Sen. John McCain used his signature snark on Thursday to warn colleagues that he will be returning soon to Washington — and 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 on Capitol Hill. 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 announced late Wednesday that he has 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.

The news sparked bipartisan calls for his swift return and public statements of support from Trump and his predecessors. Former president George W. Bush said on Thursday that he phoned his 2000 GOP presidential 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.”

Responding to the well wishes, McCain tweeted on Thursday, “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 in the statement. “Making any concession to Russia, absent a broader strategy for Syria, is irresponsible and shortsighted. The administration has yet to articulate its vision for Syria beyond the defeat of [the Islamic State], let alone a comprehensive approach to the Middle East.”

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The criticism signaled that no matter where he is, McCain intends to remain engaged. When he might return to Washington, however, remains in doubt.

The Mayo Clinic in Phoenix, which diagnosed McCain, said that the senator and his family are considering treatment options, including a combination of chemotherapy and radiation. His office did not indicate on Thursday where he will undergo treatment and when he might return to the Senate.

“We look forward to seeing our friend again soon,” Senate Majority Leader Mitch McConnell (R-Ky.) said Thursday, “and we hope he’ll be back in the very near future.”

McCain’s absence plunged the ongoing health care debate into further chaos. McConnell has vowed to hold a vote next week to begin formal debate on legislation to repeal the 2010 Affordable Care Act. With 52 Republicans and 48 Democrats in the Senate, the GOP can only afford to have one more senator absent or vote against the procedural motion to begin debate — since Democrats all plan to vote against it.

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On Thursday, one Republican holdout, Sen. Rand Paul (Ky.), said that he is willing to move ahead with the bill if certain concessions are made. But several other Republicans, including Susan Collins (Maine), Lisa Murkowski (Alaska) and Dean Heller (Nev.), have not yet signaled what they will do.

That makes McCain’s absence all the more difficult.

“Our focus is not on the missing vote. It’s on Senator McCain’s health,” Collins told reporters when asked about the stakes. “And all of us are deeply saddened and shocked by the diagnosis.”

Sen. Bill Cassidy (R-La.) said he was troubled to think that “a terrible diagnosis” was a factor in the fate of high-stakes legislation. But, he acknowledged, “Practically speaking, you know that you have to put that in the vote count.”

As chairman of the Senate Armed Services Committee, McCain is also scheduled in the coming weeks to steer floor debate on the annual defense policy bill that sets military policy. McConnell announced this month that he would cancel the first two weeks of the scheduled August recess in order to allow debate and passage of the National Defense Authorization Act.

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Sen. John Thune (R-S.D.), the third-ranking Senate Republican, indicated that other senior members of the armed services panel probably could lead debate on the bill in McCain’s absence, but “we need John here.”

Sen. Lindsey O. Graham (R-S.C.), McCain’s closest friend in Congress, added that the Senate is likely to wait for McCain’s return to consider the defense bill. He also echoed McCain’s disdain for Trump’s latest reported moves on Syria.

“The decision to stop supporting opposition forces to Assad in my view is a complete capitulation to the Russians, to the Iranians, to Assad,” he said. “It is a betrayal of the Syrian people who have been slaughtered by Assad — and Russia and Iran’s support for them — it is a strategic mistake, it puts our allies in a bad spot, and it diminishes our standing. Other than that it’s a good idea.”

In an interview, Senate Minority Leader Charles E. Schumer (D-N.Y.) recalled that he and McCain “bonded over cold pizza” as they led the bipartisan “Gang of Eight” senators who spent hours toiling over details of the 2013 comprehensive immigration reform bill. The measure ultimately died, but was heralded by members of both parties as a good-faith compromise bill.

“He was so good, he knew when to hold, he knew when to fold. He worked for the good of the eight — not just himself,” Schumer said.

“He is such a needed guy around here,” he added. “The number of people who have the ability to say what they think even if other people don’t like it — including the president — you can count on a couple of hands. He’s one of them.”

Sean Sullivan and Kelsey Snell contributed to this report.

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