The severity of Sen. Edward M. Kennedy’s illness, which most senators learned of during their weekly party luncheons, stunned the Senate on Tuesday.

Senate business took on a somber tone. Democrats choked back tears, and Republicans said they were praying for their Democratic colleague.

Robert C. Byrd, D-W.Va., the only current senator who has served longer than Kennedy, broke down several times and sobbed on the Senate floor as he offered his wishes for Kennedy’s recovery and hailed the Massachusetts Democrat’s 46 years in the Senate.

“I feel like I just got hit by a ton of bricks,” said Carl Levin, D-Mich.

“I’m having a hard time remembering a day in my 34 years here when I’ve felt this bad,” added Patrick J. Leahy, D-Vt.

“In spite of our political differences, we are one large family here,” declared Minority Leader Mitch McConnell, R-Ky.

Kennedy, 76, has been hospitalized in Boston since May 17, when he suffered a seizure at his Cape Cod home. On Tuesday, Kennedy’s physicians announced that preliminary tests indicate the seizure was caused by a malignant tumor in the upper left portion of his brain.

Even before that diagnosis, Kennedy was not expected to return to the Senate until after the weeklong Memorial Day recess, which begins at the end of the week.

Democratic aides said it is too early to know if Kennedy will step down, even temporarily, as chairman of the Health, Education, Labor and Pensions Committee.

A leadership aide said Majority Leader Harry Reid, D-Nev., would not make any decisions without consulting with Kennedy to determine how to handle the senator’s absence.

If Kennedy and his doctors determine his absence will be prolonged, Reid could designate another senator to stand in for Kennedy as head of the committee, either on a short-term basis or through the rest of this year’s session.

Senate Democrats typically make committee leadership decisions based strictly on seniority. Christopher J. Dodd, D-Conn., chairman of the Banking, Housing and Urban Affairs Committee, is the second-most senior Democrat on Kennedy’s committee, followed by Agriculture Chairman Tom Harkin of Iowa. Because both already chair major committees, Reid might look another step down the seniority ladder to Maryland’s Barbara A. Mikulski.

Democratic presidential hopefuls Hillary Rodham Clinton of New York and Barack Obama of Illinois also serve on the HELP committee, but both are junior members.

When Tim Johnson, D-S.D., who was in line to head the Ethics Committee, suffered a life-threatening brain hemorrhage in late 2006, Reid polled senators to gauge who would be interested in filling in, and eventually tapped Barbara Boxer of California. Although Johnson returned to the Senate nine months later, the Senate adopted a resolution (S Res 492) extending Boxer’s chairmanship through the end of the year.

Kennedy’s doctors at Massachusetts General Hospital announced Tuesday that preliminary results from a biopsy of Kennedy’s brain identified a malignant glioma in the left parietal lobe, a portion of the brain that plays a role in sensory processes, attention and language, according to the Society of Neuroscience.

The usual course of treatment includes various forms of radiation and chemotherapy, according to a statement by Dr. Lee Schwamm, vice chairman of the hospital’s neurology department, and Dr. Larry Ronan, Kennedy’s primary care physician.

The American Cancer Society estimates that nearly 22,000 malignant tumors of the brain or spinal cord will be diagnosed in the United States this year and that roughly half of the people who suffer those tumors will die as a result.

First elected in 1962 to the seat once held by his brother John F. Kennedy, Kennedy trails only Byrd and the late Strom Thurmond, R-S.C., on the all-time Senate tenure list.

Catharine Richert, Molly Hooper, Josh Rogin and Alan K. Ota contributed to this story.