Harriet Harman: So, farewell then, acting leader of the Labour Party

It took her 30 years to get to the top of the Labour Party but, after just 137 days, her reign is over. Harriet Harman took a final wander through the grand rooms accorded to the Leader of the Opposition in Parliament last week, looking almost wistfully at the view across the Thames, before vacating in preparation for the arrival of the new Labour leader. She will return to the other end of the shadow cabinet table with her colleagues next month, but at least she will have the consolation of having reached the top of a particularly treacherous career ladder.

But only as acting leader. The term sounds so, well, second-rate: a job for stand-ins who never quite made it to the top under their own steam; an administrative position, with a requirement not to rock the boat.

Harman bristles, an irked schoolteacher crinkling her nose at the perceived slight, and then explains why no one should belittle her role.

"I was very determined to avoid a sense that there would be a hiatus before the new leader [arrived] and the Tories would get away with murder because we would be like there was no leader, like we were in suspended animation," she says. "It was very important that we were determined and forensic in our opposition and we didn't give them a breathing space to have a honeymoon. That was really my task.

"I'd seen people's heads go down after an election defeat before, and felt it was very important that we rallied quickly and the party galvanised itself.

"So," Harman leans back slightly, a hint of a self-satisfied smile, "when you say 'administrative', I think the answer is 'no'."

The truth is, the MP for Camberwell and Peckham is inordinately proud of her performance during a period that could have developed into a crisis once Gordon Brown headed for the hills in May. Even her most trenchant critics have conceded that she has – unexpectedly – held her own in her parliamentary confrontations with David Cameron at Prime Minister's Questions.

"I've seen in the past, where we've been in opposition, that we haven't felt confident in the chamber and it had a sort of ripple effect," Harman says. "I felt I had to succeed at PMQs and be really determined. It's very overwhelming in terms of the noise and the crowdedness.

"When you get into opposition, you've got all these new MPs and they don't want to see the front bench falling down, but making sure we get our arguments clear and we don't let them crush us."

Harman, a local MP roundly derided for turning up on the streets of Peckham wearing a flak jacket – "we were going out with the police and they routinely put a jacket on you" – slides seamlessly into a riff about how she could not disappoint her constituents, the people who voted for her. It is the standard opening bars of a leadership speech – but then Harriet Harman never wanted to be leader, did she?

The nose crinkles again. "This is a very, very important period and I could have either stood for leader or done this acting leader [job]," she says, when asked whether, just once in the past 130-odd days, she had regretted not standing for the leadership. "If I'd been standing for leader, there isn't anyone else elected who could do it [acting leader]." So the party needed her, more than it needed the "very good and able people who could have stepped in", such as Jack Straw and Alistair Darling? As an answer to the "Vince Cable question", it is not wholly convincing.

Perhaps her reticence has more to do with what she would have been letting herself in for if she had tried to test her popularity in an open contest. Something about Ms "Harperson" attracts a peculiarly vitriolic reaction from elements outside the Labour Party. Her detractors are enraged, above all, by her championing of equal opportunities reforms. But even within the party, the decision – with her fellow MP and husband, Jack Dromey – to send two of their three children to selective schools has provoked lingering hostility. Diane Abbott's leadership campaign, disfigured by constant questions about her decision to send her son to the fee-paying City of London School, is a warning of what Harman could have walked into.

"I think Diane's done a great job as candidate," Harman says. "In a way, if you've been in a high-profile situation for a long period of time, people make their own minds up about you, and that's what they've done with Diane. People have kind of heard what I've said over the years and more or less made their own mind up along the way... so if they read in the paper some commentator saying 'she's rubbish', I don't think it affects their view."

It is delivered in a determinedly matter-of-fact fashion, but Harman, who turned 60 in the summer, has been bewildered by the unremitting antagonism she has faced throughout her career. Her political roots are in the left of the party but, despite her protestations ("my background is middle class rather than landed gentry"), her upbringing was undeniably posh. Harman's father was a Harley Street physician, her uncle was Lord Longford and she attended the independent St Paul's Girls' School. But it is her tenacious manner, sometimes criticised as "shrill", that is cited by some colleagues as her worst political impediment.

