Harriet Harman is a militant in the "lilac revolution". She has even coloured her new website lilac in preparation. Her campaign for Labour's deputy leadership is infused with what once would have been called political correctness, but has now entered the mainstream: the fight for women's equality, gay rights and anti-racism. "With Ségolène Royal in France, Hillary Clinton in America, the first woman president in Liberia, another woman in Chile, politics is changing for ever. The idea that you have men talking about equality for women, those days are gone. It's a very significant moment for somebody like me who fought for this and seeing people agree. The spirit of our times is equality," she says.

Harman uses the current Channel 4 race controversy to illustrate her point: "I think it is significant that 40,000 people rang in to complain about Celebrity Big Brother." She cites a Bangladeshi friend from east London who faced open hostility in the supermarket after the events of 9/11. "In that same supermarket now, she's got people coming up to her and apologising, saying: 'Actually we don't think that's all right.'"

That same spirit, she says, infuses a new public attitude to sexuality, which is why the government has introduced civil partnerships and rules to outlaw discrimination on grounds of sexual orientation in the provision of services. This has brought it into collision with the Catholic Church, which believes its adoption services should be able to deny children to gay couples. As the constitutional affairs minister responsible for the new regulations, Harman is resolute. She quashes talk of a compromise said to be backed by the Communities Secretary, Ruth Kelly, a Catholic. "We will stay true to our commitment in tackling sexual discrimination in terms of sexual orientation," Harman says. She adds, in a tart rebuke to colleagues: "You can either be against discrimination or you can allow for it. You can't be a little bit against discrimination." She insists she would not budge on the regulations.

Whether the Labour Party is ready to be painted lilac is another matter. Harman has made gender a major issue in the election. She points to polling evidence that puts her well ahead of the other four (male) candidates with the public. (Other polls put Alan Johnson and Hilary Benn in the lead.) Harman knows from experience that elements of the party remain brutish. As social security secretary in Tony Blair's first cabinet she was undermined by colleagues and special advisers. The campaign culminated in the Prime Minister's decision to sack her in 1998. After a long period on the back benches, she returned to prominence as the first woman solicitor general after the 2001 election and in 2005 became a minister of state in the Department for Constitutional Affairs. If she succeeds in her latest battle, it will mark one of the most remarkable comebacks of the new Labour era. She is still bruised by the experience. "I wouldn't wish that on anybody. I don't want to sound like one of those people in Hello! magazine," she says, "but you do learn when you get a knock back."

Apart from the equality agenda, the only other time Harman becomes passionate is about spin. "I did think it was important to be disciplined, loyal, unfragmented and clear [at the beginning]. But I've always found spin abhorrent, because it's duplicitous. It's like pulling the wool over people's eyes. It's wrong in principle and it's also wrong because people end up not trusting you."

For all this condemnation, however, for all this talk of a new openness, Harman often comes across as cautious and wooden. Time and again we ask her to say what she really thinks, to say what she and Gordon Brown would actually do - you never know, to take some risks. When we raise, in passing, the strong media coverage Peter Hain received for his interview with us last week, Harman's body language suggests a combination of disdain and possibly fear. On those big issues about which Hain spoke with such frankness, she is all evasiveness: yes, the Bush administration is not quite her cup of tea, but let's talk about the Democrats; yes, it was good to get rid of Saddam Hussein, but parliament voted for war in Iraq on the basis of weapons of mass destruction . . .

Her own plan

Like Hain, Harman has a four-point plan of her own - her "four points for a fourth term". These focus on public trust, which she concedes has been undermined by the fallout from Iraq and the "cash for honours" scandal. Everyone in Labour, she says, should focus on the following imperatives: never to take for granted the government's achievements; to be sharper in the critique of the Conservatives; to push forward the policy debate; and to rebuild the party.

Only once or twice does she come close to outlining a policy agenda. She believes, for example, that working parents should be allowed to work more flexible hours to avoid "shift parenting". At present, employers are obliged to consider proposals for flexible working arrangements but not obliged to act on them. "You could shift the onus of proof on to the employer to say why they couldn't do it," she suggests. "With the Factory Acts we didn't exhort mill owners to stop employing children, we legislated against it. Because we didn't agree with poverty pay we didn't exhort employers not to pay below a certain level. I don't think you should pass laws unless they are necessary but if they are necessary we shouldn't shrink away from them because there's a big social imperative here." She hastens to add that this is not a policy commitment.

