WASHINGTON — Senator John McCain, Republican of Arizona, used his signature snark to warn colleagues that he’ll soon return to Washington — and he chastised the Trump administration for ending assistance to moderate Syrian rebels battling the government of Bashar al-Assad.

The twin statements served as reminders of the outsized role the 80-year old senator plays. Republicans badly need him to return as they try to shore up a weeks-long debate on a health-care overhaul. And he has been more willing than most Republicans to buck his party and the president to demand more decisive action to bolster the nation’s security, particularly in areas involving the country’s cyber defenses and posture against Russia.

McCain said late Wednesday that he had been diagnosed with a tumor called a glioblastoma after surgery to remove a blood clot above his left eye last week. The tumor is an aggressive type of brain cancer, and the prognosis is generally poor.

Former president George W. Bush said he phoned his 2000 GOP campaign rival to ‘‘encourage him in his fight.’’

‘‘Instead, he encouraged me,’’ Bush said. ‘‘I was impressed by his spirit and determination. He has devoted his life to his country. Thankfully, he is committed to continuing that service.’’

McCain tweeted Thursday that ‘‘I greatly appreciate the outpouring of support — unfortunately for my sparring partners in Congress, I’ll be back soon, so stand-by!’’

In a separate statement, he blasted reports that President Trump is ending the CIA’s covert program to arm and train moderate Syrian rebels battling Assad’s government, a move long sought by Russia. If true, ‘‘the administration is playing right into the hands of Vladimir Putin,’’ McCain said.

McCain’s absence plunged the health care debate into further chaos. With 52 Republicans and 48 Democrats in the Senate, the GOP can afford to have only one more senator absent or vote against the procedural motion to begin debate — since Democrats all plan to vote against it.