John Selman Penington, 56, a staff reporter for the St. Petersburg Times who was credited by President Carter with launching his political career 18 years ago by exposing an election fraud attempt to deprive Mr. Carter of a state senate seat, died of cancer Sunday at the Bayfront Medical Center in St. Petersburg.

Born in Andersonville, Ga., Mr. Pennington eaned a bachelor's degree from the University of Georgia. He began his newspaper career in Atlanta, where he won more than two dozen state and regional awards for his investigative stories on school desegregation, Georgia prison reform and the 1962 state senate race in Southwest Georgia, which resulted in a recount and subsequent court order giving the seat to Jimmy Carter.

After a recent visit with Mr. Pennington at Bayfront, President Carter told reporters, "If it hadn't been for John's journalistic courage and effectiveness, I might have gotten out of politics 18 years ago.

Mr. Pennington, who served in the Army infantry in the Philippines in World War II, joined the St. Petersburg Times in 1978.In 1976 and 1977, he was press secretary for the director of ACTION in Washington,

Survivors include his wife, Lum, and a son, Jesse Wicker Pennington, both of St. Petersburg; three children from a previous marriage, Stuart, of Smyrma, Ga., Jeffrey Lyn of Raleigh, N.C., and Penny deCortez of Cali, Columbia; his mother, Beatrice Pennington of Plains, Ga.; three brothers, William W. Pennington Jr. of Albany, Ga., Billy Pennington of Americus, Ga., and Col. Lawrence E. Pennington of Humble, Tex., and a granddaughter.