Nevertheless, she did manage to beat the odds and win the contest to become Brown's deputy. "I remember one commentator writing that it was hard to know who thought it more ludicrous that I should put myself forward – both the Blairites and the Brownites thought it was ludicrous. People can make very scathing comments, but I won."

One of the more scathing comments recently was Tony Blair's assessment that she was "not really a policy-wonk" during her fractious period as secretary of state for social security, alongside Frank Field in 1997. Harman is an instinct politician. After her victory at the Peckham by-election in October 1982, she witnessed at first hand an ousted Labour's descent into factionalism, chaos and almost two decades of opposition. "I don't think we should lurk in the basement of a think-tank and look at graphs and then work out where the entry is to rush in," she says. "Politics is about how people feel and their concerns. We have an identification; but it's not an electoral tactic, it's because of our beliefs."

She points to an "amazing surge" of 35,000 new Labour members since the party's defeat in May, many of them from the Liberal Democrats and, significantly, many of them returning Labour supporters who had been outraged by the Iraq war.

She says: "A lot of people who care about tackling unemployment, who care about public services, who care about fairness and inequality, and who basically have been long-standing Labour [supporters] did stop voting Labour or leave over the Iraq war.

"In a way, the Tory-Lib Dem government has provided the opportunity for them to say: 'We're coming back now because we've got this government and we want to be sure it's as short as possible.' We have to understand our strengths and our weaknesses over the past 13 years."

So, at last, to the weaknesses. Harman has been accused of agitating behind Brown's back during his darkest days, particularly during the abortive "coup" attempt in January this year, so now should be the time to tell all.

"There's nobody that doesn't make mistakes, in every walk of life," she says. It is the classic tee-up – a general qualification, pointing the finger at all of us – before moving in for the kill. "I saw very much, at close hand, how Gordon reacted when there was a feeling that the global economy was on a cliff-edge and we couldn't see the bottom. His steely focus and calm was something which was incredibly important not just in this country, but internationally."

This does not sound like a Brown blunder. Harman, the consummate politician, has closed ranks, moved on. The enemies now are the members of the coalition government ranged against a Labour Party regrouping under a new leader.

"I see the Lib Dems in two different ways. I see the leadership; and then I see the members and the councillors and the voters, who are victims of having supported a manifesto that said don't cut too fast and too far. Suddenly they find themselves supporting Tory policies. The Tories are going to roll them over.

"One thing we need to learn is you look inward at your peril when you go into opposition, and you become divided at your peril."

Curriculum vitae

1950 Born in London to the Harley Street physician John Harman and his wife, Anna. Attends independent St Paul's Girls' School.

1972 Graduates in Politics from York University. Becomes legal officer for National Council for Civil Liberties.

1982 Elected as Labour MP for Peckham, south London. Marries the union activist Jack Dromey.

1984 Promoted to Labour's front bench as spokeswoman for social services, then health.

1992 Becomes shadow Chief Secretary to the Treasury; later shadows employment, health and social security.

1996 Criticised for sending son to a selective Kent grammar school after eldest son went to a London grant-maintained school.

1997 Becomes Social Security Secretary, tasked with reforming the welfare state. Sacked the following year.

2001 Back on the front bench as first female Solicitor General

2003 Fined £400 and banned from driving for seven days after doing 99mph on motorway.

2005 Becomes minister of state in Department for Constitutional Affairs

2007 Wins deputy Labour leadership. Campaign part-financed by £10,000 personal loan and £40,000 mortgage extension – leading to a parliamentary inquiry. Becomes leader of House of Commons. £60 fixed penalty notice and three penalty points for exceeding 40mph limit.

2008 Receives further three-point endorsement for speeding in a 30mph zone.

2010 Pleads guilty to driving without due care and attention after striking another vehicle while using a mobile phone behind the wheel. Becomes acting Labour leader after Gordon Brown's resignation.