Oona King

Yet this time her opponents are even more formidable. If King wants to become mayor she will have to take on not just one but two of the best-known personalities in British politics, men who make Galloway look like a political minnow. First she will have to beat Ken Livingstone, who also wants to stand as the Labour candidate, and if successful she will have to defeat the incumbent mayor, Boris Johnson.

To understand her motivation, one needs to go back to 2005 when her fight with Galloway was at its height. It was around then that she hit the lowest point of her life. This was not the day she was confronted by teenagers in a Respect stronghold screaming that they wanted her dead. Nor was it when she had her car tyres slashed. It wasn't even the rumours circulating about alleged affairs.

It was being told in the midst of the campaign that she had failed her fifth and final attempt at IVF. "Politics never made me fall to the floor and sob," she says, describing her desolation. That same day King had been asked by the health secretary to attend the opening of a family planning clinic that provided abortions for teenagers. "I remember walking in and seeing huge posters of women with babies saying, 'make the right choice for you'. I just kept thinking 'choice, choice'," says King – spitting out the words. "I thought, 'I have no choice, I'm infertile'." She didn't tell her staff, not wanting her personal trauma "writ large".

Perhaps it was this personal pain that made her more able to bear the viciousness of the Respect campaign against her. On the day the election result was announced, King was surprised by her reaction. "I was shocked, but my second emotion was a calm, quiet peace and strength. I had been worried that I would get emotional, wobbly, not able to walk on to stage. Losing your job in front of 20 million people is hard."

It could have destroyed her politically, but it didn't. "I never felt broken, so I emerged a stronger person," King says. "Nothing could be as bad as Galloway. I don't think Ken or Boris would even know how to behave like that. I have done it once and I know how to survive." So, after five years out of the political spotlight (during which she worked for Gordon Brown, wrote a book and adopted her two children), King is back.

Ten members of staff sit at laptops in the vast office in east London that is being used as King's campaign headquarters. Her own office is at the top of a spiral staircase, overlooking Heron Quays, a glittering stretch of water. Just out of sight are the towering skyscrapers of Canary Wharf.

She leans forwards as she speaks, occasionally bursting into laughter. She is convinced she can beat Livingstone and win the Labour party's nomination. But King faces two key obstacles. The first is what her team call the "three-letter brand" - Ken. Livingstone is much loved among Labour members (despite hits to his reputation during his second term as mayor). The second is King's own branding by some critics as a "Blairite" with centrist tendencies.

She prefers the term "pragmatist". King says there is no point boasting that you are "ideologically pure" if a decision - for example, to reject the use of registered social landlords in providing housing - results in families being stuck in slum conditions.

She says that Livingstone repeatedly claims he would reject all spending cuts - supporting instead moves to cancel Trident, raise the top rate of income tax to 60% and cut down on tax evasion. "My response to that is the mayor of London does not control Britain's nuclear submarines or determine the tax regime or control the Inland Revenue," she says. "So it is fantasy politics."

"Being located in the real world" does not make her rightwing, she adds, stressing her trade union background. "I have always described myself as someone on the left. For me that is about redistributing wealth. I passionately believe in the founding principles of New Labour, that you can't redistribute wealth unless you create it in the first place."

She dismisses the division into Blair and Brown camps as out of date. But when pushed she says the split has been "caricatured" as choice versus equality - and she would always be for the latter. "So if you do want to use those old-fashioned terms of Blair and Brown, then that puts me in the Brown camp." That leads to the question people keep asking but she has so far refused to answer. In the Labour leadership battle she has said that she supports "the Milibands", but which one? It is hardly surprising that it is a difficult choice for King - who has known the two brothers since childhood when she was sandwiched between them at the north London Haverstock comprehensive -sometimes called "Labour's Eton". David was two years above her and Ed (who has claimed that he wasn't cool enough to be her friend) two years below.

"I've spent time at both of their houses, round both of their kitchen tables, and also with both of their partners, who I like immensely, and it is difficult for me to not back either one of them. However, for the last three years I have been more drawn to one in political terms and that is Ed."

King says coming out on the issue has been particularly hard because her team are split, with the chair and manager both backing David. "I owe David more on a personal level," she adds. "When I was having trouble adopting I went to David as someone who had experience of having children who were adopted. He put me in touch with his social worker who became my social worker who, without a doubt, broke the bureaucratic logjam that delivered my first baby into my arms."

So she would be proud to serve either Miliband. But it is Ed who inspires her. "I remember the exact moment I became an Ed supporter – at party conference in 2006. I was walking across a hotel foyer and he was giving a speech so I stopped to listen. I saw him lift a room in a way I did not know he could do - it took my breath away." She was so moved that as soon as Gordon Brown resigned she rang him and urged him in a 45-minute phone conversation to run.

A cynic might say that backing Ed - seen as to the left of his "Blairite" brother – is a politically astute move . After all, she is trying hard to convince Labour members about her true ideology. That is why she is following such a gruelling schedule - last week there was a hustings in Dagenham, a visit to a Tesco store in Twickenham, an afternoon at the Olympics site, trips to bus companies in Hackney and New Cross, and Labour branch meetings across the capital - often followed by drinks late into the evening. And much more. She is working in the "middle of the night" so that she can have at least some time to see her children, aged five and three.

On Tuesday evening, at a branch meeting in Clapham, she spoke of how she wants to revolutionise London transport - with a "quantum leap" in levels of cycling. If elected she will reinstate the western extension of the congestion charge and use the money to keep bus fares down. "What Boris has done is give money to his mates in Kensington and Chelsea and at the same time raised bus fares by £110m last year alone," she says. As for the Boris bikes - King says it was Ken's idea anyway and all Johnson did was turn the cycle lanes from "red to blue".

She tackled Iraq that night – as she often does – and claimed her motivation had been the "genocide" taking place under Saddam Hussein. But she admits it was a mistake.

She says her leadership style is to "listen" and has already turned a suggestion into a policy (to extend the use of London's public transport Oyster cards to taxis). King also wants to focus on knife crime, attacking Johnson as a "do-nothing mayor" who has allowed violence to increase since taking over.

"I do like his gags. But I think the joke is wearing thin," she says. It is up to Labour members to decide if King should be the one who tries to silence Johnson. She believes she has a better chance than Livingstone, because the popular former mayor will struggle to win back suburban Londoners who deserted him in the last election.

As for Labour members, she had a little success on Tuesday night in Clapham. "I thought she was vacuous before I went in," says Peter O'Connell, a former Labour councillor. "I came into the room a Ken supporter, but I came out for Oona."

Written by Anushka Ansthana and published in the Observer on 22 August 2010