Andy Burnham will reach out to Jeremy Corbyn and his supporters in an appeal for party unity as the leadership election threatens to tear Labour apart.
The shadow health secretary will say it would be "unforgivable" if infighting after the result is announced on September 12 prevented Labour standing up to the Tories.
Mr Burnham has insisted he is the only candidate capable of halting front-runner Mr Corbyn's progress but promises to involve him in "rebuilding our party" if he wins.
The appeal, in a speech in Manchester, follows former prime minister Gordon Brown's passionate call for Labour not to become a "party of protest" instead of a credible electoral force.
Mr Brown said the party was "in turmoil" with polls suggesting "the one grouping in the party that is likely to get most votes is the one grouping that even its own supporters say is least likely to be able to form a government".
Ballot papers sent out by the party to the first of more than 600,000 voters will begin arriving this week and Mr Burnham will attempt to portray the contest as a straight battle between himself and Mr Corbyn.
'Two visions' for the party
Both Mr Burnham's camp and Yvette Cooper's supporters have claimed their candidate is best placed to stop veteran left-winger Mr Corbyn becoming the leader of the opposition, with polls indicating Liz Kendall will finish in fourth place.
In his speech at the People's History Museum, Mr Burnham will say that there are "two visions" for the party but only he could unite it and win power in 2020.
"The race has shown that the Labour Party is crying out for a big vision it can get behind. We can't carry on as we are and the good news is that this leadership election could bring real change to our party," he said.
"Two candidates have put forward their visions and the party now must decide which way it wants to go. I have put forward a plan that is both radical and credible, that can unite our party, that can speak to the country and lay the foundations for a Labour victory in 2020."
In an attempt to build bridges with Mr Corbyn's camp, Mr Burnham will say: "Jeremy has brought energy to this race. I want to capture that and involve Jeremy and his team in rebuilding our party from the bottom up.
"I want to show how I can take the best ideas of the other candidates, where there is common ground between us, and use them to shape my radical vision. In that way, we will come out of this contest as a strong and united team ready to take the fight to the Tories."
Mr Corbyn described the election as an "exciting and inspiring time" as he thanked his online supporters for their backing.