Politco.com, a Washington-politics website, quoted a source as saying that a tumor had been found in Specter's stomach after he complained of having trouble breathing two months ago. The source told Politco that the cancer was serious and Specter was hospitalized.
Specter's family declined comment beyond the senator's statement, and even close friends said they were pretty much in the dark on how the five-term senator is doing.
Former Gov. Edward G. Rendell, a long-time ally and one-time neighbor of Specter's in Philadelphia's East Falls section, said Tuesday: "I don't know where he is, and I don't know the extent or the nature of what he is confronting."
Rendell said he had spoken to Specter's son Shanin on the phone and had asked him to pass word to his father that he should "keep fighting."
They both laughed at that, he said. "If there's one person on this earth who doesn't need to be told to keep fighting, it's Arlen," he said. "He has twice beaten back diseases that were intended to kill him."
Specter, a fitness fanatic, has had repeated mortality scares.
He had radiation treatments in 1996 for an undisclosed ailment and a heart bypass operation in 1998. He was diagnosed with Hodgkin's disease, a lymphatic cancer, in 2005. He beat that, but the disease returned in April 2008.
In July 2008, while beginning to regain his hair after chemotherapy, he met with a small group of reporters at his hideaway office in the Senate to get out the word that he was "at the top of my game" and planned to seek a sixth term in 2010.
As a Republican-turned-Democrat, he lost the 2010 Senate primary to then-Democratic Rep. Joseph Sestak. But the reason wasn't poor health.
"You have to understand what incredible will and what incredible stamina Arlen Specter has," Rendell said. "During his last illness, on the days he didn't get chemotherapy, he would come to The Sporting Club and play squash."
Specter, who does dry stand-up comedy as a hobby and who regularly follows Philadelphia sports teams, completed his third book-length memoir this year.
Charles Robbins, co-author of "Life Among the Cannibals," the latest book, said Tuesday he, too, was out of the loop on Specter's illness.
Robbins, a former Senate aide and campaign aide for Specter, said that "for some weeks, word has been percolating through the [staff] alumni association that he was sick again. It was buttoned down to the point that some thought it was cancer but we weren't sure."
Contact Tom Infield at 610-313-8205 or firstname.lastname@example.org, or follow on Twitter @tinfield.