In April 2004, Kamala Harris was less than four months into her new job as San Francisco’s district attorney, a high-profile position that thrust her into the local headlines, when she flew to Washington, D.C., to become one face in a sea of more than a million.

People from around the country descended on the National Mall for the March for Women’s Lives, organized by groups including Planned Parenthood and the National Organization for Women, to call for the protection and expansion of reproductive health care, including abortion rights.

Ms. Harris traveled with about 30 female Bay Area leaders, including Laurene Powell Jobs, the billionaire philanthropist, and Susie Tompkins Buell, the founder of the Esprit clothing brand and the North Face outdoor recreation-wear company. They were there primarily to protest President George W. Bush’s signing of a bill that outlawed abortions late in pregnancy, typically after 20 weeks, and allowed criminal prosecutions of doctors who performed them. The women said that the president was politicizing a matter of personal health.

Twenty years later, Vice President Harris has made abortion rights — an issue that has benefited Democrats around the country since the Supreme Court overturned the national right to an abortion in 2022 — a centerpiece of her campaign for the presidency.

While some Democrats, including President Biden, have become far more outspoken about abortion rights since the fall of Roe v. Wade, interviews with friends and former associates of Ms. Harris, and a review of her work as a prosecutor, show that she has been publicly passionate about the issue for decades. For a politician who has been criticized for wavering and flipping on some issues, this is one area, former associates say, where her conviction has always been clear.

“It was always a given,” said Ms. Tompkins Buell in an interview, recalling that at the 2004 march, Ms. Harris warned the women in their “posse,” as they called it, that they could not take their reproductive freedoms for granted.