For someone reportedly "sidelined" from within during Labour's election campaign, the deputy leader, Harriet Harman, is busy. She is keeping her "ear to the ground" around the country and in her Camberwell and Peckham constituency, and reporting back to the party's headquarters in Victoria Street in Westminster.

“I'm playing a very active role," she says. "I'm out energising the troops. Well, they don't really need energising. I'm out there mobilising. This is going to be very much an on-the-doorstep, at-the-school-gates campaign. The debates are very important but there is also the person who is on the doorstep, who is on your side and knocking on your door."

Harman may now be found "on the doorstep" more than on television and at press conferences, but her position within Labour has remained strong since she won the race for the deputy leadership in 2007. Having begun that campaign as an outsider with very limited financial backing, she defied expectations in Westminster by beating Alan Johnson to the post. It was quite a comeback.

She had been sacked from Tony Blair's first administration in 1998, possibly with the encouragement of Gordon Brown, because of a dispute over welfare reform. Her aristocratic family connections - she is the niece by marriage of the late Lord Longford - had infuriated some in the party, including Blair's chief adviser, Alastair Campbell, while her dedication to the fight for equal rights alienated others in New Labour's more laddish circles. For almost a decade, Harman languished on the back benches. Now, she is talked of as a leadership contender if Labour loses power on 6 May.

On that question, she is emphatic: "I'm very clear what my obligation and also the privilege of my role is, and it is to be supporting the leader and to be deputy. I don't think you should use being deputy as a stepping stone, because it would put your head in the wrong place: your head needs to be completely on what you're doing. It's not like a staging post for me."

This corroborates what everyone who knows Harman says: she is telling the truth when she says she doesn't want to lead Labour. But would she like to remain deputy under a different leader in the future? "We're not planning to have a different leader. We are planning to fight a good election, win the election and then continue with Gordon as prime minister."

Harman seems to believe the election is winnable. "The debates have energised things and the biggest effect of the Cleggmania is that it is causing Cameron to collapse," she says. "What is quite evident about the campaign is that somehow the debates have created a sort of leap past all the cynicism of the expenses and the 'you're all the same' attitude. It has jolted things into a different kind of a zone, which is all to the good because the weary cynicism is just absent, full-stop."

“Found out"

The choice between the two main parties, she says, is clearer at last: "There is a sense out there of substantive difference between what we're saying and what the Tories are saying: the choice. We are saying 'secure the recovery' and 'the many not the few'; with the Tories, on the other hand, people see that one minute they come out with 'the big society', and the next day it's a big flop and it's gone. They see them streaking across the sky with the married tax allowance, then they see through it.

“They have a big blast on National Insurance contributions, then they realise that out there people are interpreting that as a really big warning sign about cuts. And then they see, in the debate, their man who is supposed to be doing brilliantly, really not. So I think there's a kind of steely determination among our supporters and they are going to fight hard. The campaign is very grounded."

Harman believes that Cameron is being "found out". Watching the first debate, she says, "I thought it was quite striking. You could see from the expression on David Cameron's face, his sort of petulance and the fact that it was not going his way. You could almost see the thought bubble coming out: 'It was not supposed to be like this'."

She implies that Cameron has been "spoilt" by an easy ride in the media. "He's had lots of shelter from scrutiny, he's had lots of unwarranted media support, he's had pots of money and none of it worked for him because it's the substance, that's the point."Harman continues: "I've never believed the Cameron myth. I've sat there opposite him, sitting next to Gordon in Prime Minister's Questions, and I've always thought two things about him: first, that he has a sense of being born to rule, entitlement - it's like 'my right to be in Downing Street' - which I find quite objectionable. He's arrogant. The other thing is that where we've been clear about what needs to be done, he's been tactically dodging all the time - and it doesn't add up to anything that could address people's worries."

Will she predict victory? "I don't ever predict; I would never second-guess the voters because it's disrespectful." Instead, she will say only: "There is a sense among people out there that it's a close election; that's what people seem to think. It's going to be close."