John McCain has promised to return to work while making a good-natured dig at his Republican and Democratic colleagues who were jolted by news of the senator's brain cancer diagnosis.
"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" Mr McCain said in a tweet.
I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!— John McCain (@SenJohnMcCain) July 20, 2017
The 80-year-old, 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 tumour known as a glioblastoma was associated with the blood clot," his office said in a statement late on 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 Mr 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."
Mr McCain's closest friend in the Senate, Lindsey Graham of South Carolina, said he spoke to the senator on Wednesday night and that the diagnosis was a shock.
He said Mr 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 Mr Graham, who said he expects Mr McCain to be back.
According to the American Brain Tumour Association, more than 12,000 people a year are diagnosed with glioblastoma, the same type of tumour that struck Mr 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%.
Mr 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 five-and-a-half 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 HW Bush.
Politics aside, Mr 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."
As he’s shown his entire life, don’t bet against John McCain. Best wishes to him for a swift recovery.— Bill Clinton (@BillClinton) July 20, 2017
In the past, Mr McCain had been treated for melanoma, but a primary tumour is unrelated.
Doctors said Mr McCain is recovering from his surgery "amazingly well" and his underlying health is excellent.