Sen. Ted Kennedy has been diagnosed with a malignant brain tumor and will remain hospitalized for at least several days as he and his family determine his treatment options.
The Massachusetts Democrat suffered a seizure Saturday and has since been hospitalized at Massachusetts General Hospital in Boston. He has been talking and joking with family and friends while undergoing a battery of tests that revealed the malignant tumor, a glioma in the left parietal lobe, according to the hospital.
"He has had no further seizures, remains in good overall condition and is up and walking around the hospital," Drs. Lee Schwamm and Larry Ronan said in a joint statement released by the hospital.
"The usual course of treatment includes combinations of various forms of radiation and chemotherapy," they said. "Decisions regarding the best course of treatment for Sen. Kennedy will be determined after further testing and analysis."
Kennedy, the second-longest serving senator in U.S. history, has represented Massachusetts in the Senate since 1962.
"Tough days ahead," a Kennedy family friend told ABC News, "but [Sen. Kennedy] is in fighting mood."
Another Kennedy family member is bringing in another doctor for further evaluation, saying, "This is not a good time."
Republicans, Democrats Unite in Praise
In an interview with ABC News' Jake Tapper for "World News With Charles Gibson", Sen. Barack Obama, D-Ill., who has received Kennedy's endorsement for president, said, "It's heartbreaking. Ted Kennedy is not only a giant of the Senate, but he's a good friend. You couldn't have a better supporter than Ted Kennedy."
Obama talked with Vicki Kennedy, the senator's wife, Saturday and said he was more optimistic after that converstion.
"The news came in today and it's a lot worse, but he's a fighter. He's been fighting on behalf of working families all his life. … It's a testament to how beloved he is that you are seeing well wishes from across the aisle from everyone in political life and Massachusetts."
In a statement released to the press, Sen. Hillary Clinton, D-N.Y., in Kentucky for an election night rally, said, "Ted Kennedy's courage and resolve are unmatched, and they have made him one of the greatest legislators in Senate history. Our thoughts are with him and Vicki and we are praying for a quick and full recovery."
Mrs. Kennedy informed Senate Majority Leader Harry Reid, D-Nev., Tuesday afternoon of Kennedy's diagnosis. Reid, in turn, informed the Senate Democratic caucus at its weekly luncheon on Capitol Hill.
Fighting back tears at a press conference following the luncheon, Sen. Chris Dodd, D-Conn., described by Reid as Kennedy's closest friend in the Senate, said, "We're confident he's going to be back."
Sen. Robert C. Byrd, a West Virginia Democrat and the most senior member of the Senate, openly wept while speaking from his desk on the Senate floor.
"I want to take a moment to say how distraught and terribly shaken I am over the news of my dear friend, my dear, dear friend, dear friend, Ted Kennedy," Byrd said as he began to cry. "My thoughts and my humble prayers are with Sen. Kenney, my dear friend Ted, with his wife Vicki, and with the members of the Kennedy family. I hope and pray that in an all-caring omnipotent god will watch over Ted and keep Ted here for us and for America. Ted, Ted, my dear friend, I love you and I miss you and Erma, Erma, Erma, my darling wife Erma, I want to say, thank god for you Ted, thank god for you."
Sen. John Kerry, D-Mass., called his colleague from Massachusetts "one unbelievable fighter," saying when he visited Kennedy over the weekend the senator, who endorsed Kerry's bid for the White House in 2004, was in a "fighting mood."
Gov. Arnold Schwarzenegger, R-Calif., whose wife is a member of the Kennedy family, issued a statement, saying, "Maria and I are thankful for everyone's thoughts and prayers today and over the past several days. While we are still learning the extent of Teddy's diagnosis and treatment options, what we do know is that Teddy is an incredibly courageous and tenacious man who will tackle this with the same determination with which he approaches everything in life. I encourage everyone to keep Teddy in their prayers, as we are continuing to do."
President Bush was informed by staff about Kennedy's brain tumor Tuesday afternoon. White House spokeswoman Dana Perino said the president was "deeply saddened" and would keep him in his prayers.
Sen. John McCain, R-Ariz., the presumptive Republican nominee for president, a cancer survivor, responded to the news aboard his campaign bus in Florida.
McCain's campaign also released a statement praising Kennedy, saying, "I have described Ted Kennedy as the last lion in the Senate, and I have held that view because he remains the single most effective member of the Senate."
