“If that girl had been born with balls, she would have been a hell of a politician.” Joe Kennedy, when he wasn’t busy trying to make presidents of his two oldest sons, Joe and Jack, once said that of his daughter, Eunice.

Even though she was younger, the middle child of nine, Eunice did try to compete. Her brother Ted recalled not long ago, “We had many family games and activities and Eunice always seemed to try a little harder and [was] always the one with the extra effort.”

After his sister Rosemary was institutionalized following a failed lobotomy, Eunice brought her to the family compound at Cape Cod every summer until her death.

In the ‘40s that drive led her not into politics, but into a degree in social work and volunteer work in Harlem and in a women’s prison—the sort of experiences her more sheltered siblings never sought. But she did throw herself into her brothers’ campaigns. That started in 1946 when she took the leading role in organizing dozens of tea parties and, finally, a massive reception to introduce thrilled women to her glamorous brother Jack, the candidate for Congress. Eunice ran that event and, as Doris Kearns Goodwin wrote, “At least 1,500 women came, all dressed in their finest clothes.”

Yet without ever holding public office, Eunice Kennedy Shriver, arguably the most religious of the nine children, did something few politicians ever aspire to—she changed the way America faces a moral question: how to regard the lives of the mentally retarded. She started in a time when the mentally retarded were hidden away, out of sight, and played a central role in moving toward a national approach that makes and nurtures the best of their abilities.

Eunice began by looking after her retarded older sister, Rosemary, taking her sailing and to dances. Later, after Rosemary was institutionalized following a failed lobotomy ordered by her father, Eunice brought her to the family compound at Cape Cod every summer until her death in 2005.

She took the idea of helping the mentally retarded to her brother, President John F. Kennedy. She insistently pressed him to call for and win passage of the first federal legislation to support research into mental retardation, and to upgrade state and local services. She followed that first with the creation of a summer camp for retarded children in her ample backyard, and then with the founding of the Special Olympics in 1968—a movement of lasting international impact.

Presenting her with the Medal of Freedom in 1984, President Reagan said, “With enormous conviction and unrelenting effort, Eunice Kennedy Shriver has labored on behalf of America's least powerful people, those with mental retardation…Her decency and goodness have touched the lives of many, and Eunice Kennedy Shriver deserves America's praise, gratitude, and love."

Or, as her friend, the retired political consultant Bob Shrum, put it this week, “She led her family, her nation, and the world in transforming a personal tragedy into a fundamental commitment to change how we treated mentally retarded people.”

The dramas of the lives of the Kennedys have flashed across front pages and television screens for half a century. This summer the attention has been on Ted’s struggle with brain cancer. Eunice’s decline, in a house nearby, has been overshadowed.

She never made the headlines her brothers did, but as Ted put it not long ago in a video tribute reflecting on her role in the family, “She has the biggest heart of all.”

Adam Clymer is a former chief Washington correspondent of The New York Times and author of Edward M. Kennedy: A Biography.