The bitter cup of defeat was tasted by Labour's senior ranks when they made their way to the shadow cabinet room in the House of Commons. Some of them had to ask for directions. Not so David Miliband. He could remember the way. That didn't make the destination any more pleasant for the man who had just ceased to be foreign secretary.

"I'd last been in that corridor six weeks before the 1997 general election and we were on the march. It's a very depressing room. You know then, you're out. It reeks of the absence of power. Just reeks of it. And there aren't enough seats around the table for everyone. That hit hard."

Then he strikes a more upbeat note for Labour after the party's second-worst defeat since 1918: "What's interesting is that it wasn't a funeral at the shadow cabinet meeting. It was a group of people determined to fight back."

Miliband was the first to declare that he wanted to head that fightback and became the early bookies' favourite, not always a good omen, to win the contest to be Labour's next leader. There has been a paradoxical cheerfulness in the party's depleted ranks of MPs. He finds it explicable: "The polls had us third for a significant part of the election campaign. That's a near-death experience. If you're in a car crash and you think it's going to do for you, but actually break your leg, a bit of euphoria is understandable. We've got to be clear that now is not a time for euphoria."

In southern England outside London, Labour has shrunk to 12 seats: "It was a bad defeat." The scale of the party's reverse will really be driven home, he thinks, when the Commons meets this week and Labour MPs find themselves greatly outnumbered by the coalition composed of Tories and Lib Dems.

Miliband soon dials up more reasons to be cheerful. He is an innately optimistic character as well as a clever one, and a man who needs to persuade his party not to despair.

"Our party activists and our voters are amazing people. In the midst of this barrage of money and media, they stuck with us. This party is not walking into the history books. It's determined to be a 21st-century party. The fight and the determination and the resilience of the party members, supporters and voters is a great thing. Also there's energy because we've 260 MPs. That's a 1992 level of MPs, not a 1983 level."

The "coalition of contradictions" gives Labour both a responsibility and an opportunity to "forge a progressive alliance within the Labour party of all shades of progressive opinion". But he is not among those in his party who complacently assume that they only have to sit back and wait for the Con-Libs to fall apart.

"We underestimate this coalition at our peril. The Tories have always been about power. Clegg has revealed that actually he's about power as well. After decades of moral sanctimony from the Liberal Democrats, we can now be absolutely clear that when push comes to shove they're happy to drop child poverty and the job guarantee.

"But the determination they've both shown is something we underestimate at our peril. Because there's no inevitability about the pendulum swinging. And we are going to have to be very canny about how we position ourselves. The electorate aren't going to be studying us carefully in this period but they're going to notice how we behave. And if we go back to yah-boo politics we'll make a big error.

"We've got to be ready for it to fall and we've got to be ready for it to go long. It's very, very important that we have a fighting opposition, not fighting with ourselves but fighting the government where appropriate. And that we're an alternative government. Whoever is elected is going to have to be a credible prime minister."

That is a strong strand of his pitch for the leadership: as the most senior former cabinet minister in the running, he will give Labour the most authoritative voice.

Before the party can be renewed, it must make an accurate assessment of why it has just lost: "This was a change election and we were not the party of change. I said in my conference speech last September that 'future' is the most important word in politics and we did not convince our fellow citizens that we were the party of the future."

Some blame him and other senior Labour figures for not replacing a very unpopular leader. The "bottler" label was first, if not really fairly, hung around Miliband's neck when he did not challenge Gordon Brown in 2007. "I wasn't ready," he says, and goes on to defend his subsequent reluctance to try to unseat Brown. Some members of the last cabinet believe voter aversion to Brown cost Labour as many as 40 seats. Miliband does not try to dispute that, but he argues it would be a major mistake to ascribe the scale of the defeat entirely to the failings of one man.

"I think anyone who believes that the result was just because of the leadership is kidding themselves. Gordon spoke very movingly about discovering his own strengths and his own frailties. But anyone who tells you that all we've got to do is change the leader and then everything will be fine is wrong."

Labour failed to win a fourth term because "we all said we needed to renew but we didn't sufficiently. People felt we were late to the game on issues like political reform. Antisocial behaviour – we lost focus on that. Immigration, late to the game with the Australian points system. Social care, late to the game.

"We were too timid on the role of government in the economy. We were too slow to see that climate change was not an environmental issue. It was an economic, security, foreign policy issue.

