GOP Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE's family announced on Friday that the Arizona senator has decided to discontinue medical treatment, more than a year after being diagnosed with an aggressive form of brain cancer.

"In the year since, John has surpassed expectations for his survival. But the progress of disease and the inexorable advance of age render their verdict. With his usual strength of will, he has now chosen to discontinue medical treatment," McCain's family said in a statement released by his office.

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"Our family is immensely grateful for the support and kindness of all his caregivers over the last year, and for the continuing outpouring of concern and affection from John’s many friends and associates, and the many thousands of people who are keeping him in their prayers,” they added. “God bless and thank you all."

The statement comes less than a week before the GOP senator's 82nd birthday.

"My family is deeply appreciative of all the love and generosity you have shown us during this past year,” his daughter Meghan McCain wrote in a tweet Friday. “Thank you for all your continued support and prayers. We could not have made it this far without you - you've given us strength to carry on.”

His wife, Cindy McCain, added: "I love my husband with all of my heart. God bless everyone who has cared for my husband along this journey."

McCain was diagnosed in July 2017 with brain cancer, which was discovered after he had surgery to remove a blood clot above his left eye.

He was first elected to the Senate in 1986 but rose to prominence as the party's 2008 presidential nominee and is known for a "maverick" streak that has at times rankled congressional leadership and presidents of his own party.

The Arizona Republican has had a rocky relationship with Trump dating back to the 2016 presidential campaign, when candidate Trump mocked McCain for being captured during the Vietnam War.

That tension escalated as McCain emerged as one of the loudest GOP critics of Trump in the Senate.

McCain returned to Washington, from Arizona, weeks after his diagnosis to urge senators to "trust each other" and "return to regular order" at a time when the chamber was locked in a deeply divisive debate over the GOP effort to repeal and replace ObamaCare.

Days later, he sided with GOP Sens. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Billionaire who donated to Trump in 2016 donates to Biden Credit union group to spend million on Senate, House races MORE (Maine) and Lisa Murkowski Lisa Ann MurkowskiOVERNIGHT ENERGY: House passes sweeping clean energy bill | Pebble Mine CEO resigns over secretly recorded comments about government officials | Corporations roll out climate goals amid growing pressure to deliver The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Abortion stirs GOP tensions in Supreme Court fight MORE (Alaska) to sink the Republican plan to repeal ObamaCare. The move earned him the frequent wrath of Trump, who mocked McCain as recently as this month, saying at a New York fundraiser that "one of our wonderful senators said ‘thumbs down’ at 2 o'clock in the morning," a reference to McCain voting down the ObamaCare repeal bill.

Though McCain has been absent from Capitol Hill since December — when his office announced that he had returned to Arizona for "physical therapy and rehabilitation" — he's continued to weigh in on the Senate's foreign policy and national security debates.

McCain, who is chairman of the Senate Armed Services Committee, has often tried to position himself as a counterbalance to Trump's White House on matters of U.S. foreign policy, an area where the two men have widely divergent worldviews.

He sharply criticized the president’s Russia rhetoric after the July summit with Russian President Vladimir Putin in Helsinki, Finland, saying Trump’s remarks were "disgraceful." He signed on as a cosponsor earlier this month to a "crushing" Russia sanctions bill, spearheaded by Sens. Lindsey Graham Lindsey Olin GrahamSteele Dossier sub-source was subject of FBI counterintelligence probe Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Key Democrat opposes GOP Section 230 subpoena for Facebook, Twitter, Google MORE (R-S.C.) and Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.).

He also, unsuccessfully, urged his colleagues to reject Gina Haspel's nomination to lead the CIA, saying "her refusal to acknowledge torture’s immorality is disqualifying."

The fight was deeply personal for McCain, a Vietnam War veteran who was captured and tortured after his plane was shot down in 1967.

Though the White House didn't immediately comment on the announcement from McCain's family, his longtime colleagues offered their support on social media.

GOP Sen. Lindsey Graham (S.C.), one of his closest friends and allies in the Senate, praised McCain's family for their "continuing and abiding love, care and dedication."

"The entire McCain clan is doing exactly what the McCains have done through generations — rise to the meet the challenge," Graham said in a statement.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellTrump 'no longer angry' at Romney because of Supreme Court stance On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline The Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power MORE (R-Ky.), who at times broke with McCain on issues like immigration and healthcare, said he was "very sad to hear" the announcement from McCain's family.

"We are so fortunate to call him our friend and colleague,” he said in a tweet. “John, Cindy, and the entire McCain family are in our prayers at this incredibly difficult hour.”

Sen. Jack Reed John (Jack) Francis ReedWhen 'Buy American' and common sense collide Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-R.I.), the ranking member on the Armed Services Committee, said McCain was "still inspiring us [with] his courage, grit, determination & grace."

"Scarred but never broken, John has always defied the odds,” he said in a tweet. “I’m not in denial, just in awe of my friend & a true patriot. Thank you for everything, Mr. Chairman.”

Mitt Romney Willard (Mitt) Mitt RomneyTrump 'no longer angry' at Romney because of Supreme Court stance GOP lawmakers distance themselves from Trump comments on transfer of power McConnell pushes back on Trump: 'There will be an orderly transition' MORE, a fellow GOP presidential candidate in 2008 who later endorsed McCain, said that "no man this century better exemplifies honor, patriotism, service, sacrifice, and country first than Senator John McCain."

And John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE — who served with McCain for decades in the Senate before going on to be secretary of State during the Obama administration — said the GOP senator has shown Americans what “the words grace and grit really mean."

"God bless John McCain, his family, and all who love him," Kerry said. "A brave man showing us once again what the words grace and grit really mean."

Arizona Gov. Doug Ducey (R) called McCain an "American hero."

"Always putting country before self. From Vietnam to the halls of the U.S. Senate, the spirit of service and civility that has guided Senator McCain's life stands as a model for all Americans, regardless of political affiliation," Ducey said in a statement.

Neither Ducey nor McCain's family indicated that McCain intends to resign from his Senate seat, and both sides have publicly admonished speculation about who will succeed him.

If McCain resigns or dies while in office, Ducey would be required to appoint someone to fill the seat through the next general election, in 2020.

—Last updated at 5:36 p.m.