(CNN) -- Fred Thompson, the former Republican senator from Tennessee who has said he might enter the 2008 presidential race, has been diagnosed with a treatable form of lymphoma, according to a source close to him.
Lymphoma is a type of cancer involving cells of the immune system.
The source is familiar with Thompson's deliberations on whether to jump into the 2008 presidential race and told CNN's Candy Crowley that Thompson has been discussing his illness privately "for some time."
Another source close to Thompson said the former senator has been living with the disease for 2 1/2 years.
Asked what this might do to a possible presidential bid, the source said, "This disclosure should indicate to everyone interested in seeing Fred enter the race for the White House how serious he is taking the next steps toward launching a campaign."
Thompson has played the role as gruff district attorney Arthur Branch on NBC's three "Law & Order" series.
Thompson was the minority counsel to the Senate Watergate Committee before being elected to the Senate in 1994 to fill the unexpired term of Vice President Al Gore. After chairing the Senate Governmental Affairs Committee, he stepped down in 2003 to resume his acting career.
In 2005, Thompson helped shepherd Chief Justice John Roberts through the Senate confirmation process, and most recently, he has raised money for the defense of I. Lewis "Scooter" Libby, Vice President Dick Cheney's former chief of staff. source