Rachel Maddow. Photograph: Virginia Sherwood/NBC newswire/AP

America's latest media darling - not counting the one in the White House - is kneeling on the floor like a big kid, scribbling notes on a scrap of paper. In eight hours' time 35-year-old Rachel Maddow will be sitting alone, in front of an autocue, about to present the highbrow political television programme which bears her name.

For the moment she's brainstorming ideas with her production team. What should they focus on? President Obama's proposed closure of Guantanamo? The 45,000 jobs lost before nine o'clock this morning? The banking meltdown? More troops in Afghanistan? There's a level of intense and nerdy discussion in the room which makes John Humphrys and James Naughtie look like Ant and Dec. This isn't the kind of cheery material for which American television is generally known. But these are changing times, and The Rachel Maddow Show has become an unlikely hit.

Rachel Maddow - in her own words a mannish lesbian policy wonk who doesn't own a television set - is not your average anchorwoman in America, or indeed on this side of the Atlantic. Later today when she goes live on air she must swap her Red Sox T-shirt and baggy Levi's jeans for what she calls "lady clothes" - a bland slate-grey trouser suit. (She won't say who it's by for fear of insulting the designer.) Her chunky Eric Morecambe glasses will be exchanged for contact lenses (which she's still getting used to). Reluctantly, there will be the merest smear of lipstick and blusher. She will, however, cling on to her trainers, safely out of sight under the desk. (The stylists at American Vogue recently offered her an array of extravagantly high-heeled Louboutin shoes for a photo shoot. She insisted on a pair of Converse boots.)

With their ironed hair and shrink-wrapped foreheads, women on American television news programmes all too often come across as shrill harpies or eye candy. Maddow seems to have both types on the run. She goes on air as if she's got nothing to prove, this despite the fact that she'll have done hours of preparation: a cheerful and clever geek, fluent in irony, alternately idealistic and sceptical, who doesn't believe in talking down to the viewers. Her aim? To raise the level of debate in America so that the right kind of decisions are made in the future.

From the beginning of her show, she made a policy decision to avoid the kind of bar-room slugging match which political programming in the United States is famous for. "I did that Punch and Judy stuff for so long on other people's shows. It may be kinetically entertaining but I don't think you learn very much in that environment." She's opted instead for arch analysis and informed banter. It's set a rigorous tone that has been surprisingly popular, somehow in tune with the tenor of the new administration. Within a week of the launch last September, she'd more than doubled the viewing figures. True - her timing couldn't have been better - everyone's viewing figures were enjoying a spike. However, she was regularly beating her CNN rival Larry King Live, a cable-news institution, in the sought-after, and elusive, 25 to 55-year-old demographic.

In the same way that Ann Coulter emerged as the public face of the Bush era, Maddow swiftly emerged as the go-to media figure in the most picked-over election in American history.

Suddenly on the net everyone was talking about their new girl crush. Here was someone on television with whom they connected; who, in the cosy familiarity that comes from being on screen five nights a week, they imagined that they could be friends with. They liked the fact that she can mix a mean cocktail, never shops for clothes and is more interested in her pick-up truck than lunchtime Botox injections. "My husband is in love with Rachel Maddow, I am in love with Rachel Maddow, my eight-year-old son is in love with Rachel Maddow," gushed one girl fan. Gay, straight, parents, teenagers, rabbis, Republicans: everyone, it seems, was smitten. One enterprising site began selling "Why I'm Gay For Rachel" T-shirts. As one commentator on Salon remarked: if the election campaign was indicating that America could be proved post-racial, maybe it could be post-gay too.

No one is more surprised by the attention than Maddow. "People are kind and flattering," she says, "but it is a remarkable and weird part of my life that I try not to dwell on. I try not to let it swell my head."

Two years ago she was a respected liberal pundit on the political news circuit, with a reputation for cheerfully slapping down ageing Republicans, but she didn't think she'd ever land a regular job on television, let alone her own show. She used to joke that she had a face made for radio. "I'm a big lesbian who looks like a man. I'm not Anchor Babe and I'm never going to be," she complained at the time. "I one hundred per cent believe that the reason I have not gone further in television is not only because I'm gay but because of what I look like."

