Judith Hennessee is the unfamiliar kind of New Yorker: softspoken, polite and self-deprecating. Not the sort of journalist who courts controversy. Yet this month Hennessee sparked a political and literary storm that has spread rapidly in the press and on airwaves throughout the US. Opening salvoes were launched in early April when Random House published her frank biography of the veteran feminist leader Betty Friedan.

The New York Observer accused Hennessee of "trashing" Friedan. Women who can still quote passages of Friedan's The Feminine Mystique from memory jammed talk-show lines to fulminate against the revelations in Betty Friedan: A Life. A less sanguine journalist might have begun screening her mail for lumpy packages. But Hennessee seems unscathed - even amused - by the outbursts. "It has been fairly extreme," she admits on the telephone from her Upper East Side apartment. "One review said I had fawned all over Betty. Another called me deliriously hostile."

A veteran of the women's movement in the 1960s who has written extensively about women, Hennessee anticipated a strong reaction. Her subject, after all, demanded one. "Betty is so controversial and has been so unpleasant to so many people over the years that I knew the reviewers' emotions would get in the way of their objectivity," she explains. "She has a quick temper and has always indulged it. She screams at people. Then she immediately forgets about it. But they don't. For her, it's over. For them it's often a lifelong resentment."

Stories of Friedan's confrontations - lots of shouting, lots of swearing, the odd hurled object - have been part of the oral history of feminism for decades. Practically every woman who worked with her has one to relate. What interested Hennessee, however, was not simply the bad behaviour but the apparent contradiction. How did a consummate tantrum-thrower launch the women's movement, shape its early years, and found the National Organisation of Women along with two other major political groups?

"To paraphrase Franklin Delano Roosevelt, she had a first-class mind and a second-class temperament," Hennessee writes. "Founders of movements are not necessarily nice people. She was rude and nasty, self-serving and imperious. But power has to be taken and used, and she had the major ego and drive - the sheer nerve - to do it."

In 1996, Hennessee began a series of four interviews with 75-year-old Friedan. The first, in Friedan's Washington DC apartment, went well. "We had a drink, gossiped, she answered all my questions," Hennessee recalls. A subsequent meeting in New York revealed where Friedan drew the line with her semi-official biographer. "She would not talk about her personal life," Hennessee explains, "and when I got on to it she fobbed me off with something bland."

To fill those critical gaps, Hennessee visited the Schlesinger Library at Radcliffe University, where Friedan's papers are stored. Many of those personal notes, written in a distinctive scrawl, are Friedan's emotional reflections on her early years as Bettye Naomi Goldstein, the eldest daughter of middle-class Miriam and Harry in Peoria, Illinois. "It's all there in her childhood," Hennessee stresses. "Her mother in particular was a strong, effective woman, who hated staying at home and who took her frustrations out on her husband. Betty did not want to be like Miriam."

Friedan's distinctive looks, academic brilliance and unfashionable directness guaranteed she would not turn into her mother. Genteel anti-semitism may also have shaped her ambitions. "Nobody talked about it," Hennessee observes, "but it was a strong undercurrent. The Goldsteins could not join the country club, for example. And that feeling of not belonging informed everything Betty did. All her actions can be seen as an attempt to get inside, to belong with those in power."

The price of admission to the inner circle - political and intellectual - was never negotiable in Friedan's eyes. From the outset in the 1940s, she wanted to be married, have children and make a revolution. "I dreaded being like them," she wrote of the "old-maid" librarians and teachers in Peoria. It never seemed likely.

Betty met Carl Friedan in 1946 when she was working for a union newspaper and he was just home from war. Through Hennessee's lens the marriage, which ended in divorce in 1969, was a battlefield. Jonathan, the middle of their three children, regularly sat with a friend on the stairs to watch "the Friday night fight when Betty and Carl indulged themselves throwing crockery". And there was drink, lots of drink. "The marriage was truly appalling, progressing from broken china to physical abuse," Hennessee agrees. "Yet she loved Carl. They fed each other's neuroses perfectly."

Friedan had been in therapy for years when a Freudian analyst urged her to concentrate on being a writer. The Feminine Mystique was published in 1963, but would have languished without Friedan's tireless appearances. By 1964, it was a bestseller. When Random House suggested a second book, Friedan responded: "You made me feel so Jewish for trying to sell my book. Go f . . . yourself." She became a celebrity, dyed her hair blonde and considered having her nose surgically altered. But the feminine discontent she had articulated left little time for such distractions. With Friedan's guidance, it was taking political shape.

THE National Organisation of Women was founded in Washington in 1966, an event the New York Times did not consider worth covering. They did, however, publish an interview with Friedan. It appeared on the women's page, under a story whose headline read "How to Cook Your Thanksgiving Turkey." Some feminists were equally unimpressed. "NOW was never a grass-roots movement," Hennessee explains. "It was very much Betty muscling in on the hierarchy, putting herself in a position of power. Underlying that instinct is a fear that if she's not number one, she's nothing."

Hennessee chronicles NOW's early days and Friedan's unflagging concern with equal pay and equal rights, returning us to a time when identity and gender politics were a suspect rather than a sanctified branch of feminist thinking. In Friedan's view, consciousness raising was "mental masturbation" and radical lesbians a "lavender menace". She screamed at feminist theorist Ti-Grace who had published a memorable paper on "Vaginal Orgasm as a Mass Hysterical Response".

Friedan's true nemesis, however, emerged not from the radical fringe but from the pragmatic mainstream. When she appeared on Newsweek's cover in 1971, Gloria Steinem was everything Friedan was not - slim, pretty, chic and diplomatic. At ease with the Kennedys, Hollyood directors and flashy millionaires, "this newcomer, with her hair and her empty refrigerator", as Hennessee describes her, quickly became the smiling face of feminism.

Friedan was not easily or instantly de-throned. But the 1982 publication of The Second Stage, in which she called for the subordination of women's concerns to larger social problems, confirmed the unthinkable. Friedan had mellowed. A decade later, Susan Faludi identified her as part of the backlash, "a fallen leader who is clearly distressed . . . that she wasn't allowed to be the Alpha wolf as long as she liked". Hennessee is less severe. "Her flaws cost her the leadership," she concludes. "But what she did for women outweighs the rest." The emotional reaction to Betty Friedan: Her Life seems to prove that point.

Betty Friedan: Her Life by Judith Hennessee is published in the US by Random House, price $27.95.