John McCain's son on his dad: 'He's a tough old bastard'

"He is a tough old person. I usually use the term 'bastard,' so he's a tough old bastard," Jack McCain said in an interview with The Arizona Republic on Tuesday. "He has an indomitable spirit. It is impossible to keep the man down."

The 81-year-old senator was diagnosed with a primary glioblastoma, a particularly aggressive tumor that forms in the tissue of the brain and spinal cord, according to the American Brain Tumor Association.

Jack McCain, who is a Navy lieutenant and helicopter pilot, said that while John McCain's diagnosis is "not a good one," it is far from the first challenge that his father has faced. McCain was shot down over Vietnam in 1967. He was tortured and held as a prisoner of war at the infamous "Hanoi Hilton" for more than five years.

"Of all of the things that have happened in this man's life, of all of the times that his life could have ended in the ways it could have ended, this is by far one of the least threats to him --- and that's kind of how he views it," Jack McCain noted. "He's very sober about it. He understands the implication and the weight of it. But for not one minute has he let that take over his feelings towards how hard he has to fight."

Despite the typically poor prognosis that comes with malignant glioblastoma, Jack McCain expressed confidence in his father's long-term recovery. He said he speaks with John McCain every day, and each day he sounds "better than the day before."

"He is a very strong human being with the type of will that I have not seen paralleled anywhere else, and I have no doubts that he's going to be fine," he said.