Dorothy Howell was 8 years old when her parents sent her away. It was 1927. Her mother and father, who fought violently in the Chicago boardinghouse where the family lived, divorced. Neither was willing to take care of Dorothy or her little sister.

So they put the girls on a train to California to live with their grandparents. It did not go well. Her grandmother favored black Victorian dresses and punished the girls for inexplicable infractions, like playing in the yard. (Dorothy was not allowed to leave her room for a year, other than for school, after she went trick-or-treating one Halloween.)

Unable to bear it, Dorothy left her grandparents’ home at 14, and became a housekeeper for $3 a week, always hoping to return to Chicago and reconnect with her mother. But when she finally did, a few years later, her mother spurned her again.

It took a long time for Hillary Rodham Clinton to fully understand the story of her mother’s devastating childhood. But now, four years after her death, Dorothy’s story is forming the emotional foundation of her daughter’s campaign for president, and will be a central theme in her big kickoff speech on Saturday.

Sharing that story is a shift for Mrs. Clinton, who in her 2008 campaign was fiercely protective of her mother’s privacy and eager to project an image of strength as she sought to become the first female commander in chief. And in this campaign, her mother’s story may help address one of Mrs. Clinton’s central challenges: convincing voters who feel they already know everything about her that there is, indeed, more to know, and that she is motivated by more than ambition.