While younger brother Ed has made clear he would "definitely" serve under David, the shadow foreign secretary has so far been more evasive over the question of what he would do if he loses the race to succeed Gordon Brown on September 25.
His move appears to be an attempt to draw the sting from an escalating row between their respective supporters which threatened to recast the leadership contest as a divisive scrap between advocates of Old and New Labour.
As millions of ballot papers were sent out today, David insisted that the election campaign had been "a warm comradely contest, a fraternal contest and... a contest that I think will bring credit to the Labour Party and will certainly not come in the way of our family."
Asked if he would serve under his brother's leadership, he told Five News: "Of course. You run in elections hopeful of winning but always secure in the knowledge that you might not win and that's life, but I think it's very important that now is a time for people to take responsibility for the party and the country."
Lord Mandelson, who has made little secret of his preference for David, sparked the clashes yesterday with a warning that the party would be stuck in an "electoral cul-de-sac" if it opted for a "pre-New Labour" agenda.
But Lord Kinnock - a supporter of Ed - today hit back by accusing the former business secretary of being "sadly out-of-date" in suggesting there could be no deviation from the strategy which won Tony Blair three terms in office.
Kinnock even told the Western Mail that the former spin supremo "might be having a mid-life crisis", and defended the comment in a later interview on BBC Radio 4's World at One in which he said Mandelson had shown "a real lack of wisdom" in attempting to link Ed Miliband with a backward-looking Old Labour approach.
Many of the party activists, union members and MPs who are eligible to vote in Labour's electoral college are expected to fill in their postal ballots as soon as they arrive on their doormats, making the next few days a crucial time for candidates to get their message across.
But they risk being overshadowed by the launch tomorrow of Mr Blair's memoirs, even though the former Prime Minister is thought unlikely to offer his endorsement to any candidate.
Ed Miliband has warned that Labour cannot retreat into a New Labour "comfort zone" by assuming that the key to future success is sticking to the mantras of the Blair years.
But David today defended New Labour's record, telling The Guardian's Polly Toynbee in a filmed interview for the paper's website: "We did win three elections."
Labour voters in disadvantaged areas like his South Shields constituency were depending on the party to secure power, and did not want it to be led by "heroic failures", he said, adding: "We had great electoral success and we did fundamental changes to our country.
"Could we have done more? Yes. Did we make mistakes? Yes. Do we have to do better next time? Of course we should try to do so. But, my God, after 18 years of the Tories, I think you were cheering in 1997 as well."
David Miliband accused the coalition Government of being "economically inept" in pursuing a "masochism strategy" of trying to pay off the bulk of the UK's deficit within a single parliament, and said he would maintain a strong challenge to their approach if elected leader.
Rival Ed Balls today warned that the Miliband "soap opera" was in danger of drowning out debate over the party's future policy agenda.
And shadow health secretary Andy Burnham denied the contest was a two-horse race, telling the BBC he was "in a strong third position and gaining ground" on his rivals.
Speaking as he launched a call for £6 billion investment in 100,000 new homes today, Mr Balls said: "There's a temptation to get into personalities and the past and there's been a bit of a soap opera about one candidate or another. I've resisted that."
Mr Balls insisted it "does not matter a jot" who Mr Blair backs in the leadership election and said his former mentor Mr Brown was "doing the right thing" in steering clear of comment on the contest.
He highlighted his own differences with Blair over the European single currency, noting: "He wanted to join the Euro, I didn't and I think history has shown I made the right call."
But he added: "Leaders and former leaders write their memoirs and it's about the past... People want to know who is the leader of the future, who with credibility but also with vision can say there's a better alternative to these unfair coalition cuts - that's what I'm doing."
Mr Burnham, who today launched an assault on the Government's NHS reform plans, criticised senior Labour figures for portraying the contest as "a battle between old and new Labour".
The shadow health secretary said the party had to move on from what he called "the era of self-indulgent factionalism".
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