The woman standing on the steps of the supreme court in Washington DC is nervous. A harsh sun is beating down on the white marble, threatening to bleach her out of the picture. She looks very small beneath the classical columns of America's legal shrine, with its legend: "Equal justice under the law."

"I'm scared," she says, glancing at two police officers standing nearby. "You never know if they'll recognise me. Maybe they'd arrest me."

A few minutes later, as we prepare to leave the supreme court complex, the same woman bends over and spits ostentatiously on to its bottom step. "There, I've done it!" she says. "I made sure those guards saw me."

But just a moment ago you were scared they might arrest you, I say.

"I don't care. I'm feeling militant standing in front of this mortuary."

Such is the volatile, confusing and contradictory world of Norma McCorvey. She is, or was, the Jane Roe of the US supreme court's most famous and contentious ruling, Roe v Wade. In 1973, as the anonymous pregnant plaintiff, her plight was presented to the court so that American women could win the constitutional right to an abortion.

For the past 36 years, Roe versus Wade has been a fault line across America, pitting its coasts against its heartland, state against state, woman against woman. And with the killing of the abortion doctor, George Tiller, in Kansas in May, the issue has come hurtling back into the national consciousness, its potential for vicious conflict thrown into stark relief.

For 36 years, Roe v Wade has also been the great divider for McCorvey personally. Abortion has come to dominate her life, carving it into two blocks that are so wildly at odds with each other that it is hard to imagine them being squeezed into the same human frame.

Here's what McCorvey wrote in 1994, when she was proud to be Jane Roe, a supporter of the women's rights she helped to attain: "All over the country the anti-choice fanatics are still at work, still trying to inflict their own religious views on others, still trying to hide their anti-woman feelings, still trying to keep us from controlling our own bodies and our own lives."

Yet here is McCorvey today, on the steps of the supreme court where those rights were laid down: "These steps are covered in blood! 'Equal justice under the law' - what crock! If there is no right for a child to be born, there is no justice at all."

In May, McCorvey was arrested along with 26 other anti-abortionists inside the grounds of the Catholic university of Notre Dame in Indiana. She was protesting against the appearance of Barack Obama, whom she sees as an evil advocate of what she now calls "child killing".

"It was exhilarating!" she says. "When I got arrested, I loved it! I felt like I was high. But it was a God high. I'd never been arrested before. But who better to be arrested for than the unborn children?"

McCorvey was born Norma Nelson on 22 September 1947 in a small town in Louisiana, and raised in Houston, Texas. Her father, a TV repairman, left the family when she was little. Her mother, who worked in a supermarket, used to slap her and call her an idiot.

Norma was in trouble with the law from the age of 10. Aged 16, she married a sheet-metal worker called Woody McCorvey, who turned violent after he learned she was pregnant. She left him after two months. When baby Melissa was born in 1965, Norma's own mother adopted the infant against McCorvey's wishes. Separated from her first-born, she entered into a tailspin of drinking and drug-taking in which she remained, she says, for most of the next 30 years. She bears the scars of those years: her face carries deep worry lines and her voice is grained with years of smoking. As we talk, she pulls up the sleeves of her white cardigan to reveal neat crosshatching over the inside of her wrists, the record of self harm.

Another girl, who McCorvey calls Paige but who she has not seen since birth, was given away for adoption in 1967. Then, two years later, she was pregnant again from a casual fling. Baby Jane Roe was on the way.

Aged 21, with her third child pending, McCorvey was working in a circus looking after freak animals - a giant rat, a five-legged steer, a double-headed snake. "I was a street kid. I lived with my dad part-time and on the streets part-time. I sold flowers on the street corner. These aren't high-paying jobs."

She couldn't envisage going through with another birth. Even now, a committed anti-abortionist, she communicates with burning conviction how she felt then: "I didn't want to bring a baby into the world that I was living in. I never knew where I was going to sleep at night. I never knew if I was going to eat."

She inquired about abortion, only to learn it was illegal in Texas. She was steered towards an underground abortion joint and was shocked. "It was bad. I remember a table in the middle of the room, nothing else. There was dried red stuff all over the floor. It was full of 'roaches. The epitome of filth."

Soon after, she was introduced to young lawyers, Sarah Weddington and Linda Coffee. They wanted to challenge the Texan ban on the grounds that it violated a woman's right to privacy in her reproductive choices. They were looking for a test case, and McCorvey fitted the bill. She did not want the baby, had strong economic reasons against having it, and was so poor she could not travel to the few states that in 1969 did allow abortions.

In the end the case was widened into a class action, but McCorvey remained the anonymous lead plaintiff. Legal battle was joined: Jane Roe v the Dallas district attorney, Henry Wade. The case duly rose up the legal pyramid until, on 22 January 1973, the supreme court, ruling by seven justices to two, slapped down the Texan ban and made abortion legal across the US.

Too late for McCorvey, who, in fact, has never had an abortion. She had her baby in the summer of 1970, three years before the ruling. In her 1994 autobiography, I Am Roe, she describes the harrowing moment when a nurse mistakenly handed her the girl for feeding, not knowing she was being put out for adoption. "There was a flap of cloth over its face. My entire body, my entire soul cried out to me to turn the flap down, to look at my baby's face. But my mind told me that it would be the worst thing I could ever do. I never touched the flap."

She has never seen nor heard of the baby since then, though she has given her biological daughter a name: Mariah. She prays that one day she will come and find her.

McCorvey says the whole legal fight over Roe v Wade "went right over my head". Her only reaction to the news that Weddington and Coffee had won was fear. "I thought, what the hell have I done? I had a liquid dinner that night. I had a lot of liquid dinners after that night. I went into a very deep depression."

