Paul Cotton, the lead guitarist and frequent lead singer and songwriter for the country-rock band Poco, died on July 31 near his summer home in Eugene, Ore. He was 78.

His wife, Caroline Ford Cotton, said he died unexpectedly but she did not cite a cause. His death came less than four months after that of Rusty Young, Poco’s longtime steel guitarist.

Mr. Cotton joined Poco, replacing the founding member Jim Messina in 1970, just in time to appear on the group’s third studio album, “From the Inside” (1971). Produced by Steve Cropper, the guitarist with the Memphis R&B combo Booker T. & the MGs, the project signaled a new artistic direction for the band, maybe nowhere so much as on the three songs written by Mr. Cotton.

Rooted more in rock and soul than in the country and bluegrass that had hitherto been the group’s primary influences, Mr. Cotton’s sinewy, blues-inflected guitar work and brooding baritone vocals on songs like the ballad “Bad Weather” greatly expanded Poco’s emotional and stylistic palette.

“There was no doubt that he was the guy to replace Jimmy,” Richie Furay, who founded the band with Mr. Messina and was its principal lead singer, said about Mr. Cotton’s impact on the band in a 2000 interview with soundwaves.com. “We knew that he was bringing a little bit of an edge to our sound, and we wanted to be a little more rock ’n’ roll sounding.”

Mr. Young said in the same soundwaves piece, referring to Mr. Furay and Poco’s longtime drummer, George Grantham: “You have to remember, we had some very high singing voices at the time. Paul had a much deeper voice, and he had that rock sound.”

Poco became a major influence on West Coast country-rock acts like Linda Ronstadt and the Eagles and, a generation later, on alternative-country bands like the Jayhawks and Wilco.

Formed in Los Angeles in 1968, the group originally consisted of Mr. Messina and Mr. Furay, both of them formerly with the influential rock band Buffalo Springfield, along with Mr. Young, Mr. Grantham and the bassist Randy Meisner, a future member of the Eagles. (Timothy B. Schmit, another future Eagle, replaced Mr. Meisner when he left the band in 1969.)

Mr. Furay departed in 1973, disillusioned over the group’s lack of success compared with that of his ex-bandmates in Crosby, Stills & Nash and the Eagles, especially after the release of critically acclaimed but commercially disappointing Poco albums like “A Good Feeling to Know” (1972) and “Crazy Eyes” (1973).

Poco’s remaining members carried on without Mr. Furay, with Mr. Cotton doing much of the singing and songwriting, until the group went on hiatus in 1977 and he and Mr. Young went into the studio to record as the Cotton-Young Band.

In 1978, ABC, the duo’s label, released the recordings, made with British musicians who had accompanied pop hitmakers like Leo Sayer and Al Stewart, but insisted on crediting the band as Poco.

“Legend,” the album that resulted, yielded an unanticipated pair of hits, the band’s first and only Top 40 singles: the glittering “Crazy Love,” written and sung by Mr. Young, which reached No. 1 on the adult contemporary chart, and the similarly burnished “Heart of the Night,” written and sung by Mr. Cotton. The album was certified platinum for sales of one million copies.

Poco continued to tour and release recordings into the 2000s, with Mr. Cotton, Mr. Young and Mr. Grantham anchoring the lineup.

Norman Paul Cotton, the oldest of five children, was born on Feb. 26, 1943, in Fort Rucker, Ala., in the southeast part of the state. His father, Norman, owned a line of grocery stores. His mother, Edna, kept the books for the family business. Young Norm, as he was known as the time, began playing guitar at 13.

When he was 16, the Cottons moved to Chicago, where he attended Thornton Township High School. While there he started a band, eventually known as the Rovin’ Kind, that released several singles and appeared on “American Bandstand.”

In 1968, after seeing them perform at a club in Chicago, the producer James William Guercio, best known for his work with the jazz-rock band Chicago, signed the group to Epic Records. Mr. Guercio advised them to change their name and relocate to Los Angeles, where they renamed themselves Illinois Speed Press. Mr. Cotton began billing himself as Paul rather than Norm.

Illinois Speed Press, with Mr. Cotton and Kal David as twin lead guitarists, released a pair of roots-rock albums for Epic, to little commercial effect. Mr. Cotton was invited to join Poco in 1970, shortly after the release of the band’s second and last album, “Duet.”

Besides his wife of 16 years, Mr. Cotton is survived by his sons, Chris and James; two brothers, David and Robert; two sisters, Carol and Colleen; and a grandson.

Mr. Cotton spent three decades on and off with Poco and also released a handful of solo albums between 1990 and 2014. An avid fisherman and sailor, he moved to Key West, Fla., in 2005.

Poco went through numerous lineup changes during its more than 40 years in existence, but one of the constants, from Mr. Cotton’s arrival in 1970 until his retirement in 2010, was his partnership with Mr. Young.

“There’s always been something there,” Mr. Cotton said of his relationship with Mr. Young in 2000.

Mr. Young added: “He’s never lost that voice, or that great guitar playing. I can count on him. I wouldn’t want to do this without him.”