McCain's office announced Monday night that he would return Tuesday -- a surprise to most in Washington who expected him to miss the crucial vote and return to Washington at a later date.
McCain is expected to get GOP leadership one vote closer to beginning debate on health care legislation, which is on the verge of collapsing.
"Senator McCain looks forward to returning to the United States Senate tomorrow to continue working on important legislation, including health care reform, the National Defense Authorization Act, and new sanctions on Russia, Iran and North Korea," his office said in a statement.
The Arizona Republican underwent brain surgery earlier this month and announced last week he has been diagnosed with brain cancer.
But McCain's return hardly guarantees leadership will be able to advance a health care bill now. It helps, but Senate Majority Leader Mitch McConnell still needs at least 50 votes to start debate on overhauling Obamacare, and can only lose two votes from his 52-member conference. Already, Sen. Susan Collins, a Republican from Maine, has expressed deep concerns about the motion to proceed. And several other senators including Rand Paul of Kentucky, Lisa Murkowski of Alaska, Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Dean Heller of Nevada and more remain undecided as of Monday night.
Adding to the suspense of Tuesday is the fact that leaders have yet to lay out exactly what will come next after members vote to begin debate on their health care bill. Republicans will vote to begin debate on the House-passed legislation Tuesday, but GOP members are still waiting on what their leadership plans to do after that.
Some lawmakers have said their vote to advance the health care bill Tuesday is contingent on them approving on the steps that come next.
"I'm not blindly voting," Paul told reporters Monday.
"I would like to know more as I'm sure all of you would too," Murkowski said when she was asked about where she stood on voting to advance a health care bill.
The original plan was for members to find an agreement on a Senate repeal and replace bill before the motion to proceed vote, but after two different versions were released, there still wasn't consensus on a proposal. Leadership entertained the idea last week of bringing up a repeal-only bill that the Senate voted on in 2015, but President Donald Trump made it clear he preferred senators find a path forward on simultaneously repealing and replacing Obamacare instead.
Republicans are expected to huddle for the GOP lunch Tuesday afternoon and get more information on next steps.
One thing is certain, McCain's return to Washington puts additional pressure on his colleagues. Whether it is enough to get McConnell all the votes he needs remains unclear.
McCain isn't just returning to Washington because of the health care vote. If that vote fails, the Senate is expected to take up the National Defense Authorization Act, according to two congressional aides.
That bill was marked up by the armed services committee, which McCain leads, and he would lead debate on the floor. McCain takes great pride in the legislation and does not want anyone else managing it on the floor in his absence if he can help it.
McConnell made a procedural motion on the floor Monday night to proceed to the defense bill after health care, signaling that's his backup plan this week.