In the fading light of a summer afternoon at the Long Island National Cemetery in Farmingdale, a production assistant is performing the mighty task of spinning a chair in which the 350-plus-pound John Popper sits while he plays "The Star-Spangled Banner" on a silver harmonica. In a few days, Mr. Popper and his band, Blues Traveler, will open the fifth annual Horde summer music festival, which he founded in 1992.

But today he is filming a public service announcement for the recording industry's Rock the Vote campaign. As he sits perched among thousands of headstones, the weirdness of finding the 28-year-old Mr. Popper in such surroundings isn't lost on him. "If I keel over, just put me under right here," he said.

Mr. Popper had just taken a six-month break from touring, in part to lose weight and in part to get his recently diagnosed diabetes under control, both without success. "I've got to do something about my health," he said. "What's cool is now I have the time and the resources to do something about it."

But he is not about to give up his music career to slim down. His single-mindedness carried Blues Traveler through eight years of relative obscurity. Then with the release of its latest studio album, "Four," in 1994, Blues Traveler vaulted into the mainstream, mainly on the strength of one song, the catchy, upbeat "Run Around," about a relationship gone sour. But the song, which won a Grammy, is only vaguely representative of Blues Traveler's roots-rock sound and longer jams, for which the band first gained a small but loyal neo-hippie following and a reputation as a young Grateful Dead.

"There will never be another Grateful Dead," said B. B. King, who once performed with Mr. Popper. "But John is one of a kind. He has all the potential to be the best harmonica player ever."

It's a distinction Mr. Popper appears determined to earn. Blues Traveler has played nearly 250 shows a year over most of the last decade, primarily in clubs. Now Horde (an acronym for Horizons of Rock Developing Everywhere) has given his music a more prominent home. It has also provided a place for artists like the Dave Matthews Band and Sheryl Crow. Both played Horde in previous years and have seen their popularity rise since Blues Traveler had its breakthrough album two years ago.

While Horde's alternative-rock rival, Lollapalooza, seems to be losing ground, Horde has grown each year and is expected to clear $12 million and outdraw Lollapalooza this summer, if only because it has 42 dates to Lollapalooza's 28. On Friday, Horde, which Blues Traveler is headlining for the first time this year, comes to New York at Downing Stadium on Randalls Island. Mr. Popper began his sabbatical in January by moving from an apartment in Princeton, N.J., into a 19th-century farmhouse on 32 acres in Bucks County, Pennsylvania. The move came five months after the death of Jerry Garcia, to whom Mr. Popper had often been compared not only because of his attitude toward music but also because of his size and his diabetes.

"When Jerry died, it dawned on me how much I liked the man and how much he represented endless possibility," Mr. Popper said. "I think about my mortality, but I'm more obsessed with what I want to do with my life."

Still, whether he is haunted by Garcia's early death, at the age of 53, or listening to the concerns of his doctors, Mr. Popper is checking his blood sugar three times a day and has hired a chef to travel with him. But his inability to stick to a diet reflects his willful nature: he wants it all. "John's very obsessive about things," says Chan Kinchla, Blues Traveler's guitarist. "That kind of fire has really helped to take us in the right direction."

The real test of Mr. Popper's mettle began on the night of Oct. 20, 1992, when he was in a near-fatal motorcycle accident in Louisiana. Afterward, he was confined to a wheelchair for 20 months. The band's manager, the legendary Bill Graham, had recently died and the group's label, A&M Records, had just given it the money and freedom to produce its own records. Six months after the accident, Blues Traveler had finished recording the album "Save His Soul" and was back on the road, with Mr. Popper singing from a wheelchair.

"As horrible as the accident was, John turned it into a positive experience, which is typical of his character," says Chris Barron, the lead singer of the Spin Doctors and a friend of Mr. Popper's since high school in Princeton. "He hasn't had an easy life. He was made fun of a lot as a kid. He's always had this inner strength and power. A lot of us in the business look up to him as the dean of this school of music. He's like a great emperor."

Mr. Popper, who treats touring as if he were a conquering emperor, shading the areas on a map where the band has played, is already looking to the future. "I've spent the last 10 years doing nothing but this band," he said, looping his thick thumbs into the side of his signature black leather vest, which holds not only 12 harmonicas but also a Batman grappling hook. "Now I'm discovering the world is kind of my oyster. I might go to school for history. There's a movie I'm dying to make, but for now, as far as Blues Traveler is concerned, we're going to shift our focus to Europe." He pauses, gazing over the sprawling cemetery toward the sinking sun, then adds, "And beyond. . . . "