He is also a crucial vote in their slim 52-seat majority.
With just two votes to spare, every senator matters. McCain’s inability to flash his trademark thumbs up or thumbs down when he strides into the chamber to cast his vote could seriously stymie the Republicans on key legislation.
The first test will come early next week. Senate Majority Leader Mitch McConnell of Kentucky has planned a vote on the Republican promise to repeal and replace the Affordable Care Act, known as Obamacare. Previous attempts, including one this week, collapsed when more than two senators announced their opposition. President Trump has pushed senators to press ahead on a vote anyway.
As it stands, McConnell does not appear to have enough support from his ranks to advance any version of the GOP legislation, ranging from simply repealing Obamacare to repealing and replacing it, and making major funding reforms to Medicaid at the same time. According to the nonpartisan Congressional Budget Office, the GOP plans would throw between 22 million and 32 million additional Americans into the ranks of the uninsured.
McCain’s vote on the healthcare overhaul was never guaranteed. He, too, has voiced deep reservations about its impact on residents of Arizona, whose Republican governor worries about the severe cuts to Medicaid that could leave thousands of low-income and disabled people without coverage.
But without McCain, the path to passage becomes even tougher. Since no Democrats are expected to support the plan, Republicans cannot afford to lose more than one vote. As many as 10 have expressed deep concerns, and at least four have said they would vote against each of the current plans.
“The challenges are obvious,” said Josh Holmes, a Republican strategist and former top advisor to McConnell who years ago came up with the campaign slogan to “repeal and replace” Obamacare. “You have no margin for error, and this unfortunate circumstance surrounding Sen. McCain makes that margin even thinner.”
And it’s not just healthcare that’s jeopardized. Tax reform, which leaders were also hoping to approve under a special procedure for simple-majority passage, could be at risk.
Other must-pass legislation, such as the need to raise the debt ceiling to continue paying the nation’s bills and an annual spending bill to keep the government running, will take 60 votes to overcome a filibuster. They could be even more difficult.
“Mitch has the toughest job in Washington to begin with, so he’s got to take all of this into account,” said Republican Sen. Thom Tillis, a former speaker of the North Carolina statehouse.
Exactly how long McCain will be away is unclear.
His medical team at the Mayo Clinic said he might undergo chemotherapy or radiation, so it’s possible he could be away for some weeks. For now, he is unable to fly after undergoing an operation last week to remove a blood clot above his left eye.
Even before that, there were concerns about his health. At a recent Senate hearing, McCain seemed confused during his questioning of fired FBI Director James B. Comey. He attributed it to being tired after staying up too late to watch an Arizona Diamondbacks baseball game.
Few have publicly speculated about whether McCain would step down. If he did, the balance of power in the Senate would not change. Arizona law says the governor would appoint a replacement who must be from the replaced senator’s party.
But McCain signaled Thursday that he has no intention of sitting out the Senate.
He dashed off a tweet appreciative of the warm words he has received, adding that “unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!”
The Senate has been here before, and the dynamic is a reminder that, despite the hardened partisanship that often defines modern politics, there are also individual stories of real humans and their frailty behind every vote.
Just a few years ago, when Democrats held the majority, then-Majority Leader Harry Reid (D-Nev.) was trying to pass the Affordable Care Act during President Obama’s first term. Sen. Edward Kennedy (D-Mass.) was forced to miss Senate votes during his prolonged fight against brain cancer.
The Democrats’ 60-seat supermajority at the time was further complicated when 92-year-old Sen. Robert Byrd (D-W.Va.) became ill.
When a crucial vote was scheduled, both would be on the floor. Kennedy, who was susceptible to infection during his treatment, chartered a private plane to fly to Washington when his vote was most needed, said James P. Manley, a Democratic strategist and former top aide to both Reid and Kennedy. Byrd was pushed to the floor in his wheelchair.
“Republicans keep pointing out that Democrats had 60 votes during the first part of the Obama administration, but Sen. Reid had two sick senators,” Manley said.
“He brought them in when needed for votes,” he said, “then he tried as best he could to work around it.”
Manley added, "It may sound cold, but Sen. McCain's health is something McConnell's going to have to take into account."
Even from afar, McCain is trying to stay engaged. On Thursday, he criticized the Trump administration’s decision to end a program assisting opposition forces in Syria as “irresponsible” and playing into Russia’s hands.
“The woe-is-me thing is not part of his DNA,” said longtime friend Sen. Lindsey Graham (R-S.C.), who has been speaking to McCain regularly, including three times Thursday morning. “The best medicine for John McCain is to have his voice heard.”
In the weeks ahead, leaders will need to assess when they can call on McCain to return to Washington, and the senator will have to decide when he is willing and able to make the trip.
“Sen. McCain, as we all know, has never shied away from a fight,” McConnell said Thursday as he opened the Senate. “He should know that we’re all in his corner. Every single one of us. We look forward to seeing our friend again soon and we hope he’ll be back in the very near future.”
Times taff writer Mark Z. Barabak in San Francisco contributed to this report.