Party leadership races are generally much less interesting than pundits tend to hope. In the current contest, everyone always knew David Miliband would run – and everyone knows by now that three of his challengers are very unlikely to beat him. Thus it fell to Ed Miliband to provide the excitement.

The young pretender is certainly a sexier part to play than the dauphin, but when I heard Ed Miliband speak back in late May, he seemed so manifestly awkward about fighting his older brother that it was hard to see how the contest was going to work. Remarkably, the pair's pledge to say nothing bad about one another has been maintained, disappointing Fleet Street's hopes of a family catfight. But the pledge has also created a vacuum – and in the coded world of Westminster, this has gradually been filled by a subtle but damning Blairite critique of the younger brother.

"Ed Miliband is a really nice guy," Alastair Campbell has said, "but I think you've got to differentiate between making the party feel OK about losing, and making the party face up to what it needs to do to get into shape again." The former climate change secretary isn't "up to taking difficult decisions". Just last week, the former City minister, Lord Myners, accused him of tacking too far to the left on economic policy, and the columnist Matthew d'Ancona has declared, "Ed Miliband will never be prime minister. You know it, I know it. So why would Labour choose not to know it? Because, in this case, ignorance is so much easier than undertaking the sweaty, mostly thankless task of plotting a route back to power."

When we meet at an aerospace factory in Burnley, I'm curious to see which version will appear – the bold young pretender, or the endearingly meek old Labour loser. It takes less than a minute to find out. I've met Miliband several times before, and can safely say I have never known him to sound hungrier, or angrier, or more radically combative. When I mention the whisper that he's really a Bennite, he goes off like a shotgun.

"That is such nonsense! It's pathetic, genuinely, I think it's pathetic. If we think that the way we should conduct political debate is by caricaturing people we disagree with as Bennites, I think it is an absolutely hopeless way to conduct a political debate. Are we really getting to a stage where if an aspirant leader of the Labour party has policies of the centre left, we then say they're a Bennite? Or we say then where are your rightwing policies? That is the politics of triangulation. And that is not the politics I believe in."

Miliband's politics are quintessentially centre left, and not spectacularly different from those of his brother. "I'm confronting the issue of the kind of capitalism we have in this country, which means people get stuck in low-paid work." He is campaigning for a "living wage" of £7.60 an hour and would look at making the 50% top tax rate and the tax on bankers' bonuses permanent, but stresses: "I do not want a return to penal rates of taxation." He proposes a high-pay commission to scrutinise "corporate governance – the cosy cartels which award each other high pay", but also wants Labour to be the "party of small business and the self-employed" by looking at "the way the banking system works". On the Iraq war, some of his rivals have disputed his claim to have been opposed from the outset, but a friend of his confirms to me categorically that this was his position in 2003.

What he offers seems to be less a dramatic break with the politics of the past 13 years than with its style and sensibility. He promises to revitalise the party by devolving power and inspiring grassroots participation – and it's a noble ambition. But what happens, I ask, if the party lurches back to the left, leaving him with a pre- Kinnock problem on his hands?

"The New Labour style of leadership was, 'We seize control of this party, 'cos otherwise it's going to carry on losing,' right? That had its merits for its time. But this is a different world. And actually, if we'd listened to our party more on a range of issues – housing, agency workers, tuition fees – we'd have been a better government, not a worse government. We've got to not misunderstand where the centre ground of politics is – because you know, it's not just people in the south who have aspirations, and it's not just people in the north who don't like what happened with bankers' bonuses. And sometimes we were a bit behind the public in terms of where the centre ground of politics is.

"Of course you can't just win with the converted. But the point is that a lot of the converted became unconverted, and we've got to win them back, as well as the people who came to us in 1997."

Touring the aerospace factory last week, his thesis is borne out by the striking number of working-class, low-income union men who admit they voted Tory this year. Miliband also cites a recent psephological study which found that Labour lost three times as many voters categorised as DE – the poorest of the population – as it did the most affluent ABs.

"This is fundamental," he stresses urgently. "If we think the New Labour play book is going to work for us in the 2010s, we are completely wrong, because the electoral challenge is completely different now. But I think some people in the Labour party are stuck in the New Labour comfort zone, and think let's just repeat the formulae of the past and that it's going to win us the election.

"The problem is we took the working class vote for granted. Politics isn't just about policies – it's impressionistic, it's about emotions, and whose side you're on. It's about whether you're the party that starts off with a windfall tax on the utilities monopolies, and ends up defending bankers' bonuses. I don't think we lost the last election because we were too leftwing. I think we lost the last election partly because people thought we were all the same."

Miliband concedes that he does support certain coalition policies – on ID cards, prison policy, and an inquiry into British collusion in the torture of terrorist suspects, although he quickly adds, "I know my brother more than anyone else, and I know he would never sanction torture, implicitly or explicitly." But when I suggest that we have the Lib Dems to thank, he shakes his head impatiently. "No, no, I totally disagree with that. Does the fact they're scrapping ID cards compensate somehow for the fact they're taking an axe to the welfare state? Absolutely not, in my view. I care about civil liberties, but I care massively about people finding the services they rely on done in."

So Labour under Miliband would no longer be agreeing with Nick? "Nick Clegg," he says icily, "is a betrayal of the Liberal tradition. David Cameron and Nick Clegg are texting each other like teenagers in love because they agree with each other. It's not some forced marriage, they ideologically agree with each other."

The possibility of forcing a divorce, however, he regards as very real. "I think we can stop the coalition lasting. It depends upon people like Simon Hughes, Charles Kennedy, Ming Campbell – who I believe are unhappy." He sounds so knowing that I wonder if he is in private communication with them. "I have private relationships with some Liberal Democrats that I'm not going to particularly talk about. But I think there are lots of unhappy Liberal Democrats."

