Washington (CNN) John McCain has always lived for the fight. Now he's facing his toughest battle.

The Arizona Republican senator has often seemed indestructible, despite the best efforts of his Vietnam War jailers, an earlier bout with melanoma and a list of honorable political defeats. And now he has been diagnosed with brain cancer, as CNN reported Wednesday,

He's a warrior politician who bears the scars of a lifetime of military and political campaigns and health scares on his body and across his soul. He's collected more enemies and friends than most men and is a certified national hero.

Eight years ago, McCain, one of the last giants of the Senate, stood before the flag-draped coffin of his friend and sparring partner Sen. Edward Kennedy, who had succumbed to the same disease he is now fighting, and explained their common approach to life.

"Ted and I shared the sentiment that a fight not joined was a fight not enjoyed," McCain said, recalling roiling arguments with his fellow Senate lion, but also times when they had buried their differences to forge progress for the nation.

McCain has said he's itching to get back to work after his surgery nearly a week ago and has been working the phones. But he was told not to travel for two weeks. Now, his movements could be curtailed further by treatments that could include chemotherapy and radiation.

McCain tweeted his appreciation Thursday for the outpouring of support he's received from colleagues on both sides of the aisle, as well as other political giants.

"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" he wrote.

I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by! — John McCain (@SenJohnMcCain) July 20, 2017

Long-term, McCain's outlook is daunting. His doctors told CNN's Sanjay Gupta with McCain's permission that he was suffering from a glioblastoma tumor, an aggressive form of cancer. Kennedy, who was also diagnosed during a bitter Senate showdown over health care, survived for about 15 months after a similar diagnosis. Gupta explained that average survival for malignant glioblastoma tends to be around 14 months with treatment.

Still, one 2009 study reported that almost 10% of patients with glioblastoma may live five years or longer, according to the American Brain Tumor Association.

One thing is for sure though, McCain will fight -- something one of his former opponents pointed out Wednesday night.

"John McCain is an American hero & one of the bravest fighters I've ever known," former President Barack Obama tweeted. "Cancer doesn't know what it's up against. Give it hell, John."

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

His daughter, Meghan, issued a statement paying tribute to her father when his diagnosis became public.

"He is the toughest person I know. The cruelest enemy could not break him. The aggressions of political life could not bend him ... cancer may afflict him in many ways; but it will not make him surrender. Nothing ever has."

Throughout his military and political career, McCain has been the epitome of his hero President Theodore's Roosevelt's "Man in the Arena," whose face is "marred by dust and sweat and blood" and who fails often "daring greatly" unlike timid souls who know neither victory nor defeat.

Fixture of Capitol Hill

But that combative streak has sometimes revealed a brittle, impetuous side of his personality that may have limited the ultimate height of his military and political careers and has often emerged in high pressure moments.

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Still, McCain is also a throwback, maintaining friendships with rivals across the political aisle, which often got testy, as when he confronted Hillary Clinton and fellow Vietnam War veteran John Kerry at committee hearings when they were in the Senate with him.

When he lost to Obama in the 2008 election, McCain entered a dark period of his public life, often coming across as angry and not yet having come to terms with his defeat.

But the Republican recapture of the Senate in the 2014 mid-term election gave him a chance to rewrite the final chapter of his career, as he at long last took the gavel of the Senate armed services committee, an assignment he had long coveted. Soon, he was taking the Obama administration to task over its policies in Iraq, Syria and Ukraine.

But he knew his time was limited.

"Every single day," McCain told the New York Times in 2015, "is a day less that I am going to be able to serve in the Senate."

McCain, elected just last fall to his sixth Senate term, inadvertently found himself at the center of the 2016 presidential election race when he was attacked by Donald Trump who said: "He was not a war hero," Trump said in Iowa in 2015.

"He was a war hero because he was captured. I like people who weren't captured."

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The comment triggered outrage, given McCain's war record. But it also marked an important moment in political history -- for the first time displaying Trump's capacity for getting away with behavior that would have sunk other candidates.

McCain's tense relationship with Trump was underlined on Monday, prior to the news of his cancer diagnosis, when the President said he hoped that the Arizona senator would get better soon : "Because we miss him. He is a crusty voice in Washington .... plus, we need his vote."

