Jim Seals, half of Seals & Crofts, a soft-rock duo who had a string of hits in the 1970s, including the Top 10 singles “Summer Breeze” and “Diamond Girl,” died on Monday evening at his home in Nashville. He was 79.

His wife, Ruby Jean Seals, said the cause was an unspecified “chronic ongoing illness.”

Mr. Seals and his musical partner, Dash Crofts, were still teenagers when they were asked to join an instrumental group, the Champs, which had had a No. 1 hit in 1958 with “Tequila.” By the mid-1960s they had tired of the band and of the loud, sometimes angry strains that were infusing the hard rock of the time.

Adherents of the Baha’i faith, they sought to make a calmer brand of music, mixing folk, bluegrass, country and jazz influences and delivering their lyrics in close harmony.

“Jim Seals plays acoustic guitar and fiddle,” Don Heckman wrote in The New York Times in 1970 in a brief review of their second album, “Down Home,” “and Dash Crofts plays electric mandolin and piano; together they sing coolly intertwined, and quite colorful, vocal harmony.”

With the lilting, nostalgia-seeped single “Summer Breeze,” released in 1972, the two found international stardom. They had developed a modest following, but that song changed everything, as they found out when they arrived in Ohio to play a show.

“There were kids waiting for us at the airport,” Mr. Seals told Texas Monthly in 2020. “That night we had a record crowd, maybe 40,000 people. And I remember people throwing their hats and coats in the air as far as you could see, against the moon.”

The song, written jointly by the two men, featured the kind of chorus that sticks in the brain:

“Summer breeze, makes me feel fine, / Blowing through the jasmine in my mind.”

The single reached No. 6 on the Billboard Hot 100, and a follow-up, “Hummingbird,” made the Top 20. “Diamond Girl” in 1973 reached No. 6. “Get Closer” in 1976 also reached No. 6.

But the duo’s run of success basically ended when the decade did, and they called it quits for a time.

“Around 1980, we were still drawing 10,000 to 12,000 people at concerts,” Mr. Seals told The Los Angeles Times in 1991, when the two revived the act. “But we could see, with this change coming where everybody wanted dance music, that those days were numbered.”

Six years earlier, though, the pair had begun to fall out of favor with some listeners and critics because of their sixth album, “Unborn Child,” which was released in 1974 not long after the Supreme Court’s Roe v. Wade decision on abortion rights. The title track urged women who were considering an abortion to “stop, turn around, go back, think it over.”

Mr. Seals, in a 1978 interview with The Miami Herald, acknowledged that the record damaged the duo’s career.

“It completely killed it for a while,” he said. Radio stations refused to play the record. Some Seals & Crofts concerts were picketed, although there were also hundreds of letters of support. In the 1991 Los Angeles Times interview, Mr. Seals said the pair never intended the song to be a lightning rod.

“It was our ignorance that we didn’t know that kind of thing was seething and boiling as a social issue,” he said. “On one hand we had people sending us thousands of roses, but on the other people were literally throwing rocks at us.

“If we’d known it was going to cause such disunity,” he continued, “we might have thought twice about doing it. At the time it overshadowed all the other things we were trying to say in our music.”

James Eugene Seals was born on Oct. 17, 1942, in Sidney, Texas, to Wayland and Susan Seals. His father worked in the oil fields, and Jim spent much of his childhood in Iraan, a boomtown in southwest Texas.

“There were oil rigs as far as you could see,” Mr. Seals told Texas Monthly. “And the stench was so bad you couldn’t breathe.”

His father played a little guitar and his mother played the dobro, so informal jam sessions were a common way to pass the time in the household. When a fiddler came by one evening, young Jim was taken with the instrument, and his father ordered him one from a Sears catalog.

Later he took up the saxophone, which led to an invitation to join a rockabilly band called the Crew Cats that played at dances and in local clubs. The band’s drummer quit right before a show at a junior college, and the drummer from another band on the bill sat in — Darrell Crofts, known as Dash.

The two became friends and played with the Champs for several years out of Los Angeles. Both mastered other instruments, including the guitar. Once they hit it big as a duo, they knew the image they wanted to project and tried to stay true to it. In 1973, when they were about to tour England, Mr. Seals told a reporter that they had pulled out of a previous European engagement.

“We were going to tour there earlier, but we had a last-minute change of mind when we found out that we’d be playing with Black Sabbath,” he said. “I’m sure they’re a fine band, but I’m not sure that the audience would be quite right for us.”

In addition to his wife, Mr. Seals is survived by their two sons, Joshua and Sutherland; a daughter, Juliet Crossley; and three grandchildren. A sister, Renee Staley, and a half brother, Eddie Ray Seals, also survive him. His brother, Dan Seals, a singer who had success in the late 1970s as a member of another soft-rock duo, England Dan & John Ford Coley, died in 2009. The two brothers toured together for several years before Dan Seals’s death, with Jim Seals’s two sons sometimes playing with them.

Maia Coleman contributed reporting.