"We got told that political reform was a middle-class issue and we basically stopped. We did the freedom of information, human rights act, devolution of Scotland and Wales, London. But we basically got frightened off. It was at best half a political revolution. Maybe a third. We should have done the House of Lords, for goodness' sake."

Many of those failings he ascribes to New Labour being a product of the party's "searing experiences" in the 1980s which prevented it from being bolder either about challenging the status quo or changing how politics is done: "New Labour was a reaction to the 1980s but it was trapped by the 1980s. Anyone who thinks that the future is about re-creating New Labour is wrong. I think we've got to use this period to decisively break with that. What I'm interested in is Next Labour."

It will not be escaped quite that easily. He rose as a protege of Blair; two of his competitors for the leadership were special advisers to Brown. Miliband is emphatic that this does not mean the wars of the TB-GB era are fated to be replayed by a new generation. "The whole thing about Blairites and Brownites is just wrong and gone and over because the policy agenda has changed fundamentally."

He prickles a little when we suggest it is a weakness that the contest is between apparatchiks of the Blair-Brown era. "David Miliband, Ed Miliband, Ed Balls, Jon Cruddas, Andy Burnham… we're our own people. We're going to treat the electorate as adults and we should be treated as adults."

He claims to be relaxed that his younger brother has entered the fray to turn this into the first leadership contest involving siblings: "Family is more important than politics. He's very talented. But I love him. We're not going to put that love at risk."

They were close as children, he says believably, but not competitive, he claims less plausibly. If Ed wins, "of course" David will happily serve under his younger brother. He does not much like the suggestion that Ed is more accomplished at displaying the common touch: "People have to make their own judgment about the two of us. It's better not to talk about it. It's better just to do it." It is on this subject that the normally fluent former foreign secretary is at his most closed down.

Do they disagree on things? "Of course we do. We're not twins. We're not clones." But on what they disagree he will not say. He expects their mother, a staunch Labour member, to avoid choosing: "She's not an abstaining type, but I think she'll be abstaining this time."

It will not, though, be a case of Labour being offered any leader it likes as long as he's called Miliband. Ed Balls is taking "soundings" before deciding to run. The Miliband brothers certainly love each other a lot more than either of them do the former children's secretary. When we suggest they can gang up on that rival, he chooses to laugh rather to deny it.

Another highly likely contender is Cruddas who performed very well in the last deputy leadership contest. Aware that some on the left, where Cruddas is popular, regard Miliband with suspicion, he says: "He's taught me a lot, Jon Cruddas. He's been talking about housing for a long time. He's been talking about community organising for a long time. He's fought the BNP. I think uniting different talents is an important job. Because it's not ideologically riven, this party. There's enough shades to make it interesting but I don't see incompatibilities."

That offers not so much an olive branch to the left as an olive grove.

When it is time to have his picture taken, he points to an oil painting given to him by his wife and asks: "Do we want these naked women behind me? Is that good?" The photographer thinks not.

The pictures taken, he tells one of his young sons that he will read him a story, but it will be "nighty-night time at half past six".

As one of Britain's youngest foreign secretaries, David Miliband swaggered the world stage in the company of Hillary Clinton. Now his sway is reduced to deciding on bedtime for his boys. If and when he and his party wield power again will depend on whether the contest for its leadership really does begin Labour's renewal.

Where does the Labour Party go now?



Sunder Katwala, general secretary of the Fabian Society

Labour lost the election – so has spent a week on a rollercoaster ride through the classic stages of the grieving process. Denial, as the exit polls and new Commons provided an arithmetical hope of staying in. Bargaining, over a Lib Dem deal and anger at the "vote Clegg, get Cameron" Tory-Lib Dem coalition outcome. Can Labour arrive at acceptance? Parties that lose democraticelections must acknowledge that they deserved to do so.

Will Labour now have a proper debate about its record and future? Not if the only question is which Miliband brother to vote for as the next leader. An open (and indeed fraternal) leadership contest is essential, after the 2007 coronation, but concluding this too quickly would be a big mistake. The Conservatives twice failed to have any proper inquest into election defeats: only in 2005 did Michael Howard provide more time and space. (That is also why David Cameron, not David Davis, won.)

