A statement issued by Specter's office did not specify the type of cancer he is fighting, and a spokesman declined to elaborate. Specter, 82, survived Hodgkin's lymphoma, or cancer of the lymphatic system, in 2005.

His office also declined to comment on local media reports that Specter has been hospitalized.

"I'm battling cancer," Specter said in the statement. "It's another battle I intend to win. I'm grateful for all the well-wishes I've received. I'm looking forward to getting back to work, to the comedy stage, to the squash court and to the ballpark."

In mentioning the stage, Specter was referring to his performances following his political career as an amateur stand-up comic. He appeared at Caroline's Comedy Club in Manhattan, for instance.

A political moderate, Specter was a five-term U.S. senator from Pennsylvania, serving as a Republican until he switched parties and became a Democrat in 2009. He said at the time the Republican Party had moved too far to the right.

The next year he lost Pennsylvania's Democratic primary for the seat.

Early in his career, Specter was a staffer for the Warren Commission, which investigated the assassination of President John F. Kennedy.

In a still-controversial finding, Specter was the main architect of what is known as the single-bullet theory, which argued that the president and Texas Governor John Connally, riding in the same car, were hit by the same bullet.

In 1965 he became the district attorney of Philadelphia, and was eventually elected to the U.S. Senate in 1980.

(Editing by Paul Thomasch and Philip Barbara)

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