Will David Miliband's 'lack of bottle' cost him the Labour leadership?

David Miliband condemned Gordon Brown's promise to ensure "British jobs for British workers" as the five contenders for the Labour Party leadership began to turn their fire on each other last night.

Mr Miliband, the current front-runner for Mr Brown's old job, said the pledge should be "a lesson" to Labour not "to get into a Dutch auction" over immigration. The former foreign secretary's criticism of Mr Brown's 2007 pledge came at the first hustings event.

But Mr Miliband's allies admitted his leadership campaign might have been damaged by his refusal before the election to join attempts to oust Mr Brown. They realise he will be repeatedly asked why he did not back the coups designed to install him as leader – an apparent lack of ruthlessness that some in the party fear could cost him dearly.

The most serious attempt to dethrone Mr Brown came last June, after the then pensions secretary, James Purnell, quit the Cabinet because he did not think that Labour under Mr Brown could win an election. However, Mr Miliband refused to follow his close ally.

At the time, it was thought Lord Mandelson had played a key role in talking Mr Miliband out of resigning. But Mr Miliband is expected to argue that he backed Mr Brown to keep the party united and because the Prime Minister was leading the world in the fight against the financial crisis.

Mr Miliband will be joined on the ballot paper by his brother Ed, Andy Burnham, Ed Balls and Diane Abbott. They will undertake a gruelling round of hustings over the summer. Ms Abbott was a surprise addition after a late flurry of support from David Miliband's allies helped her to secure enough nominations from MPs to go into the contest. Her presence ends the prospect of it being fought out by four Oxbridge-educated male former Cabinet ministers in their 40s.

The first debate in Westminster last night saw Mr Balls take on Ed Miliband, criticising his handling of Labour's election manifesto. He said Mr Miliband, who admitted to making "mistakes" in drawing up the document, failed to consult the party about its contents. However, Mr Balls was attacked by the other candidates for suggesting greater limits on immigration should be enforced.

David Miliband is regarded by the other candidates as the early pace-setter. While he was initially keen on a short leadership contest to ensure Labour was ready to take on a possible minority Tory government, the current stability of the coalition has his campaign team insisting a longer battle can work in his favour. In the race for nominations he was backed by 81 Labour MPs – 18 more than his brother Ed in second place.

David Miliband is still associated by many with Tony Blair, but will use the contest to reveal policies that he believes demonstrate social democratic instincts to the left of the former prime minster.

Ed Miliband has positioned himself to the left of his older brother. His allies believe he has a wider appeal to party activists and trade unionists, groups which each comprise one-third of Labour's electoral college, along with MPs and MEPs.

He will argue in a speech today that he is the best candidate to "turn the page from Blair and Brown". He says the party needs to shed its top-down leadership style and "become a campaigning force for social change – making a difference to people's lives at local level".

His appeal to the Labour left could be undermined, however, by Ms Abbott, who has 33 nominations and was given a major boost when her fellow left-winger, John McDonnell, dropped out of the race yesterday and threw his support behind her.

She was still about eight nominations short an hour before the deadline, but frenzied lobbying led to several Miliband sympathisers backing Ms Abbott in the interests of widening the field of candidates. The decisive 33rd nomination came from the former justice secretary, Jack Straw.

Ms Abbott, the MP for Hackney North and Stoke Newington, was as surprised by her success as some of her supporters. Last night, she had still to appoint a campaign team or design a strategy. One sympathiser said she would galvanise the contest, adding: "More people will come to hustings. Like her or loathe her, she is a celebrity."

Mr Balls's allies predicted the ex-schools secretary would do well among unions and party members.

Simon Carr's verdict: The contenders

David Miliband

He and his front-running brother have spawned the most nicknames. The Miliboys. The Milkiband Kids. DMil and EMil. He has a little rabbity tuft of hair going up in front. And if he was a woman you'd say it was a moustache on his upper lip. Intelligent, confident, fluent, popular, and high in the precedence of Labour. He's backed by Mandelson, and Blair is said to be active on his behalf. Full grasp of policy. But he has to win every trick. You can see him trying to find something funny to say. He's constructed, and work isn't complete.

Tory fear factor 5/10

Ed Miliband

Cuter than David, has younger hair and a more welcoming attitude. He is Gonk to his brother's Geek. Also, wider appeal (he says). Relates well to people (he says). Will bring people back to the party (he says). Proper leaders have other people saying this stuff for them. There is a major problem with his mouth. Depending on what he's saying it balloons on the right side. The left side is normal and then whoahhh! suddenly the right gets huge and almost circular. It looks shifty. The idea of either Miliboy managing the Labour Party by himself is a stretch.

Tory fear factor 4/10

Ed Balls

Passionately wants to win, passionately supported by the Tories to do so. Strange, bulging eyes prove that Myxomatosis can jump the species barrier. Has a monotonous thumping voice and wonderful capacity for loathing. He is the Manichean candidate (he's right, everyone else wrong). Michael Gove mocked him for outflanking the Tories on immigration and Europe – "something no one has done since Enoch Powell". Relishes power and plots. Dirigiste, had to claim he had access to secret source of knowledge. Serious though, homme serieux.

Tory fear factor 0/10

Diane Abbott

Rank outsider with Harriet Harman's backing. However, she has heart, humour and public profile. People have seen her on TV, so she's real. She can wallop. She's funny. Cameron would be least comfortable dealing with her. She may be a bit bonkers on the box but these things can be reined in when the candidate is groomed. She has three months to play herself in. If she became a contender she'd electrify the contest and Harriet's bequest to the party – the first woman, the first black person to lead it – would make history.

Tory fear factor 7/10

Andy Burnham

The mystery candidate. No one knows who he is. Minister of Local Communities, possibly, something like that? Tories defeat him in the noisy House but hate going on TV with him – because they always lose in the quiet and intimate battle. Pleasant, benevolent, well set-up in his background, nice eyes, nobody's first choice, everyone's second? Some voting systems couldn't stop him winning every time. As for popular appeal? His appearance at the Hillsborough anniversary can only be watched from behind the sofa.

Tory fear factor 3, maybe 4/10