The Sunday congregation rocked with the steady pulse of two words, "Lord God, Lord God, Lord God," as bodies swayed and hands clapped.

"This house is filled with Jesus," the singers exploded in joyful harmony.

Enter Elmer Mackall.

With two partially amputated legs -- the longer one, ending just below the knee, bobbing to the beat -- 80-year-old Mackall was wheeled to the front of this makeshift church inside a small Jazzercise studio in Southern Maryland. He held the microphone like an ice cream cone. Then his raspy voice led the congregation of more than 75 people, his wide eyes searching upward and a broad smile slipping across his face.

"In his voice, you hear years of joy and hardship," said Rory Turner, the program director of folk and traditional arts for the Maryland State Arts Council. "You hear a great sense of personal integrity and spirituality. It's contagious. He makes you feel happy."

Today, Mackall will perform at the Smithsonian Folklife Festival on the Mall, a setting far removed from the Whole Heart Deliverance Temple, his nephew's church in Prince Frederick.

After decades of singing in close-knit churches -- "Every time somebody dies in Calvert County, they call him," joked one of his daughters -- Mackall (pronounced MAKE-ALL) has drawn the attention of regionwide music experts and historians. He recently received an arts council award for best solo vocal performance and, Turner said, should have won a lifetime award.

With the help of friends, Mackall has even recorded a CD.

"He says without music he'll die," said daughter Thelma Claggett, 49, who lives with him in Prince Frederick. She and two of her sisters, Margaret Copeland, 50, and Ollie Williams, 56, will accompany Mackall when he performs at the festival at 3:30 and 5 p.m.

Mackall sings about hope and faith, but he has endured a life of hardship. The descendant of slaves in Calvert, he eked out a living as a sharecropper and tobacco farm laborer. Severe health problems began in the mid-1990s, when complications from poor circulation led doctors to amputate his left leg below the knee. He lost much of his right leg to surgery last summer. Mackall also suffers from emphysema, and in January doctors diagnosed early stages of dementia, Claggett said.

"He sings about God . . . and finding strength of spirit," said Carrie Kline, a folklorist with the Southern Maryland Folklife Project, who has become a close friend of Mackall's and helped bring him to the attention of Smithsonian festival organizers.

Many of the spirituals he performs were passed on from his mother, Rosie, who sang in churches across Southern Maryland with her 17 children. Mackall, one of the youngest siblings, taught himself to play piano as a child by listening to his mother sing.

"I heard music in the air," he said of his childhood.

As a young boy, he'd walk miles each Sunday to worship and sing with his mother in church choirs. For a time in his twenties -- the "wild days," he calls them -- he strayed from his mother's teachings and sang in local bars. But he put a stop to that, he said, because, "I thought I was going to hell if I kept on."

In between sharecropping and, later, maintenance work, Mackall helped raise nine children: two boys and seven girls. "We didn't have money," Claggett said, "but we had a pretty good life."

He would gather his children in a circle around a potbellied stove in the kitchen of their home, Claggett said, and assign each child a different note to sing. Then he'd tell them to cover their ears and sing a cappella. "He put our hands over our ears to make us sing our own tune, and harmony came out," his daughter said.

At the Sunday church service last week, Claggett watched her father intently as she sat poised on the end of her folding chair. He tires easily, she said, but stubbornly refuses to rest.

The times when his voice fails him, he makes music on the piano, which he calls "singing with a feeling."

These days, Mackall rarely sings alone -- but only with his daughters -- for fear of forgetting the lyrics. "But," he said, grinning, "I'm always in charge."

Elmer Mackall sings at his nephew's church, Whole Heart Deliverance Temple. He has been singing since childhood, mostly at small churches.