In recent weeks, McCain has emerged as a source of colorful quotes for reporters over the Russia cloud swirling around Trump, repeatedly predicting that "there will be more shoes to drop" in what he has painted as a classic Washington scandal. He has also made clear his disdain for any efforts to improve relations with Russian President Vladimir Putin.

The Arizona senator's absence from Washington in recent days has come at a politically inopportune time for Senate Majority Leader Mitch McConnell as he has desperately tried to find a majority to repeal and replace Obamacare.

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Any prolonged absences from Washington by the Arizona senator will further reduce McConnell's already wafer-thin majority.

This week, after the health care push floundered, McCain broke ranks and called for discussions with Democrats and a committee process to finally provide "Americans with access to quality and affordable health care."

McCain has often joked he is "older than dirt."

But though he is now 80, McCain has made few concessions to his age. He has maintained a punishing schedule of world travel that would exhaust a man half his age. He often pops up on Sunday talk shows, direct from Arizona in the dawn hours. The Washington Post reported last month that McCain had traveled 75,000 miles to more than 15 nations so far this year alone.

CNN town hall with US Sens. John MCcain and Lindsey Graham on Wednesday, March 1.

His bosom buddy, South Carolina Sen. Lindsey Graham, told CNN earlier this week that the hectic pace had taken a toll.

"You know he just wore himself out traveling all around the world," Graham said.

It is unknown whether McCain's condition contributed to his somewhat confused line of questioning during fired FBI Director James Comey's appearance on Capitol Hill in June.

There are constant reminders of his previous battles with mortality however.

His puffy left cheek and a scar on his face date from melanoma surgery in 2000. During his 2008 campaign, aides had to comb his hair, as he still cannot lift his arms above his head -- thanks to injuries that date from his plane crash and years as a prisoner of war.

But McCain's experience in Vietnam hardly made him less enamored of the use of military power abroad. Sometimes his outrage over some conflict or example of mass human rights abuses seems to boil out of him, in the conviction that American leadership has been lacking.

It's an attitude that has seen Democratic critics claim that he is the embodiment of a Republican reflex to respond to every global problem with military force, which led America into misadventures like the war in Iraq.

His hawkish worldview is reflected on the walls of McCain's Senate conference room, which features letters and photos from the likes of Ronald Reagan and Margaret Thatcher, leaders who didn't suffer critics gladly.

From Hanoi Hilton to Washington

McCain's legend is well known. The son and grandson of admirals, he refused the offer of a preferential release from the infamous Hanoi Hilton prison camp until his comrades could also go home. After his Skyhawk jet was shot down over North Vietnam, McCain was a prisoner for five-and-a-half years, several of them in solitary confinement. He returned home to a nation torn by the war in 1973 and finally retired from the US Navy in 1981. He served in the House of Representatives from 1983 to 1987 as a down-the-line conservative and then succeeded the retiring Barry Goldwater in the Senate.

His Senate career almost crashed to earth before it began. In 1989 he was among the Keating Five group of senators accused of interfering with regulators in a campaign finance case. He was cleared of wrongdoing, but the Senate ethics committee accused him of poor judgment, an experience which led to him becoming a pioneer of campaign finance reform.

He joined Kerry to facilitate the reconciliation and the eventual re-opening of diplomatic relations between the United States and Vietnam.

By 2000, he set his sights on the White House and ran as a maverick Republican holding court for hours in candid back-and-forth sessions with reporters on his campaign bus. His effort was eventually crushed by then-Texas Gov. George W. Bush after a brutal South Carolina primary campaign and he returned to the Senate, in time for a deeply consequential period that saw national security policy dominate Washington with the 9/11 attacks in 2001 and the Iraq and Afghanistan wars, of which McCain was a strong supporter.

McCain, who is often seen on Capitol Hill at the center of a gaggle of reporters, was once so beloved by the press that he once joked the media was his "base."

By late 2007, he was ready for his next battle -- another presidential campaign, and this time won the Republican nomination but ended up coming up short against Obama in 2008.

History may best remember his campaign for his selection of Alaska Gov. Sarah Palin as his vice presidential pick, a move that was hailed as bold at the time, but was later criticized as his running mate's inexperience led critics to charge she was unqualified to be a heartbeat away from the presidency.

If the past is any guide, McCain will be reacting to his diagnosis with his characteristic humor. One of his favorite lines on the stump tells how he got over his defeat to Obama in 2008.

"After I lost, my friends, I slept like a baby — sleep two hours, wake up and cry."