Sen. John McCain's glioblastoma diagnosis has devastated his Senate colleagues. But he's told them he's not gone yet. McCain told South Carolina Sen. Lindsey Graham to keep it together as he grapples with brain cancer.

WASHINGTON — Sen. John McCain is telling his best friend in the Senate to keep it together as he grapples with brain cancer.

South Carolina Sen. Lindsey Graham says McCain called him three times on Thursday about immigration legislation. Graham says McCain “is yelling at me to buck up so I’m gonna buck up.”

McCain’s glioblastoma diagnosis has devastated his Senate colleagues. But he’s told them he’s not gone yet.

The resilience is classic McCain, the Navy pilot and Vietnam prisoner of war turned congressman and senator. He twice ran for president, losing a GOP primary to George W. Bush and a general election to Barack Obama. Still a force in Congress, the 80-year-old six-term Arizona Republican is chairman of the powerful Armed Services Committee.

The grave medical diagnosis hit McCain just as he was settling into the latest notable role in his storied career, emerging as a voice for what some Republicans feel is a party lost in the Trump era. He’s lambasted President Donald Trump as a defamer of military personnel, recoiled from Trump’s willingness to cozy up to Russian President Vladimir Putin and rejected the president’s self-described boorishness toward women.

After audio surfaced in October of Trump talking about groping women, McCain broke with the candidate and said he’d write in Graham’s name on Election Day. When Trump won, McCain called for a special committee to investigate Russian meddling in the election. He has lamented that the Russia issue is “a challenge to Washington, D.C., the way we do business, a challenge to bipartisanship and a challenge to the effectiveness of this newly elected president.”

McCain says he received sensitive information last year and turned it over to the FBI, an apparent reference to an unsubstantiated report that Russia had compromising personal and financial information about Trump.

On Thursday, from his home in Arizona, McCain said the administration would be “playing right into the hands of Vladimir Putin” if, as The Washington Post reported, Trump was ending a program to back the Syrian opposition.

“I think John is a force that is unique to him. He has done things that most people could not do,” said Graham. “Going forward he’s excited, quite frankly, about getting a second chance to finish things that have been stuck.”

Yet for all of his confrontational style, McCain has voted with Trump most of the time. He backed most of the president’s Cabinet nominees and sided with Trump against several Obama-era regulations.

Longtime colleagues, even those McCain has called names, say he developed his fearlessness as a Navy aviator held as a prisoner for more than five years in Vietnam. Resilience, they say, has fueled his long Senate career and helped him overcome two failed presidential campaigns. For some, McCain has become the moral voice of the Republican Party, whose leaders have not always said out loud what they really think about Trump.

On Thursday, McCain warned his colleagues, and Trump, not to get too comfortable in his absence.

He tweeted from afar: “I’ll be back soon, so stand-by!”