South Carolina Sen. Lindsey Graham has urged U.S. citizens to pray for John McCain after he was diagnosed with an aggressive form of brain cancer.

McCain, the 2008 Republican presidential nominee and longtime Arizona Senator, reportedly told Graham he expects to return to Washington once he's been treated.

"Yeah, I'm going to have to stay here a little bit longer, take some treatments. I'll be back," the 80-year-old politician said. 

Graham, who shared the conversation with reporters on Wednesday, added, "So pray. God knows how this ends, not me. But I do know this: This disease has never had a more worthy opponent."

McCain, chairman of the Senate's Armed Services Committee, has glioblastoma, an aggressive cancer, according to doctors at the Mayo Clinic in Phoenix. On Friday, he had a blood clot removed from above his left eye, and doctors have said he is recovering from his surgery "amazingly well" and that his underlying health was excellent.

His treatment options include a combination of chemotherapy and radiation, his office said. McCain was already a cancer survivor, having had non-invasive melanomas removed at least three times.

In a statement on Twitter, the senator's daughter, Meghan McCain, spoke of the shock of the news and said it has "affected every one of us in the McCain family."

"My grandmother, mother, brothers, sister, and I have all endured the shock of the news, and now we live with the anxiety about what comes next," she said.

Meghan added that the family was familiar with the experience of battling cancer since her father had fought it previously, and that through all of this, McCain himself was the "most confident and calm" of all of them.

"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," she said.

Meghan also described her father as a "warrior at dusk" who would not be defeated by the disease.

McCain is no stranger to overcoming obstacles; during his time as a U.S. Navy pilot, his plane was shot down over Vietnam in 1967, and he spent 5-1/2 years as a prisoner of war. The injuries he sustained from being tortured at the hands of his Communist captor left him unable to lift his arms above his head 

According to Fox News, a group of senators prayed together Wednesday night after learning that McCain had been diagnosed with a brain tumor.

Sen. John Hoeven, R-N.D., said he asked Sen. James Lankford, R-Okla., who has a master's degree in divinity, to lead the group in prayer.

"It was very emotional," Hoeven said.

House Speaker Paul Ryan also offered prayers for the Senator, saying, "all of us, not as Republicans or Democrats, but as Americans, are behind him. The prayers of the whole House are with Senator McCain and his family."