Dolores Huerta, a celebrated 1960s labor union leader and civil rights activist, died on Wednesday in California at the age of 96, her family said.

Huerta was co-founder with Cesar Chavez, another veteran unionist and political activist, of the United Farm Workers union (UFW), which was credited with improving pay and conditions for agricultural workers through a series of strikes, boycotts and protests in the 1960s and 1970s.

Huerta was born in New Mexico in 1930, and followed her father, a miner and farm worker, into union activism. As a political operative, she worked on the presidential campaigns of several Democratic party candidates, including helping Robert F Kennedy to secure the party’s 1968 nomination before he was assassinated, and aiding the elections of Barack Obama and Joe Biden.

“We are deeply saddened to announce that our beloved mother, Dolores Huerta, has passed away today at the age of 96,” her family said in a statement on social media.

“As you can imagine, this is an incredibly difficult time for us, and we ask for privacy as we grieve this incredible loss to our family. We know that many of you around the world share in our mourning, and we will be following up on plans to recognize and celebrate her incredible life and legacy.”

Huerta is credited with coining the phrase “Sí, se puede!” (yes, we can), which became the slogan of the UFW, and which Obama adapted as a rallying cry for his successful 2008 campaign for the White House.

As well as serving as Chavez’s right hand at the UFW, the union said on its website, “she could also be the greatest thorn in his side.”

It said: “The two were infamous for their blow out arguments, an element that was a natural part of their working relationship. Dolores viewed this as a healthy and necessary part of the growth process of any worthwhile collaboration.”

It emerged only in interviews Huerta gave to the New York Times, published in March, one month before her 96th birthday, that their relationship was much darker than it was publicly portrayed. As allegations emerged that Chavez, who died in 1993, groomed and sexually abused a number of teenage girls, Huerta revealed that he had raped her during the winter of 1966 in a vehicle in a secluded California grape field.

Huerta, who was 36 at the time, told the Times she did not report the assault because of authorities’ hostility to the UFW, and a fear that nobody in the union would believe her.

Huerta was later engaged in a long-term domestic partnership and had four children with Chavez’s brother, Richard, who died in 2011.

The UFW said Huerta was also “seemingly unaware of the tremendous impact she was having on not only farm worker women, but also young women everywhere” as a fierce advocate for women’s rights.

Four elementary schools in California, and one in Fort Worth, Texas, as well as a high school in Pueblo, Colorado, are named after her.

In 2012, she was among 13 people awarded the Presidential Medal of Honor, the highest civilian award in the US, by Obama in a ceremony at the White House.