Labour, too, should run its leadership contest through the autumn – using the party conference as a debating showcase – and take the chance to bring many more people back to Labour to take part. That will take a different, more open party culture and to restore its instinct for civil liberties too.

We may learn little if, before anybody has properly studied this complex election, everybody just says what they thought already, repeating their favourite leftist or New Labour mantras, about losing C2s over immigration, or failing to inspire with Labour values.

So we must not be frozen in a timewarp debate about whether we stay "New Labour" or not. New Labour achieved a great deal – but that was a long time ago. A broad winning majority for the Britain of 2015, not 1995, means imagining and mobilising the next left, not trying to reconstruct the last political generation.

Tessa Jowell, MP for Dulwich and West Norwood

We lost the election in the marginal constituencies along the M1, M2 and M6, the spinal cords of middle England.

In 2010 there was nothing much to lighten the hearts of those who had flocked to vote for us in the heady days of 13 years ago. We had huge achievements to our credit – new hospitals, rebuilt schools, renewed infrastructure, rising standards in education and health, all undeniable – but voters don't thank you for what you've done; they rightlywant to know how you will make their future better.

In an adverse economic climate, in the face of a largely hostile press, and against the easy claim that it was time for a change, our account of a Labour future was timid. We were hearing, but not listening to, people's fears about migration, immigration and housing shortages.

The election results were spotty, and what stood out were the results of MPs who worked hard in their constituencies, championed local causes, knew their patch intimately, worked to make real lives better, and listened and responded to what local people had to say.

While politics may be becoming more presidential at the top, it's solid campaigning and community work year round in the constituencies that will hold the line in a bad year and advance the cause in a good one. From this also come the best ideas for the future, authentic and rooted in real lives.

For the future, how we do politics will be as important as what we do. But to own the future we must stop and listen, and admit in humility that the answers will be found in the values we have always believed in and articulated in the mouths of the people we hope once again to serve.

Chuka Umunna, newly elected MP for Streatham

I wouldn't wish Conservative government on my constituents, but we are where we are and opposition gives Labour the chance to refresh and renew, which eluded us in government.

Clearly there is much to be proud of and we must build on our achievements. But we also made mistakes and lost the election: we need to acknowledge that and explore why, in a cool, calm way, without entering into some factional blame game.

In some respects, the generation of politicians ahead of me came to be defined by their loyalty to one of our last two leaders – now both have moved on. Unshackled by the travails of office, the leaders-to-be can tell us what they are all about. As a humble new MP, I'm excited at the prospect.

There should no rushed coronation, beauty contest or stitch-up. In Harriet Harman, we have an excellent caretaker leader in place – this affords us the time to have a proper debate, so as many people as possible should throw their hats into the ring.

The scale of transformation required to resolve the economic, democratic and environmental crises is a challenge. How do we go about tackling them? We want to know the answers each leadership candidate is offering.

Moreover, the contest gives us a golden opportunity to become a mass membership party once more. Anyone joining now will be able to participate in the contest. To those who left the party over the years – in some cases drifting to the Lib Dems in the hope of finding something more progressive – we should say: come back and shape the future direction of the Labour party so you feel at one with it again

Fiona Millar, journalist and former adviser to Cherie Blair

The Labour party has a real opportunity. We can't shirk the fact that we lost but, for all the slick Clegg/Cameron rhetoric, this will be a socially conservative government, made up of people who won't use the public services they are cutting and are not really concerned with fairness and equality. Many Liberal Democrat voters feel betrayed. We should encourage them to join us and build a much broader coalition.

We must do better at defending our record in government and explaining it in language people understand. We all have policies that we didn't agree with, but overall the Blair/Brown governments did much good. Where were the big arguments about childcare and family-friendly working in the election campaign?

We need to build on the best of the last 13 years and politely park the worst. I fear the leadership election will be quickly styled as yet another battle between the "modernisers" and the "dinosaurs", with pressure for us to start second-guessing where the Tory/Libs will go and become a "Cameron-lite" party. That would be a disaster.

I'd like to see an end to the idea that we can run everything like a private market. People want high-quality, flexible, local, accountable public services, good enough to be used by all. Personally I want policies that genuinely help the poorest children. I am sceptical about the pupil premium, which doesn't address the fact that about 80% of a child's life chances are determined by their circumstances outside school. Labour began as a party of aspiration. That is still so, but it needs to be explicitly married to progressive values.