The Weavers perform in a 25th anniversary reunion concert at Carnegie Hall in New York in 1980. From left are Pete Seeger, Lee Hays, Ronnie Gilbert and Fred Hellerman. (Richard Drew/AP)

Ronnie Gilbert, who lent her majestic contralto voice to the revival of folk music as a founder and sole female member of the Weavers, the celebrated quartet led by Pete Seeger, died June 6 at a nursing facility in Mill Valley, Calif. She was 88.

Her partner of 30 years, Donna Korones, confirmed her death and declined to disclose the cause.

Ms. Gilbert began her musical career in the late 1940s, in an era that she described as “that strange time after World War II, when already the world was preparing for cold war.”

The Weavers — formed by Seeger, Ms. Gilbert, Lee Hays and Fred Hellerman — became one of the most influential musical groups of its era, both musically and politically, and was credited with inspiring the resurgence of popular folk music in the 1960s.

Seeger once described the group’s sound as that of “two low baritones,” a “split tenor” and “one brilliant alto” — Ms. Gilbert. Her voice could be heard, blending with but also rising over the others, in Weavers standards such as “This Land Is Your Land ,” “If I Had a Hammer ,” “On Top of Old Smoky ,” “Goodnight, Irene ,” “Kisses Sweeter than Wine ” and “Tzena, Tzena, Tzena .”

Some of their songs became unofficial anthems of the movements for civil rights, workers’ rights and other progressive causes.“We still had the feeling that if we could sing loud enough and strong enough and hopefully enough, it would make a difference,” Ms. Gilbert said in the 1982 documentary “The Weavers: Wasn’t That a Time!”

The Weavers gained popularity slowly, initially by word of mouth, through their performances at hootenannies and union halls. They sang prolifically but not profitably until 1949, when they began an extended engagement at the Village Vanguard nightclub in New York City.

During the Red Scare of the 1950s, the Weavers were among the many performers in the entertainment industry who were blacklisted amid anti-communist fervor.

“I have sung in hobo jungles and I have sung for the Rockefellers, and I am proud that I have never refused to sing for anybody,” Seeger memorably declared, defending himself before the House Un-American Activities Committee. “I have never done anything of any conspiratorial nature. . . . I love my country very deeply.”

The scarcity of performance and recording opportunities forced the musicians to break up, but they reconvened for an acclaimed sold-out concert in New York’s Carnegie Hall in 1955. After Seeger embarked on a solo career, the remaining Weavers performed with Erik Darling and other musicians before disbanding in the mid-1960s.

Ms. Gilbert went on to a theatrical career, performing under directors including Joseph Chaikin and Harold Pinter. Later, she played the labor activist Mary Harris “Mother” Jones in a one-woman show. Ms. Gilbert also received a degree in clinical psychology, practicing for periods in Northern California and British Columbia.

But she remained a musician throughout her life, performing and recording with the singer-songwriter Holly Near and releasing several solo albums, including “Alone With Ronnie Gilbert,” “Love Will Find a Way” and “The Spirit is Free.” The Weavers performed at Carnegie Hall in 1980, commemorating their performance of 25 years earlier. It too was a sell-out.

Ruth Alice Gilbert was born in New York on Sept. 7, 1926. Her parents, a factory worker and a dressmaker, were Eastern European immigrants.

“I come from a long, honorable tradition of political singers which goes back to the troubadours,” she told The Washington Post in 1985, recalling her family’s involvement in union work in Poland.

Ms. Gilbert came to Washington during World War II and met Library of Congress folklorist Alan Lomax and Woody Guthrie, along with other folk singers. She attended Anacostia High School, where The Post reported that she was nearly kicked out over her refusal to perform in a minstrel show. She sang in the early 1940s with the Priority Ramblers before forming the Weavers with Seeger, who died last year.

Her marriage to Martin Weg ended in divorce. She and Korones were married in 2004, when then-San Francisco Mayor Gavin Newsom allowed same-sex marriages in that city. Those marriages later were ruled invalid. Besides Korones, of Berkeley, Calif., survivors include a daughter from her first marriage, Lisa Weg of Caspar, Calif.; and a granddaughter.

The memoir “Ronnie Gilbert: A Radical Life in Song” is forthcoming.

“We were four very politically motivated people, interested in doing what we could for the music we loved and for social action,” Ms. Gilbert once told the Boston Globe, recalling her time with the Weavers. “Part of social action at that time was to raise the consciousness of any audience to folk music, just to the idea that these songs existed.”