There are no votes to spare in the contentious effort to pass a new health care bill through Congress, where Republicans are desperate to fulfill Trump's campaign pledge.
There are no votes to spare in the contentious effort to pass a new health care bill through Congress, where Republican leaders are desperate to fulfill President Donald Trump's campaign pledge to dismantle the 2010 reforms of his predecessor Barack Obama.
So when McCain, 80, announced over the weekend that doctors in Phoenix removed a five-centimeter (two-inch) blood clot above his eye, Senate Majority Leader Mitch McConnell said he would "defer" the upcoming vote on the bill by at least a week.
Republicans hold 52 of the chamber's 100 seats, but two members of the party -- one conservative and one moderate -- have already declared their opposition. Democrats are united against the legislation.
Several others Republicans have expressed reservations, leaving the bill hanging by a thread just weeks before the Senate decamps for its summer recess -- a break already delayed in a bid to wrangle support for the controversial plan.
Unlike in some legislatures around the world, US lawmakers must be present in Congress to vote.
Senate leadership held to the idea that a vote was forthcoming.
"I believe as soon as we have a full contingent of senators, that we'll have that vote," number two Senate Republican John Cornyn told NBC's "Meet the Press" on Sunday.
But as lawmakers returned to Washington Monday, divisions remained apparent, particularly among senators whose states opted under Obamacare to expand Medicaid, the health insurance program for the poor and the disabled.
The new bill would gradually roll back the program, a move that some Republicans warn could lead to millions losing coverage.
Senator Rob Portman of Ohio said lawmakers concerned about the Medicaid provisions were "still having discussions" with leadership about a path forward.
"Still trying to improve it," he told a crush of reporters.
Trump, who has had his differences with McCain, on Monday wished the iconic senator a speedy recovery.
"We miss him," Trump said of the 2008 Republican presidential candidate, who has not hesitated to criticize the current leader.
"He's a crusty voice in Washington -- plus, we need his vote," Trump added.
Spokesman Sean Spicer said the president would invite several senators to the White House later Monday to discuss the measure.
But questions swirled about how much recovery time McCain may need, after doctors performed a craniotomy, an opening of the skull in order to access tissue or blood near the brain.
While McCain's office said he will be recuperating in Arizona this week, neurosurgeons said typical healing may take longer.
"I think it's going to take him a few weeks to recover from this," Norberto Andaluz, a neurosurgeon and craniotomy specialist with Mayfield Clinic in Cincinnati, Ohio, told AFP.
One major factor for recovery time is the level of impact the operation had on the brain, said Andaluz, who is not involved in McCain's care.
Another issue is the underlying pathology.
Doctors removed several malignant melanomas on McCain's skin in the 1990s and 2000s, including an invasive melanoma in 2000.
Experts say McCain's latest operation suggests the possibility of a return of skin cancer. His office said test results are expected in the coming days.
"John had not been feeling good," Senator Lindsey Graham told CNN, adding that his friend McCain was maintaining a busy travel schedule and was becoming "forgetful."
"I'm glad they found what I think was the cause."
A delayed health care vote would undoubtedly complicate McConnell's effort to ram the legislation through the Senate.
Republican leaders are eager to notch a major legislative victory for Trump, but there is a logjam of important business in the Senate, including consideration of raising the limit on federal borrowing.
Achieving Trump's goal of ditching Obama's sweeping health care reforms has been elusive as his administration nears the six-month mark later this week.
Some fear that repealing the Affordable Care Act could adversely impact millions of Americans on Medicaid, or make health costs soar for people with pre-existing conditions.
There is little institutional support for the bill, and even less for an amendment introduced by Senator Ted Cruz that would allow insurers to offer bare-bones plans that do not comply with Obamacare's coverage requirements.
The chief executives of America's Health Insurance Plans and the Blue Cross Blue Shield Association called it "simply unworkable in any form."
Former vice president Joe Biden lambasted the plan Monday.
"They want to drag us back to a time -- not all that long ago -- when Americans could be denied basic health care because they were unable to afford it," he wrote in The Washington Post.
A Post poll released Sunday showed Americans preferred Obamacare to the Republican plan by 50 percent to 24 percent.