank outside to frontrunner by a wave of support for a non-mainstream candidate.
The bulk of the 550,000-plus available votes are thought already to have been cast but Mr Burnham will tell a rally on Monday he is confident many of the undecided are switching back to his cause.
He will accuse "the Tory press" of falsely giving the impression the result is a foregone conclusion and claim he is the only one of the veteran's rivals to stand a chance of stopping him.
The other candidates are fellow shadow cabinet minister Yvette Cooper and Liz Kendall.
"Tens of thousands of people are still to vote and there is everything to play for," he will say - though his camp said they did not have any details of the number of votes cast.
"The future of the Labour Party hangs in the balance. I believe I am the only candidate in this race that can stand in the way of Jeremy Corbyn.
"Over this long weekend, I have spoken to Jeremy Corbyn supporters who are now moving to my campaign. I feel encouraged going into the final 10 days and will be fighting for every vote.
"Millions of vulnerable people, suffering under the Tories, are in need of a Labour government. Those left to vote must think who is best placed to win for Labour in 2020.
"The Tory press are so desperate for Jeremy Corbyn to win that they're making up stories to give the impression that he already has.
His push follows another hard-hitting intervention in the race from former Tony Blair, who renewed his warning that Mr Corbyn's "Alice in Wonderland" appeal would make the party unelectable.
Mr Blair conceded that his public opposition - and that of other senior figures such as Gordon Brown and Lord Kinnock - appeared only to be encouraging support for Mr Corbyn.
But he mocked those behind it for embracing a "politics of parallel reality" and failing to learn the lessons of the party's bruising internal struggles of the 1980s Mr Burnham said anyone who failed to listen to the ex-prime minister had "lost the plot".
The Corbyn camp said it now had more than 15,000 registered volunteers and had raised £180,000 in donations via the campaign website - with the average donation being just over £23. More than 30,000 had attended his rallies so far, they added.