A year ago, David Miliband had a bad Labour Party conference. Even before he was photographed with a banana, the Crown Prince of Blairism had given the impression of leading people up the garden path and then down again. Mostly, it wasn't his fault: he had set out his stall as a young, modernising alternative to Gordon Brown in the summer, only for Lehman Brothers to go bust the week before the party assembled in Manchester. All of a sudden, Britain needed an economics prime minister and the leadership speculation lever was thrown into reverse.

This year, the Foreign Secretary comes to conference from a different position. He gives every impression of having resolved in his own mind that his ambition for the leadership has been deferred until after the election. Over the past year, Alan Johnson, the Home Secretary, has emerged as the most credible interim leader should the party dump Brown in the next few months. But when Miliband spoke to me last week by telephone from the British "cubicle" in the United Nations building in New York, he said he would not be joining the "Alan Johnson for PM" campaign. "I'm joining the 'Gordon Brown Stays as PM' campaign," he said.

"What the polls show is that we're not popular, but they don't show that the Tories have broken through either. We've got to break through the froth and use party conference to debate the manifesto; it gives us the opportunity to show what we stand for for the future – at the end of the week showing that we've got a vision for the future as well as a proper recognition, a proper assessment of the last 12 years, and also exposing the Tories. The future is the key and that means having a mental map that is looking forwards not backwards."

Download the new Independent Premium app Sharing the full story, not just the headlines

Last week, Labour cheerleaders from Alastair Campbell to Roy Hattersley demanded that Cabinet ministers should come out fighting. Miliband is happy to oblige. "My very strong view is that we do have three tasks at this party conference. One is to defend the record, and to fight declinism about Britain. The second is to expose the Tories and fight the easy ride they're getting. Thirdly and most importantly we have to show what we're offering to the future, and fight the myth that New Labour doesn't have ideas."

Defending the record means refusing to apologise for New Labour. He said: "New Labour has been the most successful government since the 1945 government. It powered us forward. We have got to keep powering forward, and what we have shown during the economic crisis is the ability to do that. What's happened on the economy over the last year shows that we can renew." That does not mean pretending that everything has been perfect: "We've got to recognise we were too late on the climate stuff. We've got to regain lost momentum on the political reform agenda." But it was really important for the party to fight what he called "declinism" – "the self-flagellation that everything is going to the dogs".

He implied that the Labour Party itself could be a victim of declinism: "When New Labour has been successful is when it has challenged the inertia of both Britain and of the Labour Party. That marks out the most successful part of both the Blair and Brown premierships."

Note that Blair is part of the record, even if he is not the sole point of reference: "Tony Blair is the last person that would want to be the buddha that people go to bow down in front of." (And Miliband is notably reluctant to talk about Blair as European Union president – although that could be put down to a diplomatic reluctance to take the result of this Friday's Irish referendum for granted.)

Defending the record includes some murkier areas of realpolitik. Since he became Foreign Secretary he has had to field questions about whether his predecessors asked enough questions about intelligence information that may have been obtained by torture. He bristled: "I've seen absolutely nothing to suggest that. We have profound national and international obligations which we uphold. After 2001 there was a profoundly different situation." He said "new mechanisms" that had been put in place long before he became Foreign Secretary. But isn't it the case that, in the immediate aftermath of 9/11, we didn't try to find out what the Americans were up to? "The Americans certainly didn't tell anyone what they were up to. We didn't know exactly what the Americans were doing [and] what they then did."

He moved from defending the record to attacking the Conservatives with relish. "One of our tasks is to expose the rhetoric and the reality of the Tories, because when you look at what they are actually proposing it seems to have a very hollow centre. One can point out in the least shrill and most Independent on Sunday way that if people get fundamental decisions wrong it says a lot about their instincts.

"If you get something as wrong as a global economic crisis, it says a lot. The very simple budgetary arithmetic for next year has got every economist, right, left and centre, in Britain saying the Tories are wrong to think that 2010 is the time to cut public spending."

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

His main attack, though, was reserved for the issue of Europe. He admitted that "we haven't made Europe popular in the last 12 years", but claimed that "we have led Europe to play a much more progressive role in the world". There is now, though, "a fundamental choice over the direction of Britain: either that you fight Europe, as the Tories want to do, or you shape Europe, as we are doing. And you know, on the Irish referendum, if they vote No, the [Lisbon] Treaty doesn't go through; if they vote Yes, we say, 'Let's make the most of it and here are our ideas for dealing with it.' The question for David Cameron is: Will he live with it or will he fight it?"

