Sen. John McCain (R-Ariz.) has been diagnosed with brain cancer, his office announced Wednesday.

The 80-year-old Republican leader had been away from Washington the past several days, recovering in his home state from a surgery he underwent last Friday in Phoenix to remove a blood clot above his left eye. Following the operation, tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot.

McCain’s office said in a statement on Wednesday that the senator remained optimistic, despite the difficult news.

Senator McCain appreciates the outpouring of support he has received over the last few days. He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective.

Doctors at the Mayo Clinic are currently reviewing treatment options, and those may include a combination of chemotherapy and radiation. They say the senator is “recovering from his surgery ‘amazingly well’ and his underlying health is excellent.”

McCain’s daughter, Meghan, on Wednesday released a heartfelt tribute to her father following the news of his diagnosis, writing candidly about her father’s character and strengths. She also referred to McCain’s battle against skin cancer in the early 2000s.

“It won’t surprise you to learn that in all this, the one of us who is most confident and calm is my father,” she wrote. “He is the toughest person I know. The cruelest enemy could not break him.”

McCain’s surgery made headlines last week when the Senate decided to delay a vote on the GOP health care bill until he had recovered.

The Arizona senator has had a long career in public service. He has served the state of Arizona since 1982, when he was elected to the House of Representatives. Four years later, he won a seat in the U.S. Senate, where he has served as chairman of the Senate Commerce Committee and the Senate Armed Services Committee.

McCain ran a memorable presidential campaign in 2008, choosing Sarah Palin, then governor of Alaska, as his running mate. The two were defeated by then-Sens. Barack Obama and Joe Biden. Both of McCain’s 2008 Democratic opponents tweeted their support for him Wednesday evening:

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. — Barack Obama (@BarackObama) July 20, 2017

John and I have been friends for 40 years. He's gotten through so much difficulty with so much grace. He is strong - and he will beat this. — Joe Biden (@JoeBiden) July 20, 2017

President Donald Trump and the first lady sent their thoughts and prayers in a statement shortly after the announcement. “Senator John McCain has always been a fighter,” the president said. “Get well soon.”

And heartfelt well-wishes from politicians across the aisle poured in throughout the night.

John McCain is as tough as they come. Thinking of John, Cindy, their wonderful children, & their whole family tonight. — Hillary Clinton (@HillaryClinton) July 20, 2017

John McCain is one tough fighter - we know he'll face this diagnosis with courage and strength. Our family... https://t.co/XXTUltNGey — Sarah Palin (@SarahPalinUSA) July 20, 2017

Lindsey Graham responds to news of John McCain's brain cancer diagnosis.



"This disease has never had a more worthy opponent." pic.twitter.com/4S6cssfJCv — Matt Fuller (@MEPFuller) July 20, 2017

McCain responded Thursday, tweeting his gratitude for “the outpouring of support” and promising his “sparring partners in Congress” that he would return soon.

This story has been updated with McCain’s response.