(CNN) -- Former longtime U.S. Sen. Arlen Specter said Tuesday he plans to win his fight against cancer.
A source close to his office said Specter was battling for his life early Tuesday after he was recently diagnosed with a "serious form of cancer." He had a "big flare-up" of the disease Monday night, the source said.
Specter, who has overcome numerous serious illnesses over the past two decades, including a brain tumor and non-Hodgkins lymphoma, was diagnosed six weeks ago with the new form of cancer, the specifics of which are being closely shielded by his family.
"I'm battling cancer. It's another battle I intend to win," Specter said in a statement released Tuesday. "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."
Specter, 82, served in U.S. Senate from 1980 until 2011, making him the longest-serving senator from Pennsylvania.
Nearly all of those years, the political moderate was a member of the GOP, but he switched parties and became a Democrat in 2009, saying Republicans had moved too far to the right.
The move gave Democrats a 60-seat filibuster-proof majority in the Senate.
Two years later, when he ran for re-election, Specter lost in the Democratic primary, ending his political career.
After the loss, he moved from the halls of Congress to those of academia, taking on a new role at the University of Pennsylvania Law School.
"Arlen's knowledge of the inner workings of the government and lawmaking is second to none," said Michael Fitts, the law school's dean. "The insight he brings from his career in public service, particularly as a leader on judicial issues, will be invaluable to our students as they prepare for their own careers in the law."
Specter also returned to his law practice after leaving the Senate, according to his official online biography.
Specter was born in Wichita, Kansas, in 1930. He is the youngest child of Lillie Shanin and Harry Specter, an immigrant from the Ukraine.
After graduating from Russell High School in Kansas in 1947, he went to the University of Pennsylvania, where he majored in international relations and graduated Phi Beta Kappa four years later.
Specter served stateside in the U.S. Air Force during the Korean War from 1951 to 1953, before returning to college. He graduated from Yale Law School in 1956.
Specter earned some of his first political stripes in the early 1960s while serving on the Warren Commission, which investigated the assassination of President John F. Kennedy.
He is credited with co-authoring the "single bullet theory," which suggested that some of the wounds to Kennedy and then-Texas Gov. John Connally were caused by the same bullet.