Just back from a week in Cornwall, Ed Miliband seems relaxed and confident as he perches on a window seat in his Westminster office, with a view on to the Thames. He knows he has many disparagers - both from among the supporters of his main rival for the Labour leadership, his elder brother, David Miliband, and from the professional commentariat, who caricature him as an unreconstructed Brownite statist with limited appeal to the wider electorate, especially in England. "I don't think we can just recycle New Labour solutions," he says, when we put some of this to him. "I think we've got to realise that we are not going to win the next election with the old formulas and I don't think a political party can succeed if departures from perceived orthodoxy end up being seen as 'Old Labour' and not part of the debate."

We meet Miliband on the day Lord Myners, the former banker and City minister under Gordon Brown, has used a newspaper interview to criticise the candidate's position on progressive taxation as "wrong" and suggest that he has moved away from the "classic centre left", from which elections are won.

“I don't think a 50 per cent tax rate is a retreat or a move from the centre ground of politics," Miliband counters. "I think a 50 per cent tax rate at £150,000 is actually in the centre ground of politics, because I think that one of the things people thought about us, as a Labour government, was that we were behind where the public was on some of the excesses we have seen in society. And people do believe that those at the top should pay their fair share."

Miliband has been the subject of a whispering campaign by Labour MPs from rival camps that accuse him of running a 1980s-style “left-wing", even "Bennite", campaign. His response? "I think all that stuff about being a Bennite is not really worthy of proper debate." What about the dreaded S-word? Does the younger son of the Marxist intellectual Ralph Miliband and former aide to Gordon Brown see himself as a socialist? "I'm happy to be defined as a socialist. My socialism is about being willing to critique the injustices of capitalism. We're not about to replace it, but there are different forms of capitalism we can have . . . My socialism is not about a blueprint for the perfect society, but it is about saying we can have a more equal, just and fair society."

Miliband says he blames the Tory-supporting media in this country for skewing the terms of political debate. "We have got to get out of a paradigm, defined by the right-wing press, which says that if you talk about Labour values and talk about what you believe, then somehow we are not going to win."

In recent weeks, various senior columnists and commentators have urged Labour members to back the "more authoritative" David Miliband, claiming that Ed is "on course to destroy the Labour Party" and is "leading Labour on a march into insignificance".

The younger Miliband says of these commentators that they "have their own agenda . . . They want to define the ground of politics on the right. It is part of our job to define the ground of politics in a different place and not to take advice from people who don't have the less well-off's interests at heart."

New Labour invested much time and energy in courting and cultivating the right-wing press and, in particular, the Murdoch-owned newspapers. Tony Blair, on becoming Lab­our leader in 1994, flew to Australia for an audience with Rupert Murdoch. Does Miliband have similar plans to woo the next generation of Murdochs? He smiles. "I think you want as many people supporting you as possible." Does this include the editors of the Sun and the News of the World? "I think the Murdoch press has less influence than it used to," he says. "I don't think the Sun had a particularly good election. Twenty-three front pages supporting the Tories and the Tories got 36 per cent of the vote. Let's see where they are, at the next election."

Miliband defines the Labour Party he wishes to lead as "a coalition of lower- and middle-income earners". But what of those more affluent voters, especially in the south of England, whom Labour must attract if it is to win again? Are they excluded from this coalition?

“If you think about a lot of the affluent voters who left us - well, they went for the Lib Dems, not for the Tories," he says. "Some of that was about civil liberties and tuition fees and Iraq. They weren't simply about issues of higher tax, for example. I think there are a lot of people out there who can be won back to our cause - I want to win back Tory voters, but I want to win back Liberal Democrat voters, too. And they would come back to us, but only if we have the right formula for the future - and that's about a progressive economic policy, but also about defending liberties and putting civil liberties at the centre of what the party cares about."

Miliband is keen to address the argument that a Labour leader must necessarily challenge the prejudices and complacencies of his party; must, in effect, be defined against his party, in order to demonstrate his moderate or centrist credentials. "I talked to a Labour member, Nick in Cornwall, who said to me: 'Do you agree with those who say it is the job of the Labour leader to protect the country from the views of Labour Party members?' I thought that was a fantastic way of putting it."

Miliband points out that Neil Kinnock's defining battle with the Militant Tendency inside the party took place 25 years ago. "Are we going to live with the ghosts of the 1980s, or are we going to exorcise those ghosts?"

For Miliband, "of course, the Labour leader is going to disagree with his or her party at times, but does the Labour Party have something to say? Do the members connect us with the public? Yes. Do we need a more outward-looking party? Yes." He rejects the view that "not disagreeing" with his party is equivalent to "pandering" and suggests that New Labour, under Blair and Brown, may have looked as if it had "contempt" for the views of party members.

