Sandra Feldman, the blunt-spoken former president of the American Federation of Teachers and its local in New York, who emerged from the shadow of her mentor, Albert Shanker, to become a deft and intimidating labor powerhouse on her own, died Sunday night at her home in Manhattan. She was 65.

Alex Wohl, a spokesman for the national union, which Ms. Feldman led from 1997 to 2004, said the cause was breast cancer, which recurred last year and deterred Ms. Feldman from seeking re-election.

A scrappy fighter who rarely minced words, the Brooklyn-born Ms. Feldman took on mayors, school chancellors and American presidents with the same inattention to ceremony with which she dispatched rivals for her union leadership.

As head of the American Federation of Teachers, the nation's second-largest union, with 1.3 million members, she was an impassioned advocate for the public schools that educated her. She worked with members of the Bush administration and members of Congress to write the education law known as No Child Left Behind, which imposed stricter performance standards on all schools receiving federal aid. But she was not timid about assailing President Bush when she thought he was not funneling enough resources to sustain those standards.

She put the power of the United Federation of Teachers, the New York local, with 110,000 members at the time, behind the election of David N. Dinkins as mayor in 1989, an effort that helped him garner white votes. But when he disappointed her by letting a year pass without a new contract, she publicly let him know. "I feel angry at him," she said, and added, "The mayor had better understand that I'd fight him to the end for the needs of my members."

At one point in the late 1980's, she fought so ardently for Bernard Gifford to become New York City schools chancellor that Robert F. Wagner Jr., the president of the Board of Education, threatened to resign unless Ms. Feldman backed off.

"She is part shop steward, part visionary," Mr. Wagner said in a comment at the time that seemed to capture her earthy charm.

Yet, even as she could be fierce in fighting battles like smaller class sizes, she had an artful touch in private dealings over the brass-tacks issues of pensions and benefits and often veiled her thoughts.

Randi Weingarten, who succeeded Ms. Feldman as president of the United Federation of Teachers, said, "People remember her take-no-prisoners kind of attitude, but Sandy was far more pragmatic than people give her credit for." She recalled how in 1991 the Dinkins administration planned to lay off 4,000 paraprofessionals, and to save those jobs Ms. Feldman agreed to defer a $40 million portion of a 5½ percent wage increase she had negotiated not long before. "She was courageous in explaining the difficulties to her members," Ms. Weingarten said.

The person to whom she was most wedded in the public eye was Mr. Shanker, who built a rudderless collection of local unions into the United Federation of Teachers, which came to have perhaps the most powerful voice in who would become chancellor. As a 29-year-old union field representative, she was often at Mr. Shanker's side in 1968 when he locked horns with an experimental, largely black community school district in Ocean Hill-Brownsville, Brooklyn, that tried to transfer 13 white, mostly Jewish teachers and administrators.

The conflict was fraught with charges of anti-Semitism and racism, and Ms. Feldman was in a difficult position, having been an advocate for civil rights, working to integrate Howard Johnson restaurants along Route 40 in Maryland. Her instincts were at first to avoid confrontation, Ms. Weingarten recalled.

Although the battle culminated in legislation creating a diluted version of community control in a decentralized school system, the union, by going on strike three times, had proved it could shut down the schools at will and, in a fragmented system, it emerged more powerful than ever with increased job protections for its members.

Hardened in that caldron, Ms. Feldman became the union's secretary and took over its presidency in 1986, when Mr. Shanker became head of the national union. She battled chancellors and mayors for significantly higher pay and improved working conditions for her members.

"Having lived through the contentious teachers' strikes of the late 1960's, she was wary of any effort to remove due process protections from her membership," said Diane Ravitch, the education scholar.

Detractors in government said she sometimes protected teachers' rights at the expense of students, by, for example, winning concessions that put aides rather than teachers in charge of discipline in hallways and cafeterias. Critics also pointed out that she never abandoned her insistence that the school system should not have the right to reassign seasoned teachers to failing schools, even if they were needed there most.

As head of the American Federation of Teachers, she worked to increase federal and state spending for programs for the youngest children in a program she called Kindergarten Plus. She also tried unsuccessfully to devise a merger with the largest national union, the National Education Association.

Ms. Feldman, whom almost everyone called Sandy, was born Sandra Abramowitz in Coney Island in October 1939, about six weeks after the outbreak of war in Europe. The daughter of a milkman and a sickly woman who worked part time in a bakery, she grew up in a tenement and later in a public housing project.

She went to James Madison High School and studied English at Brooklyn College, where she immersed herself in socialist politics and civil rights activities. After her freshman year she married Paul Feldman, whom she met at the college literary magazine. They divorced 17 years later.

Her first full-time appointment was to Public School 34 on the Lower East Side, where she taught fourth grade for three years. "When I came into that school, there was only one union member," she said. "I organized it immediately."

She loved to read fiction and sometimes went dancing in salsa and jazz clubs with her second husband, Arthur H. Barnes, senior vice president for external affairs of HIP Health Plan of New York.

Besides Mr. Barnes, she is survived by a brother, Larry Abramowitz of New York City; a sister, Helen Berliner of Wylie, Tex.; two stepchildren, David Barnes and Donna Marie Barnes; and two grandchildren.