McCain has been undergoing targeted radiation and chemotherapy treatments at the local Mayo Clinic on weekday mornings before going about his day with vigor.
In the past two weeks, the Republican has discussed a development project with Arizona mayors, given a radio interview and held a Facebook town hall. The chairman of the Senate Armed Services Committee outlined a military strategy for Afghanistan, attended an Arizona Diamondbacks baseball game and went hiking several times with his family. He has been active on Twitter, including condemning the white nationalist attack in Virginia while criticizing President Donald Trump's response to the violence.
Those who know the former POW well say they aren't surprised by McCain's upbeat and feisty approach to his latest challenge.
"This is all so characteristic of him, going back to his early days in Arizona politics," said Grant Woods, the state's former attorney general who served as McCain's administrative assistant while he campaigned in 1982 for a seat in Congress' lower house. "He outworked everyone, went door-to-door all summer in 110 temperatures."
McCain's spokeswoman in Washington, Julie Tarallo, said the senator was not available for an interview with The Associated Press. His daughter, Meghan McCain, tweeted Friday that he had just finished his first round of chemotherapy.
"His resilience & strength is incredible," she wrote. "Fight goes on, here's to small wins."
The senator's trip home during the August congressional recess comes as other lawmakers have returned to hostile constituents amid debates in Washington over health care and other elements of the president's agenda. Despite his busy schedule, McCain has avoided town hall meetings. He has no upcoming election to worry about, having won a sixth term in November.
Still, during McCain's time in Arizona, tensions have increased with Trump, who recently criticized the senator again for voting against the GOP health-care bill he backed. "You mean Senator McCain, who voted against us getting good health care?" Trump asked when his name came up during a news conference.
Trump's remark came a day after McCain criticized him for saying both the white nationalists and counterprotesters bear responsibility for the violence last weekend in Charlottesville, Virginia. The senator insisted in a tweet that "there's no moral equivalency between racists & Americans standing up to defy hate and bigotry" and the president should say so.
Trump also has sharpened his criticism of McCain's fellow Arizona Republican Sen. Jeff Flake in the lead-up to a Tuesday night rally Trump has planned in Phoenix, calling the senator "WEAK" on the border and crime.
McCain's friend Woods said McCain hasn't slowed down since undergoing surgery in mid-July to remove a 2-inch (51-millimeter) blood clot in his brain and being diagnosed with an aggressive tumor called a glioblastoma. It's the same type of tumor that killed Sen. Edward M. Kennedy at age 77 in 2009 and Beau Biden, son of then-Vice President Joe Biden, at 46 in 2015.
Glioblastoma is a somewhat unusual cancer, with the American Brain Tumor Association estimating only about 12,400 new cases will be diagnosed this year.
McCain's natural energy aside, his upbeat attitude is typical of people who've recently had a brain tumor removed, said Dr. Michael Lawton, chairman of neurosurgery and CEO and president at Phoenix's Barrow Neurological Institute.
"They experience a lot of relief from the problems the tumor caused," such as headaches or seizures from pressure on the brain.
Patients usually do well in the early post-operative stage, Lawton said. "But there could be some tough things down the road," he added, speaking generally about typical experiences with glioblastoma because Barrow is not involved in McCain's treatment.
Typical treatment of a glioblastoma involves chemotherapy and radiation to halt division of any possible remaining cancer cells and shrink any existing mass, followed by an MRI every two months to monitor for a recurrence, he said.
McCain's three-week round of treatments that ended Friday forced the globe-trotter to stay near home rather than travel to meet with troops or international leaders as he normally does each August. The senator said during his Facebook appearance that after this round, doctors will "see if there is anything additional that needs to be done."
In the meantime, "I feel good. I have plenty of energy." McCain warned friends and foes with a laugh: "I'm coming back!"
Survival amid seemingly unsurmountable odds has been a constant in the life of this son and grandson of four-star admirals.
As a Navy pilot, McCain lived through a July 1967 fire that killed 134 sailors aboard the aircraft carrier USS Forrestal in the Gulf of Tonkin during the Vietnam War. The following October, his plane was shot down during a bombing mission over Hanoi. He endured more than five years as a prisoner of war.
McCain also has survived several bouts with melanoma, a dangerous skin cancer.
He was first elected to the Senate in 1986. Over his six terms, McCain carved out a reputation as a maverick and became one of the best-known figures in American politics. He ran unsuccessfully for the Republican presidential nomination in 2000 and won it in 2008, but then lost to Barack Obama.
McCain returned to Washington after his operation, entering the Senate on July 25 to a standing ovation from his colleagues. He sported a wound from the surgery above his left brow and bruising under his eye.
In a widely praised speech, McCain complained to his fellow senators they had been "getting nothing done" because of partisanship and called the U.S. health care system a "mess." He then cast a thumbs-down vote against the latest attempt to repeal and replace Obamacare, winning praise from Democrats and scorn from the right.
Michael O'Neil, an Arizona pollster who writes a political column and has a local radio show, said facing such a serious illness will likely bring McCain even greater freedom to act on his beliefs. "All the political constraints are now gone," he said.
The American Cancer Society says the odds of surviving for five years or more are only 4 percent for people over 55.
Despite the diagnosis, the senator seems to be a man filled with gratitude as he approaches his 81st birthday on Aug. 29.
"Even those that want me to die don't want me to die right away," he said during his Facebook appearance. "Thank you for everything you've done for literally the luckiest guy on Earth."
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