The most contentious issue over which the Labour leadership diverged from the views of the membership, and the country at large, was the invasion of Iraq. Seven years on, and in the midst of a prolonged leadership election, Iraq remains the event from which the party cannot escape. Miliband has sought to distinguish himself from his elder brother by arguing that he opposed the war at the time of the invasion, but was on a sabbatical in the United States then, and so his views were not widely known back home in Britain.

Throughout this campaign, however, both David and Ed Balls have repeatedly challenged Ed's claim, the elder Miliband telling the New Statesman in July: "If the only candidate you can vote for is someone who resigned or made clear their opposition to Iraq in 2003, then Diane [Abbott] is your woman," adding, "What I'm not willing to do is rewrite history."

Is Ed guilty of rewriting history? "I was very clear with people I talked to at the time in 2003 [about] what my view was: that the UN weapons inspectors should have been given more time. If you talked to anyone who knew me at the time, they would say that."

He believes the next Labour leader must be "willing to accept that Iraq was a mistake". What about the argument from David and others that it is time to draw a line under the whole debacle? "I think, as a party, we do need to draw a line under Iraq. But I think that's hard if you are saying the war was the right thing to do," he says - as his brother does.

At husting after dreary husting, all five leadership candidates have rehearsed platitudes about the need to move "beyond Blair and Brown". More specifically, Ed Miliband, who is 40, believes it is essential for the party to "move on from New Labour". He sees himself as being at the forefront of such a move. "I think it is definitely time for a new generation . . . there is no question about that. I'm very proud of the backing I've got from people like Rachel Reeves, Chuka Umunna, Emma Reynolds, Luciana Berger and a whole range of new MPs. They are the future of Labour politics."

Critics of the younger Miliband question whether he has the strength of will and stamina to lead what can be a fractious and disunited party. They mention, too, his lack of support among senior former cabinet ministers such as Jack Straw, Alan Johnson and Alistair Darling, all of whom are supporting David. "Well, I don't have the backing of the New Labour establishment, that's true," he says, smiling. He knows it is part of his appeal.

But the younger Miliband has struggled to raise funds for his campaign, especially compared to his brother. According to figures from the Electoral Commission, Ed raised just £46,450 in July, against David's £138,835. "I think there is a lesson for future leadership contests: that there is a limit on spending. This is not special pleading for me, but you have to find a way so that you have proper expenditure limits. Someone like Diane has clearly been able to raise the least amount of money - it does make it look like an unlevel playing field. The Labour Party was smart to have a limit on the amount of money you could spend on Labour Party members, but it's a partial cap, not a complete cap. I suspect David might well acknowledge that, too."

Ed Miliband has a distinct position on equality, on civil liberties and on the need to transcend the old New Labour orthodoxies. But has he thought hard enough about the relationship between the individual and the state and about how the Conservatives have, through the "big society", appropriated the language of mutualism, reciprocity and association? When we ask him about the overweening state, he merely reiterates his position on civil liberties. We press him: surely the country is suffering from the consequences of both market and state failure?

“The state needs to be less overbearing, more democratic, more open," he says, after a pause. "On a basic level, the frustrations I see in my constituency surgery are as much about the state, which people feel has let them down. The Tory theory is: get the state out of the way and the big society will flourish. I would give more voice to local authorities and introduce better mechanisms of redress when the state lets you down. I think we need to be defenders and reformers of the state." He cites the Bill Clinton motto: Mend it, don't end it.

“The kind of attack the Con-Dem coalition is making on the state is a right-wing, Republican-style attempt to, as they put it, 'starve the beast'. Basically, you take away some of the middle-class benefits from the state. Then you make it into a sort of safety net which is essentially full of holes - and you further undermine support for it." For Miliband, both the Conservatives and the Liberal Democrats "have a very residual view of the state".

He is especially scornful of the role played by the Liberal Democrats in the coalition government, and suggests that "there is a lot of disquiet among the Lib Dems. Next May's elections will be bad for them."

He also reveals, for the first time, that he would demand the resignation of Nick Clegg before doing any deal with the Lib Dems in any future hung parliament. "Given what he is supporting, I think it is pretty hard to go into coalition with him."

So you wouldn't work with Nick Clegg?

“That's right. No."

Miliband may be in favour of regime change for the Liberal Democrats, but how secure would his own position be as Labour leader, if he is able to defeat his brother, the favourite? On this, all he will say is: "On 25 September, all speculation about the leader has to end."

To decode - for Miliband, there will be no repeat playing out of the Blair-Brown wars, neither in the next generation of Labour politicians nor in his own family.