Ed Balls, Labour's toughest street-fighter both inside and outside the party, is fiddling with his shoelaces. Sitting in his office at the House of Commons, we have reached the stage of the interview where he must tackle the difficult questions. Has he ever briefed against colleagues? He stares straight at me for the first time during our hour-long conversation. "No," he says, barely moving his lips. Has he ever asked a friend or a member of staff to brief against a colleague for him? "No." And that's that. The tension has risen, but we move on.
I have a confession to make: like some other journalists and politicians, I have at times been a little wary of Balls. In February 2009, I wrote an article suggesting that he was vying for Alistair Darling's job as chancellor and wanted to be leader. When it was published, I was called in to Balls's office for an hour-long meeting, during which forceful denials were made. Since then, Darling has described how "the forces of hell" were unleashed on him by Gordon Brown's allies; Balls has admitted that he did want to be chancellor; and former cabinet colleagues of his told me that Balls had planted stories against them.
But that is now history. The Labour leadership contest has turned Balls's ruthlessness into a strength, and as shadow education secretary he has emerged as a formidable force in the fight against the Tory-Lib Dem coalition. He was persistent in opposing Michael Gove's moves to scrap Labour's school building programme, culminating in the Education Secretary issuing a humiliating apology when it transpired that he had wrongly claimed that certain schools would be exempt. The affair was a huge embarrassment for the coalition.
Balls's attacks on the government have not been limited to education, however. He was the first to denounce the VAT rise announced in last month's Budget. But then, such matters are comfortable terrain for him. A former Financial Times journalist, Balls was influential in preparing some of New Labour's most eye-catching early economic initiatives - including independence for the Bank of England. An economic adviser to Gordon Brown from 1994 to 1999, he went on to be chief economic adviser to the Treasury from 1999 to 2004, and then economic secretary from 2006 to 2007.
So New Labour's record on tax seems a good place to start our interview. In 2005, Tony Blair summed up his relaxed approach to the rich getting richer by telling Jeremy Paxman: "It's not a burning ambition for me to make sure that David Beckham earns less money." Balls winces at the memory. "I think many people in the cabinet and the Labour Party were uncomfortable with those comments . . . I don't think that's where our politics and our values take us. I don't see how you can be comfortable, ever, with a widening gap between the rich and the poor, because that undermines not just the justice of our society, but its cohesiveness."
Does he accept his share of responsibility for the stance against City regulation? There is a long pause. "You're right," he says eventually. "In that period, I hugely regret that we weren't tougher on regulation - we definitely should have been. But the prevailing mood was so much in the other direction that it was a fight to hold our own."
He wishes Labour had drawn clearer dividing lines on "investment v cuts" during the election campaign, and points out that, as education secretary, "I went out of my way to try to show how you could make an argument, 'Yes, we're making savings . . . but we can have rising school spending.'" In taking this line, he implies, he was on his own. "The trouble was, unless the government was making that argument . . . it wasn't really an argument we were ever willing to put with any confidence." Balls condemns those in his party who argued that "the public don't want to hear about dividing lines". He goes on: "I went through this long period when, whenever I would try and set out the differences between the parties, people would say, 'Core vote, class warfare.' Then we got to the election and suddenly there are all these people saying, 'You're all the same.' Very frustrating."
Today, Balls claims that he was opposed to Darling's plan to reduce the deficit in four years, but concedes that he did not make a case against it at the time. "No . . . Through the whole period that Alistair was chancellor, I never ever said, in public or in private, anything that was other than fully supportive of him. Now that we're out of government and other colleagues are going into detail about what they thought, I think it's different, totally."
The leadership contest has been gruelling, and there is some way to go yet. Balls argued that the race should go on until September, but his eyes betray tiredness - inevitable after dozens of hustings. "Let's be honest, the whole thing is pretty weird. We've been doing it for so long now, I continually have to resist the temptation to give Ed Miliband's speech rather than my own, I've heard it so many times."
Was he surprised that Ed ran against his brother, David? "Yes, it was a surprise. I think: up to him, up to them. It doesn't make me feel uncomfortable, but that's probably because I don't have to sit in the audience." Unexpectedly, he compares the Milibands to himself and his wife, Yvette Cooper, whom many saw as a potential leadership contender. "Yvette and I had our conversation and decided it would have been weird [to run against each other]. In fact, I don't think we really needed to talk about it."
Whether or not he is suggesting that the Milibands' own situation is odd, the brothers are both strong candidates. In David's favour is the possibility that the Tory-Lib Dem coalition could fall apart in a year or two. Labour would then need to be ready with a prime-minister-in-waiting. But Balls, who perhaps knows that he is seen more as an opposition leader than as a potential PM, predicts the government will survive longer than that. "Longer than we would like, is my fear, because I think once you've decided as a group of politicians to throw in your lot in the way the Lib Dems have . . . it's not very easy to then ditch power to re-establish a bunch of principles you've already shown were for the birds." With a rare, if rather fixed smile, he says he cannot understand how Lib Dem ministers "sleep at night".
Disillusioned Lib Dem voters might well agree with him. Nevertheless, Balls faces the criticism that he is not enough of an instinctive pluralist to win them over to Labour. His apparent opposition to the planned referendum on the Alternative Vote (AV) has been seen as one example of the problem. Balls confirms he is "very" - he says the word five times - "wary". He cites concerns over the timing of the referendum and the government's plan to couple it with a redrawing of constituency boundaries, which would reduce the number of MPs and threaten safe Labour seats. "The idea that we do this on local election day [in May 2011] - which we all know is Nick Clegg's moment of truth on local government cuts - plus as part of a package that means the most undemocratic attempt to gerrymander boundaries . . . What can I say?"
Balls has been criticised for his tough line on immigration. He makes a strong moral and economic case for his position, but then argues: "There is a view some people try to put around that we lost in the south because we didn't do enough to appeal to Middle England, to upper-income voters. That's not what happened. What happened in the mill towns of Lancashire was what happened in the Medway towns of Kent: it was about lower-income voters feeling Labour wasn't standing up for them."
Unafraid of taking a firm stance on an issue, Balls is also arguably unrivalled in his ability to take the fight to his opponents. If there is a flaw in his profile as a potential leader, it is that he lacks a clear vision for Labour's future. But he argues that the leadership contest consists of three stages: looking back, tackling the coalition and, lastly, looking ahead. This final stage, Balls suggests, should largely take place after the new leader has been elected.
“The choice people have got to make is not who's got the better vision or values. It's actually the job - of leading the opposition and being prime minister - and who can take the enormous strain and the heaviness from the right-wing press, and not get knocked over by it."
As he says this last sentence, it strikes me that while Balls may indeed lack vision, he is the only candidate willing to take on the voices of the right - including those of the Tory-supporting media - in those unqualified terms.
If you weren't a politician, what would you be?
A teacher, or in a leadership team in a school.
What's your favourite meal?
Roast beef and Yorkshire pudding.
What does God mean to you?
I'm firmly in the Church of England tradition - which is to say that it's perfectly acceptable to grapple.
Have you ever taken drugs?
What's your most valuable possession?
A first edition of Keynes's The Economic Consequences of Mr Churchill, published in 1925.
What would you consider your biggest fault or character flaw?
Being late for meetings.
Who would you advise your supporters to give their second preference to?
I'd advise them not to rush to early decisions, but to make their minds up carefully and not make a decision until 1 September.