BOSTON — A cancerous brain tumor caused the seizure Sen. Edward M. Kennedy suffered over the weekend, doctors said Tuesday in a grim diagnosis for one of American politics' most enduring figures.

Doctors for the Massachusetts Democrat say tests conducted after Kennedy suffered a seizure this weekend show a tumor in his left parietal lobe. Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma.

His treatment will be decided after more tests but the usual course includes combinations of radiation and chemotherapy.

"I'm really sad," former Sen. Bob Kerrey, D-Neb., said when told in a Senate hallway about Kennedy's condition. "He's the one politician who brings tears to my eyes when he speaks."

The 76-year-old senator has been hospitalized in Boston since Saturday, when he was airlifted from Cape Cod after a seizure at his home.

"He has had no further seizures, remains in good overall condition, and is up and walking around the hospital," said a joint statement issued by Dr. Lee Schwamm, vice chairman of the Department of Neurology at Massachusetts General Hospital and Dr. Larry Ronan, Kennedy's primary care physician.

The doctors said Kennedy will remain in the hospital "for the next couple of days according to routine protocol."

"He remains in good spirits and full of energy," they said.

Kennedy's wife and children have been with him each day but have made no public statements.

Malignant gliomas are a type of brain cancer diagnosed in about 9,000 Americans a year — and the most common type among adults. It's a starting diagnosis: How well patients fare depends on what specific tumor type is determined by further testing.

Average survival can range from less than a year for very advanced and aggressive types — such as glioblastomas — or to about five years for different types that are slower growing.

Kennedy, the second-longest serving member of the Senate and a dominant figure in national Democratic Party politics, was elected in 1962, filling out the term won by his brother, John F. Kennedy.

Kennedy eldest brother, Joseph, was killed in a World War II airplane crash. President John Kennedy was assassinated in 1963 and his brother Robert was assassinated in 1968.

Kennedy is active for his age, maintaining an aggressive schedule on Capitol Hill and across Massachusetts. He has made several campaign appearances for the Illinois senator in February, and most recently another in April.

Kennedy, the senior senator from Massachusetts and the Senate's second-longest serving member, was re-elected in 2006 and is not up for election again until 2012.

Were he to resign or die in office, state law requires a special election for the seat no sooner than 145 days and no later than 160 days after the vacancy occurs.