WASHINGTON -- John McCain couldn't bring himself to vote for Donald Trump -- so he talked about writing in his best friend's name for president. After the election, he's been the leading Senate Republican critic of Trump's posture toward Russia. And from his Arizona home, where he's battling brain cancer, the Arizona senator on Thursday lobbed a new attack at the White House over its Syria policy.

The grave medical diagnosis hit the six-term senator just as he was settling into the latest notable role in his storied career.

The ex-prisoner of war, former GOP presidential nominee and onetime standard-bearer of the political Straight Talk Express has emerged as a voice for what some Republicans feel is a party lost in the Trump era.

He's lambasted Trump as a defamer of military personnel, recoiled from Trump's willingness to cozy up to Russian President Vladimir Putin and rejected Trump's self-described boorishness toward women.

On Thursday, less than 24 hours after announcing he'd be undergoing treatment for glioblastoma, McCain promised -- warned, really -- he won't be gone for long.

"He is yelling at me to buck up, so I'm gonna buck up," said Sen. Lindsey Graham of South Carolina, McCain's close friend in the Senate.

It was classic McCain, whose candor offers a dose of authenticity and moxie at a time when his fellow Republicans control Congress and the presidency but are struggling to govern.

His absence, however long, raises the prospect of a Senate without its sometimes trash-talking, yet also self-effacing, senator from Arizona for the first time in more than three decades.

In the short term, McCain's treatment deprives Senate Republicans of a vote they need for a controversial health-care rewrite in the narrowly divided chamber.

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, he called for a special committee to investigate Russian meddling in the election, recently lamenting 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, chairman of the Armed Services Committee, said he received sensitive information last year and turned it over to the FBI, an apparent reference to an unsubstantiated report 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.

Graham said McCain had called him three times Thursday on immigration legislation.

Yet for all his confrontational style, McCain has voted with Trump most of the time.

He voted in favor of most of the president's Cabinet nominees and with Trump against several Obama-era regulations.

Longtime colleagues, even those McCain has called names, said he developed his fearlessness as a navy aviator held as a prisoner for more than five years in Vietnam.

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!"