The trade unions are racing to sign up "affiliated members" to take part in Labour's leadership election, with some officials predicting that union members could cast more than half of the votes in the contest.
David Miliband, former foreign secretary, ruled himself out of the battle on Monday, saying he had made a commitment to working for New York-based charity International Rescue Committee.
He criticised his brother Ed, who beat him to the leadership in 2010 - prompting his self-imposed exile abroad - for allowing the perception that Labour was moving backwards and did not understand "aspiration".
The Blairite former MP said he hoped his brother's successor would "speak to millions of people around the country" with a more centrist "dynamic progressive politics".
With spirits already at a low ebb after last week's devastating election result, Labour sought to rally morale, announcing that it had signed up 20,000 new members since the election. That takes the total to 221,247 - echoing an increase in Lib Dem membership.
Under Labour's new election process, the result of last year's "Collins Review", those members will each have the same voting rights as "affiliated supporters" from the trade unions.
The unions are rapidly trying to sign up affiliated supporters ahead of the contest in the summer or autumn.
One official estimated that 400,000 people might vote in the leadership contest. Of these 250,000 could be union members - roughly in line with 2010 - he predicted.
In previous elections union members were limited to only a third of the electoral college, with another third for MPs and a final third for members.
By introducing "one member one vote" Ed Miliband, the former leader, may have handed more power to the union grassroots - contrary to his rhetoric about reducing their power.
Unite said the party had only just started signing up affiliated members but the process had been "very positive" so far. Unison said it would encourage members to sign up as fast as possible.
One difference is that the unions will no longer send out the ballot papers: in 2010 they stuffed those envelopes with pro-Miliband leaflets.
However, they would still be able to use their money and organising muscle to help their favoured candidate: widely expected to be Andy Burnham, shadow health secretary.
The timing of the contest - to be decided at a meeting of the national executive committee on Wednesday - could prove crucial.
Liz Kendall, shadow social care minister, is the only Labour MP to have publicly declared that she will run for the leadership.
Chuka Umunna, shadow business secretary, and Yvette Cooper, shadow home secretary, are also expected to enter the race.
The Financial Times revealed on Saturday that the separate contest for the deputy leadership is likely to be between Caroline Flint, shadow energy secretary, and Tom Watson, a prominent backbencher.
The battle between the Blairite Ms Flint and Mr Watson, who is close to the unions, will be seen as a proxy battle for the soul of the party - rekindling memories of the 1981 race between Denis Healey and Tony Benn.
Harriet Harman, acting leader, appointed Chris Leslie as shadow chancellor and Hilary Benn as shadow foreign secretary on Monday in what are likely to be caretaker roles ahead of the new leader's selection.
Meanwhile Lord Sugar, one of the party's few high-profile business supporters, said he was quitting his membership.
The peer said he had joined "New Labour" in 1997 but had more recently sensed a policy shift back towards what "Old Labour" stood for. Lord Sugar said he had waited until the result of the election before resigning because: "I have no wish to stick the boot into the party."
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