WASHINGTON (AP) — Battling brain cancer, Sen. John McCain on Thursday promised to return to work, making a good-natured dig at his Republican and Democratic colleagues who were jolted by news of the six-term lawmaker's diagnosis.

"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" McCain said in a tweet.

The 80-year-old McCain, the GOP's presidential nominee in 2008, was diagnosed with glioblastoma, an aggressive type of brain cancer, according to doctors at the Mayo Clinic in Phoenix, who had removed a blood clot above his left eye last Friday.

"Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot," his office said in a statement late Wednesday.

The senator and his family are considering further treatment, including chemotherapy and radiation, as he recuperates at his home in Arizona.

Prayers and words of encouragement multiplied on Thursday from presidents and Senate colleagues past and present.

"I called Senator John McCain this morning to wish him well and encourage him in his fight. Instead, he encouraged me," said former President George W. Bush, who prevailed over McCain for the GOP presidential nomination in 2000. "I was impressed by his spirit and determination."

Former Sen. Bob Dole of Kansas said: "Having known John for many decades, I am certain that he is as tough as they come - if anyone can defeat this, it's him. John is a true American hero."

McCain's closest friend in the Senate, Lindsey Graham of South Carolina, said he spoke to the senator Wednesday night and that the diagnosis was a shock to McCain. He said McCain is fighting the illness, and "woe is me" is not in his DNA. "One thing John has never been afraid of is death," said Graham, who said he expects McCain to be back.

According to the American Brain Tumor Association, more than 12,000 people a year are diagnosed with glioblastoma, the same type of tumor that struck McCain's close Democratic colleague in legislative battles, the late Ted Kennedy of Massachusetts. The American Cancer Society puts the five-year survival rate for patients over 55 at about 4 percent.

McCain has a lifetime of near-death experiences — surviving the July 1967 fire and explosion on the USS Forrestal that killed 134 sailors; flying into power lines in Spain; the October 1967 shoot-down of his Navy aircraft and fall into Truc Bach Lake in Hanoi; and 5 1/2 years in a North Vietnamese prison.

"The Hanoi Hilton couldn't break John McCain's spirit many years ago, so Barbara and I know — with confidence — he and his family will meet this latest battle in his singular life of service with courage and determination," said former President George H.W. Bush.

The absence of the senator and chairman of the Armed Services Committee forced Majority Leader Mitch McConnell, R-Ky., to delay action on health care legislation.

Politics aside, McCain and Bill Clinton developed a strong friendship, and the former president said: "As he's shown his entire life, don't bet against John McCain. Best wishes to him for a swift recovery."

The junior senator from Arizona, Republican Jeff Flake, said Thursday that McCain told him about his tumor only at the end of a telephone conversation, saying he was "feeling fine, but I might have some chemotherapy in my future." Flake said on ABC's "Good Morning America" that his colleague is "optimistic, obviously. He's John McCain. That's what we'd expect."

In the past, McCain had been treated for melanoma, but a primary tumor is unrelated. Doctors said McCain is recovering from his surgery "amazingly well" and his underlying health is excellent.

With his irascible grin and fighter-pilot moxie, McCain was elected to the Senate from Arizona six times, most recently last year, but was twice thwarted in seeking the presidency.

An upstart presidential bid in 2000 didn't last long. Eight years later, he fought back from the brink of defeat to win the GOP nomination, only to be overpowered by Obama. McCain chose a little-known Alaska governor as his running mate in that race, and helped turn Sarah Palin into a national political figure.

After losing to Obama in an electoral landslide, McCain returned to the Senate, determined not to be defined by a failed presidential campaign. And when Republicans took control of the Senate in 2015, McCain embraced his new job as chairman of the powerful Armed Services Committee, eager to play a big role "in defeating the forces of radical Islam that want to destroy America."

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AP Medical Writer Lauran Neergaard and writers Nancy Benac and Alan Fram contributed to this report.