Kennedy Suffers Seizure, Rushed to Hospital Last Weekend
Kennedy, the last of three brothers who changed the American political scene in 1960s, was taken by ambulance to a Cape Cod hospital Saturday morning and later flown to Massachusetts General Hospital by helicopter.
"Preliminary tests have determined that he has not suffered a stroke and is not in any immediate danger," Kennedy's physician, Dr. Larry Ronan, said Saturday evening. "He's resting comfortably and watching the Red Sox game with his family."
Kennedy's wife, Vicki, was with him at the hospital, Kennedy family spokeswoman Stephanie Cutter said.
Several other members of the senator's family visited him at the hospital, including his three children, Kara, Edward Jr. and Rep. Patrick Kennedy, D-R.I., niece Caroline Kennedy and nephew, former Rep. Joseph P. Kennedy II.
He spent much of the afternoon watching sports on television: first the Boston Red Sox game, then the Boston Celtics' playoff game against the Cleveland Cavaliers. Kennedy went to Cape Cod Hospital Saturday "after feeling ill at his home" in Hyannisport, Cutter said.
"We got a 911 call at 8:19 a.m. [Saturday] morning for a request for an ambulance," Lt. Bill Rex, a spokesman for Hyannisport Fire and Rescue, confirmed to ABC News Saturday. "We transferred one male to Cape Cod Hospital in Hyannis."
Kennedy's doctors in Boston were contacted, and it was decided he should be taken to Massachusetts General Hospital for further testing.
The Cape Cod Times first reported this weekend's medical emergency and has posted a photo of Kennedy being taken to a MediFlight on a stretcher.
A Democratic Senate source told ABC News at the time, "The situation is serious. We just don't know how serious."
The Kennedy family was set to host the annual Best Buddies Challenge event Saturday evening at the Kennedy family compound.
Kennedy's Close Calls
Kennedy is no stranger to brushes with death.
In 1964, only a year after the assassination of President Kennedy, the newly elected senator and Sen. Birch Bayh, D-Ind., were nearly killed in a horrific plane crash that claimed the life of the pilot and one of Kennedy's aides.
Kennedy suffered two fractured ribs and three broken vertebrae in his back, an injury that would cause him a lifetime of back pain.
In October 2007, Kennedy had surgery to repair blockage in the left carotid artery in the neck believed to be connected to the injuries he suffered in the crash more than 40 years ago.
In July 1969, a year after the assassination of his brother, Sen. Robert F. Kennedy, the senator once again escaped a fatal crash.
After a night of partying, Kennedy drove his mother's 1967 Oldsmobile over a wooden bridge on Chappaquiddick, an island off of Martha's Vineyard, killing passenger Mary Jo Kopechne.
Questions surrounding how Kennedy survived while Kopechne drowned have haunted the senator ever since.
He was convicted of leaving the scene of an accident and lost his driver's license for a year, but the Kopechne family never sued the Kennedys or the senator for his role in the incident.
In more recent years, Kennedy was traveling with the late Sen. Paul Wellstone, D-Minn., on the day Wellstone's campaign plane crashed just outside Eveleth, Minn.
Kennedy had been on several flights with Wellstone on the twin-engine turboprop but did not accompany him on the fatal flight, as Wellstone was traveling to a funeral and not a campaign event.
Wellstone, his wife, daughter, three staff members and the two pilots were killed in the crash.
'Liberal Lion' Still Roars
Even though he is the third longest-serving senator in the history of the body -- with 45 years and two months in office, behind only the current Sen. Robert Byrd of West Virginia and Strom Thurmond of South Carolina -- Kennedy is one of the hardest-working members of Congress.
As the chairman of the Health Education and Labor Committee, he was the Democrat who ran the Senate floor last week when senators were considering a bill to give collective bargaining rights to all police and firemen. He was also the chief Democrat who fought for a recently passed student loan improvement bill.
Kennedy chaired a widely covered hearing May 12 about how to fight cancer. Among the people who testified during that hearing were Lance Armstrong, Elizabeth Edwards and Steve Case.
Kennedy was also set to play a pitvotal role for Democrats as they consider the supplemental war funding bill next week. Kennedy was one of the loudest critics of the war in Iraq from its outset.
Before his illness this weekend, Kennedy was apparently working; his office released a statement in his name, congratulating Benjamin Jealous on his election as head of the NAACP.
The Associated Press and ABC News' Jen Duck, Eloise Harper, Bret Hovell, Dean Schabner, Stephanie Z. Smith, George Stephanopoulos and Sunlen Miller contributed to this report.