Even now she says: "I don't honestly know why MSNBC hired me. I guess we got good ratings. We didn't suck. All of a sudden it seemed I might be able to co-host sometimes." The emphasis on the way she looks remains a frustration. (Under pressure to glam up, she's joked in the past that programme makers "would brooch me if they could".) She says: "I get exasperated. I don't think very much about my appearance and when I do it's not, how would you say... strategic." She laughs. "But it's a visual medium and it really does matter what you look like. I don't care and I don't want to have to care. I want my appearance to be beside the point. My goal is to do the physical appearance stuff in such a way that it is not comment-worthy."

The irony is, perhaps, that part of her success is exactly because she doesn't look like all the other cookie-cutter women on television. She is 6ft tall and boyish (so much so that for a radio show stunt she was once persuaded to go out onto the street and ask people to guess what gender she was). "It's true. I am an unusual-looking person. Maybe people tell themselves: 'Well, she isn't getting by on her looks, for sure. Maybe she's got something to say.'"

The woman I meet after the show has reverted to skateboarding third grader as opposed to television personality. However, she's actually far more lovely-looking than she likes to make out. The make-up has been scrubbed away once again; the sensible grey jacket has, quite possibly, been rolled up into a ball and abandoned in the corner of her dressing room. She's been on the go since five o'clock this morning - before she starts preparing for her TV programme she will have recorded her daily show for the Air America radio station - and she is, she says, a little bit wired.

Only four weeks into the new television show Barack Obama's team called up saying that the Democratic candidate was willing to give her an interview. (She describes the new president as calm, confident, wonky. "I like wonky in a politician.") Last month she was part of the channel's high-profile inauguration team in Washington. "It's a pressure, now, having a hit show. It's the notion that people are really paying attention," she says. "A lot of the way I have proceeded through my broadcast life has been to pretend that it's just me and my conscience. I would study the material and essentially write an essay on a scrap of paper and memorise it. The fact that I am starting to cotton on to the fact that people can hear me is unsettling."

While she's eminently likeable - self-deprecating, quick to belly laugh, unfailingly polite, chummy even - I suspect she finds interviews a whole new layer of public exposure. She'd far rather ask a question than have to answer one. Ten years ago she was an academic and committed Aids activist, a graduate of Stanford and a Rhodes scholar, trying to finish her doctorate of philosophy in political science at Oxford University. She seemed, if anything, hellbent on a life as a serious campaigner and academic, as far from the media spotlight as possible. When she was awarded the Rhodes scholarship it's said she dyed her hair blue as a sign that she wasn't part of the establishment. At Oxford she disliked rarefied collegiate life so much so that she escaped to a squat in a basement flat near Arsenal football ground in London and took a job for an Aids charity. Back in America she relocated to a particularly remote part of Massachusetts, where she hoped she'd be miserable (she's still based there): "I figured if I wasn't happy I would get my doctorate finished."

To make ends meet she took odd jobs. "I was a waitress, bike messenger, bucket washer at a coffee-bean factory, yard help, landscaping labourer, handyman - I was very bad at that. I went for a job at a video store and got turned down because I didn't know enough about movies."

It was a friend who persuaded her to go for an audition as the sidekick for a morning show at the local radio station for a dare "and because we thought it might pay more than the minimum wage". As soon as she was on air, something clicked. "I'd never done anything like it before but I can remember thinking: 'I like this'." In the early days the station would make her do stunts like dress up as an inflatable calculator (how this worked on radio I'm not entirely sure). Ten years later she's being courted by the likes of Vanity Fair and Newsweek. She's clearly relishing having her own serious platform on which to debate the issues of the day, but the last thing she seems to be interested in is being on television for the sake of it. "I don't think being on television, in and of itself, has any value. The only reason to be on TV is to say something worthwhile."