She talks earnestly about the culture of "remittances", whereby immigrants in Britain send money back to their families in their country of origin. "They work often in two or three jobs. They work incredibly hard, they're low-paid, they pay their taxes, they bring up their children and they are the welfare state for their village in Africa." Harman points to the injustice of such poor people paying what are in effect development grants out of taxed income with large charges for international money transfers on top. So does this mean she is proposing some sort of tax relief for them? "Well, I'm not going to say anything about that, no."

On Labour's human-rights record she is similarly hesitant. As a former head of the National Council for Civil Liberties, the forerunner of today's Liberty, she might be expected to have concerns about her government's draconian anti-terrorism legislation, antisocial behaviour legislation or proposed limits to the Freedom of Information Act. Her response is off-the-shelf new Labour. It comes down to the Human Rights Act. "The government has got a responsibility to keep people safe, but we have put the mechanisms in to make sure that if the government does overstep the mark and parliament oversteps the mark by agreeing to something that the government has put forward, then there is a remedy. So I think we have the right checks and balances." The rationale strikes us as bizarre. In effect, ministers are under no obligation to calibrate their actions against the civil-liberties consequences, because the Human Rights Act is there to do it for them. But what about the immediate effect of state actions, and the ethics?

No criticism

Where she does depart from the government line (or rather the Blairite line), is over the issue of the cash-for-honours scandal and its implication for the future of party funding. Harman's husband, Jack Dromey, is deputy general secretary of the Transport and General Workers' Union. He also happens to be the Labour Party treasurer and the man who blew the lid on the secret system of loans set up by Blair's inner circle in advance of the 2005 election, so it is perhaps not surprising that she has strong feelings on the subject. Harman does not join some of her cabinet colleagues in condemning the police approach to the criminal investigation, particularly its dawn knock on the door to Blair's senior aide Ruth Turner. "I think the police have to go about their investigation as they see fit," she says. "They've got to be fair in how they treat people, and whatever the circumstances people have, they've got to deal with them equally. The police have their job to do and they've got to do it. That's what everyone would expect them to do. That's very important."

As for the scandal itself: "I think it has undermined public confidence and trust, and it has dismayed party members. Tony Blair did say that he took the view that it was wrong that the party wasn't told and I think he was right to say that." She supports changes to the law to make future loans disclosable, but is adamant that any cap on future donations should not apply to the trade unions. "I can't understand why some people purport not to be able to tell the difference between 800,000 members of the Transport and General Workers' Union and one millionaire," she says.

The union link, she suggests, should not be loosened further, as some around Blair suggest, but enhanced. "We need to make sure that we work with the trade unions to make sure that more branches are affiliated to local Labour parties. Obviously unions are very important at election time, not just with financial donations, but with people coming out and helping. But actually we need to make it a living link."

In many ways Harriet Harman is the obvious foil to Gordon Brown, not just because she is a woman, but because of other qualities she would bring to the job, such as her record on family issues and the sympathy she has with party members and the wider public. Her lilac revolution indeed chimes with the spirit of the times, as David Cameron has been so quick to realise. But in order for her to reach the political pinnacle she seeks, she needs to be as assertive as she would wish other women to be.

Harriet Harman: The CV

Born 30 July 1950, London
1974 Employed as a solicitor at Brent Law Centre
1981 Found guilty of contempt for disclosing Home Office documents exposing prison "control units". Later cleared
1982 Elected MP, one of only ten Labour women in the Commons
1984 Appointed to Labour's front bench. Succession of posts over next decade
1996 Attracts criticism from Labour ranks for sending her children to selective state schools
1997 Appointed secretary of state for social security and minister for women
1998 Abruptly sacked in Blair's first reshuffle following high-profile disputes with fellow minister Frank Field
2001 Appointed solicitor general, the first woman to hold the title
March 2004 Describes Gordon Brown as prime minister on BBC's Question Time
2005 Appointed minister for justice at the Department for Constitutional Affairs
March 2006 Her husband, Jack Dromey, Labour's treasurer, says he is kept in the dark about loans. Harman gives up ministerial responsibility for party funding to avoid conflict of interest
September 2006 Announces bid to run for Labour deputy leadership
Research by Sophie Pearce