The 80-year-old McCain had a blood clot removed about his left eye earlier this month and was diagnosed with glioblastoma, an aggressive type of brain cancer.
He returned to the Senate on Tuesday, casting a crucial vote to move ahead on health care legislation. Early Friday morning, he dashed GOP hopes on legislation by opposing a repeal measure.
People who knew McCain well, and those who have worked with McCain over the years said the "No" vote on Thursday night was not a shocker, saying McCain stood on principle.
"McCain's point is, 'listen, we need regular order. Health care is too important an issue. Everybody is involved. It needs to be thoughtful and the stakes are too high if we do it wrong,'" said Doug Cole with Highground Public Affairs Consultants.
People who knew McCain best said the Senator will get through this latest cancer diagnosis.
Sen. John McCain, recently diagnosed with an aggressive form of brain cancer, is returning to Arizona to begin radiation and chemotherapy.
In a statement Friday, his office said he will undergo further treatment at the Mayo Clinic in Phoenix while maintaining his work schedule. The senator, who won a sixth term last year, plans to return to Washington at the end of Congress' August recess.
"He has been treated for cancer before and survived that, and he has survived a lot when he wasn't supposed to on many occasions, and I think he is going to survive this as well," said Wes Gullett with Oh Strategic Communications.
Gullett went on to say part of what powers Sen. McCain is his personal toughness, and also the sence that the country needs him right now.
"A lot of people are like, he is 80, why not retire?" said Gullett. "He is at an important point in American history. Unique waters never at this point with President with no government background, and I think McCain believes it is important time for him to be part of the process."