While plenty of people in the media are coy about revealing their sexuality, not being out was never an option. "I've been out the majority of my life. I could not go back in," she says. "It would have looked very strange. It wasn't something I was going to lie about." As it happens, being at ease with who she is seems to have worked in her favour, because it comes across on air. However, it was not always thus. She grew up in Castro Valley which (despite being near San Francisco) was, she says, a narrow-minded kind of place where racism and homophobia were rife.

"I was a weird, depressive little kid who never really thought they would get to be an adult. I never thought I'd reach drinking age." Her father, a lawyer, had been in the air force, her mother was a school administrator. She doesn't recall the household being particularly bookish or academic. Her parents remember a daughter who somehow taught herself to read by the time she was four. As a teenager she had hopes of being an Olympic athlete until a run of injuries forced a rethink and she turned her attention to political activism instead. "I wouldn't wish a torn biceps tendon on anyone, but it was a blessing in disguise."

Before she knew she was gay, she'd volunteered at a centre for Aids services. This was 1987, six years after the first cases of Aids and HIV had come to public attention, at a time when the San Francisco area seemed to be at the epicentre of the tragedy that was unfolding. "I had a very acute sense that something was happening to 'my' people even before I knew I was gay. I was very moved by what was going on. Growing up in the Bay Area as a gay kid was devastating. It defined the world in a very serious way for me, in a life-or-death sort of way. I had a lot of older friends and many of them died. There was a sense of: look, your life is happening now. This may be all you get."

Maddow came out at Stanford, aged 17, six months after she realised she was gay. "I knew there was something. My whole childhood I knew there was something, but I didn't know it was that." The only other woman she knew who was gay was the daughter of a Liberian fundamentalist Christian minister. "I thought: if she can do it, so can I." However, appalled by the casual homophobia she'd encountered on campus, she outed herself by broadcasting the fact on handmade posters which she pinned up in all the bathrooms in the student accommodation in the sure knowledge that by the end of the day everyone would have seen the poster at least once.

"It was confrontational, funny, theatrical. On-my-own-terms aggressive." The student newspaper picked up the story; the next thing she knew someone had sent a cutting to her parents, who she hadn't yet told.

"By the time I got home it was really bad. But they are great now. They could not be more supportive. They talk to my girlfriend every day."

She met her partner Susan Mikula, a photographic artist, 10 years ago, when she was still broke and trying to finish her dissertation. Mikula, who is 15 years older than Maddow, had just bought a run-down, ramshackle house and needed help doing it up. "I drove out there, she answered the door, and it was love at first sight. I had never had a monogamous relationship. I had never wanted to. But this was different. We both had to extricate ourselves from other things. Fortunately it was mutual." Their first date was at a firing range. Despite living in Massachusetts, where gay marriage is legal, they have no plans to get married. "I believe in the right. I just don't think you have to exercise it. I like the idea of a subculture," she says, and pulls a face. "I'm not a weddings person."

The Rachel Maddow Show is plainly absorbing and all-encompassing - "I'm motivated by fear of failure and a desire to make my existence useful" - but if Mikula asked her to give up her media career, she would, she says. For now, her workload means that the couple divide the week - four nights together, three nights apart. On top of everything else, there's a book to write about America's relationship with war. Weekends are spent in the house where they first met. It's the only place, she says, where she really switches off. During the week she stays in their tiny tenement apartment - the size of a van, she says - in the West Village. She's lived there for four years and is rather proud of the fact that she's yet to use the oven, which she uses as a makeshift bookshelf instead.

Friends wish she'd find an apartment with a doorman, because for all the positive fan mail she receives there has also been an anti-gay reaction to her new high profile. "Most homophobic stuff has a violent tone to it. It's like: 'I'm going to kill you.' I get that all the time. I work predictable hours, so if someone was going to be really creepy..." She pauses. "I don't enjoy being contactable. I purposely make it hard to contact me. I don't have a landline phone. I don't have an office phone. I don't open my own mail." All evening strangers have come up to congratulate this big-hearted, serious-minded woman on her show. The buzz around the media's latest darling seems to be real. As for reports of a post-gay America - they may be, for the moment at least, greatly exaggerated.