Former US presidential candidate Senator John McCain has promised to beat the brain cancer he has been diagnosed with after having a blood clot removed.
Doctors said the Arizona senator has glioblastoma, an aggressive cancer.
The 80-year-old war hero and his family are considering further treatment, including a combination of chemotherapy and radiation.
The senator has been recuperating at his Arizona home after doctors at the Mayo Clinic removed a blood clot above his left eye.
The doctors said Mr McCain is recovering from his surgery amazingly well and his underlying health is excellent.
Mr McCain is a long-term survivor of melanoma, a deadly skin cancer. But doctors classified this new cancer as a "primary tumour," meaning it is not related to his former malignancies.
South Carolina Republican Senator Lindsey Graham said he spoke to Mr McCain by phone on Wednesday evening.
He said his close friend told him: "Yeah, I'm going to have to stay here a little bit longer, take some treatments. I'll be back."
Mr McCain was the Republican Party's presidential nominee in 2008, when he was defeated by Barack Obama.
Mr Obama tweeted: "John McCain is an American hero & one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John."
President Donald Trump said: "Senator John McCain has always been a fighter. Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon."
Mr McCain's absence from Washington had forced Majority Leader Mitch McConnell to delay action on health care legislation.
Mr McConnell said: "John McCain is a hero to our conference and a hero to our country. He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterised his life.
"The entire Senate family's prayers are with John, Cindy and his family, his staff, and the people of Arizona he represents so well."
Mr McCain's daughter Meghan said: "My love for my father is boundless and like any daughter I cannot and do not wish to be in a world without him. I have faith that those days remain far away."
Mr McCain, who served in Vietnam, has a lifetime of near-death experiences, including surviving the July 1967 fire and explosion on the USS Forrestal that killed 134 sailors.
In October 1967 his navy aircraft was shot down into Truc Bach Lake in Hanoi, and he spent more than five 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.
Mr McCain was elected to the Senate from Arizona six times, but twice thwarted in seeking the presidency.
A presidential bid in 2000 did not last long, but eight years later, he fought back from the brink of defeat to win the Republican nomination, only to be overpowered by Mr Obama.
Mr 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 Mr Obama in an electoral landslide, Mr 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, he 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".
Early in the 2016 campaign, Mr McCain largely held his tongue when Mr Trump said: "He was a war hero because he was captured. I like people who weren't captured."
He stuck by Mr Trump at times seemingly through gritted teeth - until the release a month before the election of a lewd audio tape in which the tycoon said he could kiss and grab women.
Declaring that the breaking point, Mr McCain withdrew his support and said he would back "some good conservative Republican who's qualified to be president".