Helen Gahagan Douglas, 79, a former actress and Democratic member of Congress whose political career came to a halt in 1950 when she was defeated in her bid for a Senate seat by Richard M. Nixon, died of cancer Saturday at Memorial Sloan-Kettering Medical Center in New York.

A family spokeswoman said Mrs. Douglas underwent a mastectomy seven years ago. Her cancer went into remission but recurred three years ago, the spokeswoman said.

Mrs. Douglas was first elected to Congress from the 14th District in Los Angeles in 1944. She was reelected to two more terms and then decided to try for a Senate seat in 1950.

The California Democrat ran up against Nixon, a Republican, who had represented the 12th district in the Los Angeles area since his election in 1946.

The campaign was a heated one. Nixon, who became known for his role on the House Un-American Activities Commission, charged that Mrs. Douglas had strong left-wing tendencies and that her views were considered pro-Communist. He accused her of pink leanings.

She in turn, accused him of witch hunting. She lost the election and he went on to become vice president two years later.

Because of Nixon's attacks on her, mrs. Douglas became a martyr in the eyes of embittered Democrats, and Nixon a target of criticism because of his tactics for years to come.

In later years, Mrs. Douglas, who became a lecturer and author and returned for a period to the theater, said she felt no bitterness. But in 1974, at the height of the Watergate scandal when then President Nixon was forced to quit the White House, she suggested that he be impeached.

Mrs. Douglas was born in Boonton, N.J., and grew up in Brooklyn, N.Y.

She attended Barnard College but dropped out of school to go on the Broadway stage.

She appeared in such plays as "Young Woodley," "Trelawney of the Wells" and "Tonight or Never," where she met actor Melvyn Douglas. They were married in 1931. He was with her when she died.

During the late 1920s, she also studied operatic singing and sang opera on several stages in Europe. She went to Hollywood in 1935 and starred in a movie, "She."

While in California, Mrs. Douglas became interested in the plight of the poor and became involved in politics. She was named a Democratic national committeewoman from California in 1940, when she also became vice chairman of the Democratic State Central Committee and chairman of the women's division, positions she held until her election to Congress.

Active in president Franklin D. Roosevelt's New Deal, Mrs. Douglas had served as a member of the national advisory committee of the Works Progress Addministration and the state committee of the National Youth Administration.

President Roosevelt appointed her to the voluntary participation committee in the Office of Civil Defense. Later President Harry S. Truman named her an alternate delegate to the United Nations Assembly.

In 1964, President Lyndon Johnson appointed Mrs. Douglas as special ambassador to head a United States delegation to the inauguration ceremonies of President V. S. Tubman of Liberia.

Mrs. Douglas, who lived in New York, was working on her memoirs at the time of her death, the family spokeswoman said.

In addition to her husband, she is survived by a son, Peter, and a daughter, Mary Helen.