She remained disengaged for several years. But from 1980 she began to become more involved in the pro-abortion movement, attending rallies and gradually letting it be known that she was Jane Roe. She started working in abortion clinics and, when anti-abortionists began picketing the clinics, she turned their own tactics against them.

"I used to follow them around the country. Do you remember that song I Am the God of Hellfire from the 60s? I would drive up to the abortion clinic where they were picketing and I would jam in the cassette and crank it up to full blast. Just to let them know I was there."

For her pains, McCorvey became the target of aggression. She received hate mail calling her a baby killer. Eggs were thrown at her door. Dolls' clothing and parts - arms, legs, torsos - would be scattered over her lawn. Then one night a truck drove past and a marksman shot out her front window and the windshield of her car. In I Am Roe, she describes these as the acts of "pro-lifer terrorists". Now, however, she blames the harassment on pro-abortionists: "The pro-aborts didn't like me. I spoke my mind. I never supported them. They never got a penny out of me."

How she made such a gigantic leap of faith from one side of the abortion divide to the other illustrates the extraordinary persistence and persuasiveness of the anti-abortionists. Operation Rescue, one of the leading US anti-abortion groups devoted to overturning Roe v Wade, turned its considerable firepower onto her. At the time, she called them a "pack of vultures". Now she prefers to see them as "rescuers".

When her book came out in 1994 they picketed her signings, shouting at her that she was responsible for the deaths of 35 million unborn babies. Then Operation Rescue took out a lease on the house next door to the abortion clinic where McCorvey worked. They began talking to her, befriending her, offering her lunch.

"I started watching the rescuers and wondering what makes them tick. They were down to earth, they weren't telling me I was going to fry in hell, though I'm sure they were thinking that. They were very kind to me."

Slowly, they turned her ideas around. In August 1995 she allowed herself to be baptised in a backyard swimming pool in Dallas. Three years later, she attended her first Catholic mass. She moved from the clinic - or "abortion mill" as it now was to her - to Operation Rescue next door. One of her new friends summed up the shift: "The poster girl of the anti-abortion movement has walked off the poster."

Randall Terry is recording his daily radio show. He begins by introducing his guest, the "living, breathing, walking demonstration of the truth of God's mercy, Norma McCorvey."

We've driven out to Terry's impressively large house in a suburb in Virginia. Terry cuts a beguiling figure, with his attractive smile and alligator-skin cowboy boots. But underestimate him at your peril.

He founded Operation Rescue, and for 20 years has been a leading figure in the struggle to rescind Roe v Wade. He has a loyal following of anti-abortionists, McCorvey among them, who he drills through his daily broadcast which is beamed to nine major cities across America.

In 1991, Terry organised mass pickets outside George Tiller's abortion clinic in Kansas; almost 3,000 of these protesters were arrested. And when the same clinic was permanently closed last month following Tiller's murder, Terry rejoiced. "The mortar of that building is meshed with the blood of the innocent," he said at the time.

In the wake of Tiller's death, Terry convened an emergency war council of anti-abortionist leaders, and called for a wave of social unrest on a scale not seen since the civil rights days. He calls it Insurrecta Nex - Latin for "insurrection against death". He tells his listeners: "We are looking for people who will use the weaponry of social revolutions past so that child killing can be brought to an end."

During the past decade, however, the real battleground over Roe v Wade has been at the level of individual states. Elizabeth Nash of the pro-choice thinktank the Guttmacher Institute says access to abortion has been progressively whittled down under a constant barrage of legal challenges. Some 400 or more anti-abortion bills are put before state legislatures every year, of which 30 on average succeed.

The outcome is that women are now required to undergo waiting periods before terminating a pregnancy. They must be given information about their foetus that is often patently designed to act as a deterrent. In some states, women are told of a link between abortion and breast cancer, though such a link has never been proved. Other states emphasise the pain felt by foetuses.

"Nothing is solid," Nash says. "Even without Roe being overturned, many women already don't have a right to abortion as they just can't access it." McCorvey sees these mini-victories as moves in the right direction.

With Terry as her guide, she has been moving increasingly on to the national stage. Terry has grasped the PR worth of having Jane Roe on board, describing her conversion as of "inestimable value" to the cause. When she was arrested at Notre Dame last month, he was right beside her. Her court case for trespassing comes up in October, and she says she is relishing the prospect of going to prison.

Where McCorvey holds back from the full fire-and-brimstone rhetoric of Terry is over the death of Tiller. She says she was shocked when she heard that the doctor had been gunned down inside his church one Sunday. The news was poignant for her as she used to live in Wichita and regularly prayed the rosary outside Tiller's clinic. "Even though he was a monster, it still didn't give the right to take his life."

Not for a minute, though, is she taking her eye off the main prize. Her latest obsession is Sonia Sotomayor, Obama's nomination for the new ninth member of the supreme court. Though Sotomayor has not been outspoken on the subject of abortion, it is assumed she would maintain the current balance of the court at five justices to four upholding the right to abortion.

McCorvey is methodically gathering testimony from women who say their life was ruined by having had an abortion, to present to the supreme court whenever the time comes. And later this month she will be back on those court steps, demonstrating against Sotomayor's nomination. "That's when I'm at my best," she says with real elation. "When I'm all fired up!"

No doubt there will be more nerves, followed by more spitting; Norma McCorvey will always be a mass of contradictions. But grant her this: the woman who was once so proud to be Jane Roe will not stop until she has wiped from the face of America the judgment that bears her name.

• This article was amended on Thursday 9 July 2009. We referred to Norma McCorvey taking her first Catholic mass; we should have said she attended it. This has been corrected.