Miliband's claim to be the leader most likely to draw disillusioned Lib Dems to Labour sounds quite plausible. The prospect of an election in less than five years, however, poses perhaps his greatest problem – the perception that only his older brother is oven-ready for office.

"Well obviously I don't agree with that." But many Labour members struggle to picture the 40-year-old in No 10, so I wonder when he first imagined it himself. "I think I was a late adopter, actually," he smiles. "Others got there before me." Does he mean he was talked into it? "No, I'm not saying that. But you do judge the support, otherwise you wouldn't run. And lots of people said to me, I think we need someone who represents more of a change from the past."

Alistair Darling has warned candidates against being over-critical of Labour's record, in a bid to sell themselves as a fresh start. "Well I don't know who Alistair's talking about, but I'm certainly not dumping on the record. I'm absolutely not. But if you say, when you lose an election, the show was great but the audience was poor, you're going to keep losing elections."

He also dismisses any suggestion that his backing from the three biggest trade unions risks reviving an impression of a Labour leader in the pocket of Unite. "I'm not defending everything the trade unions do, nor would I as Labour leader. I don't think we're about to go back to the 1970s, and I'm not planning to take us there. But I do defend the role of trade unions in our society. And I think it's surprising that that's surprising, coming from someone who wants to be leader of the Labour party. Politics has basically become a middle-class pursuit – a London-middle-class pursuit, detached from ordinary people's lives – and it's actually the link with the trade unions which helps make us relevant to people's lives."

Miliband himself, of course, is a middle-class Londoner whose entire life has been spent in politics. An Oxford PPE graduate, with an MSc from the LSE, he worked for Harriet Harman before becoming an adviser to Gordon Brown, then won Doncaster in 2005, and joined the government only a year later. "Well I am the person that I am," he says simply. "But there are people who have worked in a whole range of jobs who aren't very empathetic, and can't understand people's lives. And there are people who have worked in politics for a large part of their lives who can, and that in the end is the test."

I get to test Miliband's famous empathy for myself when we visit a vast building site in Salford, where he tours the staff canteen meeting construction workers. Most of them seem to have no idea who he is, and it's a vintage scene of dauntingly tough masculinity – hard hats and red tops, pie and chips and tattoos. Any trace of the slightly geeky awkwardness Miliband sometimes used to convey has vanished, but there is nothing ostentatious about him either as he moves from table to table: no polished quips to bridge the class divide, but instead an unusually respectful intimacy. "What did we do wrong?" he asks them, listening carefully to their answers. "What do we need to do to win back the voters?"

The only self-consciousness comes when the photographer wants him to pose holding a hard hat in the yard, before an amused audience of smoking men. "Any politician that claims to you that they're an ordinary person is not telling you the truth," Miliband mutters, half smiling and wincing. "Because politicians don't lead ordinary or normal lives."

Miliband lives with his pregnant partner Justine Thornton, a barrister, and their 15-month-old son Daniel. Never part of the glamorous 90s New Labour social scene, he admits that even his own pretty austere idea of a work-life balance has been out the window ever since the Copenhagen summit in 2008. He has never looked after Daniel by himself for 24 hours – "I'm not looking for special pleading, but, yes, it is a big sacrifice" – and describes Justine as "brilliant", but feels no personal or political need to marry. "We'll get round to it at some point, but I don't think people would mind if we didn't." As party leader, he would guard his family life less ferociously than Brown, but more privately than Tony Blair, and "under no circumstances" will his sons go to private school.

He has to think quite hard to remember the last time he danced – "Oh yes, at Sue Nye's party after the election, to something from Glee" – and on holiday in Cornwall this month he listened to Robbie Williams, and read The Girl With the Dragon Tattoo. He seldom cooks, buys his suits off-the-peg from TM Lewin, and when I ask if it's true he once played in a punk band called Squashed Psyche, he laughs – "Oh I wish it was!" – but admits that the first he heard of this fiction was when it appeared on Wikipedia. In truth, reports of Ann Widdecombe joining Girls Aloud would be less surprising – and apart from driving too fast ("according to Justine"), his only discernible vice is an addiction to Desperate Housewives.

Miliband's unracy style has led some observers to cast him in the Brown rather than Blair model of leadership. But when I ask when he last lost his temper, he says, "I very rarely do that. I'm not someone who spends their time shouting, and I think it's important not to. I think you need calmness, stoicism and optimism. I think warmth and humanity are undervalued qualities in politics." Is that why people like Campbell doubt he has the mettle to win? "Warmth and humanity don't mean you can't make difficult decisions. I think listening is an undervalued quality in politics. And that's something I would do as leader."

After 16 years of Blair and Brown, and New versus old Labour, Miliband may well be a genuinely new and untested quantity – a pragmatic young progressive. He clearly lacks the New Labour instinct for triangulation, spin and soundbites, but displays none of the self-indulgently defeatist idealism of old Labour either. I wonder whether it might be his refusal to criticise his brother that has created an impression of a lovable loser.

"I don't care. I don't care. There are certain prices I'm not willing to pay for this. If I thought I could win this election by taking lumps out of David, I wouldn't do it. I'm going to win this with a clear conscience."

If he won't attack one David, is it just possible that people think he won't attack the other one?

"That is nonsense!" he erupts. "I hate what David Cameron's doing to this country. Alastair Campbell is wrong. He is wrong. Just to clear up any doubt, I don't want a Labour party that feels comfortable about losing. I want a Labour party that's back in power."