WASHINGTON (CNN) -- The news that Sen. Edward Kennedy had been diagnosed with a brain tumor brought an onslaught of emotional response from Capitol Hill and the presidential campaign trail Tuesday.
Senate Democrats and Republicans were having their weekly closed policy lunches in the Capitol when the news broke.
When Majority Leader Harry Reid, D-Nevada, told his Democratic colleagues about Kennedy, there was "stunned silence," one senator said.
Capitol police officers and Senate staffers gathered around television screens off the Senate floor and watched news reports about Kennedy.
Kennedy, 76, was airlifted Saturday to Massachusetts General Hospital in Boston after suffering a seizure. Following a battery of tests, doctors determined that a malignant tumor in the brain caused the seizure, the hospital said Tuesday. Learn more about Kennedy's condition »
The bitter partisanship on the campaign trail somewhat dissolved Tuesday as Sens. Barack Obama, Hillary Clinton and John McCain responded to their colleague's illness.
Obama, the front-runner for the Democratic nomination for president, said the news about Kennedy is "heartbreaking." Watch more of the interview »
"It's our job now to support him in the way that the supported us for so many years. He's not just a great senator, he is a great friend. He is beloved by me and beloved by colleagues," Obama added.
Clinton, speaking on the campaign trail, called the news "very difficult."
"Obviously, he's a fighter. There isn't anybody like him. He gets up and goes out and does battle on behalf of all of us every single day. And I know he's a fighter when it comes to the challenges he's facing right now," she added. Watch Clinton speak about her colleague »
McCain, the GOP's presumptive presidential nominee, said, "Our thoughts and prayers go out to his family and to him. We hope and pray that they will be able to treat it and that he will experience a full recovery."
Watch more of McCain's interview »
Sen. John Kerry, a Kennedy friend and fellow Massachusetts Democrat, arrived late and entered a back door to the lunch. Looking drained, he declined to comment except to nod that he had spoken to Kennedy's family.
Later, Kerry said "every one of us knows what a big heart this fellow has. He's helped millions and millions of people in so many way in so many times."
"Now, everybody needs to pull for him and his family ... and remember, this guy is one unbelievable fighter," he added.
Sen. Chris Dodd, D-Connecticut, appeared alongside Kerry and added that Kennedy's a "strong guy and has great heart, and we're confident he'll be back here."
Sen. Dick Durbin, D-Illinois, released a statement saying, "Ted Kennedy has spent his life caring for those in need. Now it's time for those who love Ted and his family to care for them and join in prayer to give them strength." Watch more reaction from the Senate floor »
Sen. Patrick Leahy, D-Vermont, said it's "one of the worst days I've spent in the Senate to hear this news."
A distraught and shaken Sen. Robert Byrd, D-West Virginia, teared up on the Senate floor, saying "Thank God for you."
"I hope and pray that an all-caring, omnipotent God will watch over Ted and keep Ted here for us and for America. Ted, my dear friend, I love you, and I miss you." Watch Byrd's emotional response »
Kennedy's older brothers, President Kennedy and Sen. Robert F. Kennedy, were both assassinated. The oldest brother, Joseph Kennedy Jr., was killed in World War II. Kennedy was elected to the Senate in 1962. Learn more about Kennedy »
California Gov. Arnold Schwarzenegger, whose wife, Maria Shriver, is Kennedy's niece, said in a statement Tuesday afternoon, "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."
Another California resident, former first lady Nancy Reagan, released a statement saying, "I was so very sad today."
"We have always been good friends, and he and his family are all in my prayers. ... My heart goes out to all of them at this time," she said.
On the other side of the political aisle, Republicans expressed concern for Kennedy, perhaps the best-known Senate liberal.
Sen. Orrin Hatch, R-Utah, offered "heartfelt support and prayers" for his "good friend" on Tuesday.
"Ted and I have sparred a lot. So I know firsthand what a formidable fighter he is, and I know that he will do all he can to battle this disease. I join my Senate colleagues, Massachusetts residents and a grateful nation in wishing him a full and speedy recovery," Hatch said in a news release.
Minority Leader Mitch McConnell, R-Kentucky, said he was "hoping for the best and for a speedy recovery for our colleague Sen. Kennedy. And I know I speak for every Republican in the conference that this was a development of a great concern and sadness for all of our members." Watch McConnell discuss Kennedy »
Sen. Arlen Specter, R-Pennsylvania, who was diagnosed with a malignant brain tumor two years ago and is battling Hodgkin's disease, said "Kennedy is a real fighter. We all know that. And ... I'm betting on it," he said. Watch more of Specter's comments »
President Bush also released a statement, saying "Laura and I are concerned to learn of our friend Sen. Kennedy's diagnosis. Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit. Our thoughts are with Sen. Kennedy and his family during this difficult period. We join our fellow Americans in praying for his full recovery."
Meanwhile, House Speaker Nancy Pelosi, a California Democrat, said the news of Kennedy's brain tumor "has been met throughout the country with great concern. I have confidence, though, because Sen. Kennedy has been a fighter all his life. I know that this fighting spirit will hold him in good stead." Watch Pelosi speak about Kennedy »
House Majority Leader Steny Hoyer of Maryland released a statement saying, "Kennedy is a giant in the Senate, a wonderful public servant, and a great American."
Rep. Edward Markey, D-Massachusetts, called Kennedy "the greatest senator in American history and a tireless fighter for America's families."
CNN's Ted Barrett, Deirdre Walsh, Ed Hornick and Anastasia Diakides contributed to this report.