The first votes in Labour’s leadership contest are due to be cast, with ballot papers expected to arrive by post on Monday morning.
Jeremy Corbyn is still regarded as a clear favourite, by both his opponents and bookmakers.
Fellow candidate Andy Burnham will later announce he wants to involve the left winger in rebuilding the party.
Former PM Gordon Brown has said Labour must be “credible” and “electable” in order to win back power.
Online ballots, which is the way union-affiliated supporters and newly registered voters for the ballot will vote, are to be emailed out on Monday.
The other candidates in the contest are Yvette Cooper and Liz Kendall.
Mr Burnham will say in a speech in Manchester that there are “two visions” for the party but only he could unite it and win power at the next general election.
“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 will say.
“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.”
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers were sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters – including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Mr Burnham will also promise to involve Mr Corbyn in “rebuilding our party” if he wins.
“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 Brown, speaking on Sunday, did not use Mr Corbyn’s name but made it clear he disagreed with many of his economic and social policies.
The ex-PM’s comments came as a poll suggested Mr Corbyn is considered the candidate most likely to worsen Labour’s prospects of winning the next election.
Meanwhile, Yvette Cooper will say that Parliament should set a deadline by which Sir John Chilcot would be required to publish his report on the Iraq War.
She will say the Labour government made mistakes, both in believing weapons of mass destruction existed in Iraq, and in its strategy after the conflict.