Having just spoken to senior Conservative sources, I suggested that Cameron was hoping that the Czechs would delay ratification. "That is a total absence of leadership. It says a lot, this issue, about whether you're embracing the future or whether you're stuck in the past. It's a very good example. It's not an easy one for us to argue. It's not immediately popular. But it's right.

"It is a fundamental part of the national interest to be strong in Europe, and this is a massive strategic weakness of the Tories: they say they want to do stuff on climate change, but you can't without Europe; they say they want to do stuff on the Balkans but you can't without Europe." A note of contempt tinged his voice as he spoke of Cameron's dilemma: "It looks like the Tebbits et cetera won't let him say, 'Of course we've got to live with Lisbon.'"

On this, he speaks with the authority of someone greeted warmly and spontaneously that day by Barack Obama; someone central to the world's response to Iran's nuclear programme. He denied that the Iranian regime is "playing us for fools". He said: "You've got real unity and clarity from the international community. And this is the first year of an American administration that wants to be part of an engagement strategy as well as a pressure strategy."

The third part of what he called "my pitch" is to set out what Labour is offering for the future. "I now think 'future' is the most important word in politics," he said. This is double-edged, of course. Surely it would be easier for a younger leader to present such an image for the party, I said. "No, I don't think that's right. I think that the economic response gives you the answer to that. I think that on the green agenda Gordon and Ed [his brother, Secretary of State for Energy and Climate Change] have really pushed forward very strongly in the last year. So I don't accept that."

All the same, as if to exemplify his claim to the future, he started using Twitter last week. Because some newspapers recently fell for a spoof Twitter account in his name that commented in improbable terms on the death of Michael Jackson, he announced: "OK this is really me."

Before our interview, he and I had a Twitter exchange. He commented from his BlackBerry, possibly during one of the less riveting sessions of the UN General Assembly, on something I had said about Nick Clegg's ambition to replace Labour as the main alternative to the Conservatives. He wrote: "Let's not rehash 1920s. Labour is place to combine progressive thought." It was a bit compressed, but you could tell what he meant. He believes that Labour will see off the Liberal Democrat challenge, which suggests to me that he is thinking ahead beyond the next few difficult months.

And there is a strong element of a personal manifesto in his "empowerment agenda", which he describes as "deeply progressive". He explained: "You can't stand for empowerment unless you are an egalitarian. That's the platform we then use to stand up for a strategic role of government, but also stand for decentralisation. We stand up for social mobility and we see public service reform as critical to that, and welfare reform. We stand up for the diversity of Britain but we know it has to be founded on strong rights and responsibilities. And, very importantly, although there's no point in pretending it's popular, you have to stand up for internationalism, and you have to stand up for the need to share power in Europe, to be influential in the world. That's basically my pitch."

While Alan Johnson would probably take over if Brown stood down before the election, Miliband must be out in front in the race for the Labour leadership afterwards. Those who wrote off his leadership chances last year were doing so prematurely. It would be absurd if the choice of a party leader and potential prime minister were decided by something so superficial as a single encounter with a banana.

Curriculum vitae: From think tank to the Cabinet

1965 Father Ralph was the grand old man of Marxist political theory and refugee from the Nazis. His mother, Marion, is a historian. Younger brother Ed, pictured right with David, followed him into politics, and is now Secretary for State for Energy and Climate Change.

1980s After Haverstock comprehensive school, north London, David enrols at Corpus Christi College Oxford, gaining a First in Philosophy, Politics and Economics. Becomes a Kennedy scholar at Massachusetts Institute of Technology.

1989-94 His big break – working for New Labour think tank, the Institute for Public Policy Research.

1992-94 Secretary of the Commission on Social Justice, appointed by then Labour leader John Smith.

1994-2001 Head of policy for Tony Blair, in opposition and in government. Best line: "It's the Third Way between having your cake and eating it."

1998 Marries Louise Shackelton, a violinist with the London Symphony Orchestra. They met on a plane. Two sons.

2001 MP for South Shields.

2002-04 Minister for Schools.

2004-05 Cabinet Office minister.

2005-06 Minister for Communities and Local Government, in Cabinet.

2006-07 Secretary of State for the Environment, Food and Rural Affairs. Helps put climate change on the political map.

2007 Appointed Foreign Secretary by Gordon Brown.