Nedda Casei, who in the 1960s and ’70s could be reliably heard as Suzuki, Maddalena, Lola and other bread-and-butter mezzo-soprano characters at the Metropolitan Opera before transforming herself into a pathbreaking labor leader, died on Jan. 20 in hospice care in Manhattan. She was 87.

A niece, Janice Arponen, said the cause was a stroke.

Ms. Casei sang in some 280 performances at the Met, from her debut as Maddalena in Verdi’s “Rigoletto” in 1964 until her final curtain, in 1984, as Larina in Tchaikovsky’s “Eugene Onegin.”

She also sang “Carmen” there. In one performance, in 1978, as the understudy for Elena Obraztsova, she had to go on at the last minute.

After her first scene, “‘Boo’ rang out like a clap of thunder, full-throated, resonant and shocking,” the critic Jack Hiemenz wrote in the newspaper The News World. “For a moment there was stunned silence, then the audience, outraged, shot back vehement applause and a few bravos.” Mr. Hiemenz suggested that a plant in the audience might have emitted the “boo” to generate sympathy for Ms. Casei.