YORK, PA - AUGUST 12: Republican Presidential Candidate Sen. John McCain (R-AZ) speaks at a Town Hall Meeting while on the campaign trail in the Toyota Arena August 12, 2008 in York, Pennsylvania. Over one thousand people attended the Town Hall.  (Photo by William Thomas Cain/Getty Images)
William Thomas Cain/Getty Images
YORK, PA - AUGUST 12: Republican Presidential Candidate Sen. John McCain (R-AZ) speaks at a Town Hall Meeting while on the campaign trail in the Toyota Arena August 12, 2008 in York, Pennsylvania. Over one thousand people attended the Town Hall. (Photo by William Thomas Cain/Getty Images)
Now playing
01:30
McCain returning to Senate for critical vote
CNN
Now playing
01:08
Military flyover honors John McCain
Now playing
02:36
Obama makes guests laugh during McCain's eulogy
CNN
Now playing
02:12
Meghan McCain: A few resented my father's fire
Now playing
01:58
Meghan McCain: My dad was defined by love
Now playing
01:32
Lieberman: McCain floated idea of me on ticket
WASHINGTON, DC - SEPTEMBER 1: Cindy McCain looks on as a joint military service casket team carries the casket of the late Senator John McCain following his funeral service at the Washington National Cathedral, September 1, 2018 in Washington, DC. Former presidents Barack Obama and George W. Bush delivered eulogies for McCain in front of the 2,500 invited guests. McCain will be buried on Sunday at the U.S. Naval Academy Cemetery. (Photo by Drew Angerer/Getty Images)
Drew Angerer/Getty Images
WASHINGTON, DC - SEPTEMBER 1: Cindy McCain looks on as a joint military service casket team carries the casket of the late Senator John McCain following his funeral service at the Washington National Cathedral, September 1, 2018 in Washington, DC. Former presidents Barack Obama and George W. Bush delivered eulogies for McCain in front of the 2,500 invited guests. McCain will be buried on Sunday at the U.S. Naval Academy Cemetery. (Photo by Drew Angerer/Getty Images)
Now playing
03:31
Friends and family pay final tribute to McCain
Now playing
00:55
Mike Pence's remarks make McCain family smile
mccain lie in state capitol steps rain cupp vpx_00011010.jpg
CNN
mccain lie in state capitol steps rain cupp vpx_00011010.jpg
Now playing
01:45
Rain suddenly falls as McCain's casket escorted into Capitol
Larson Family Photos
Now playing
00:59
McCain will be buried next to his 'wingman'
Carolyn Kaster/AP
Now playing
02:35
Former Presidents honor John McCain
john mccain ernest hemingway
CNN
john mccain ernest hemingway
Now playing
03:03
How McCain helped save Ernest Hemingway's home
CNNI
Now playing
01:26
John McCain's hearse leaves his home
Chip Somodevilla / Getty Images
Now playing
01:12
Obama on McCain: We are all in his debt
CNN
Now playing
02:10
Former staffer jokes about McCain's driving
CNN
Now playing
00:58
McCain's former aide: He was an urgent man
Secretary of Defense James N. Mattis and Chairman of the Joint Chiefs of Staff Marine Gen. Joseph F. Dunford Jr. will brief the media on Tuesday, Aug. 28, at 10 a.m. EDT, in the Pentagon Briefing Room (2E973).  DAILY SCHEDULE: Secretary of Defense James N. Mattis and Chairman of the Joint Chiefs of Staff Marine Gen. Joseph F. Dunford Jr. brief the media at 10 a.m. EDT in the Pentagon Briefing Room (2E973). Both U.S. and foreign journalists without a Pentagon building pass must be pre-registered in the new Pentagon Visitor Management System to attend this briefing; plan on being escorted from the River Entrance Pedestrian Bridge or the Pentagon Metro Entrance Facility only. Please arrive no later than 45 minutes before the briefing; have proof of affiliation and photo identification. Please call 703-697-5131 for any questions and escort into the building. The briefing will also be streamed live on www.defense.gov/live.  Secretary of Defense James N. Mattis hosts an enhanced honor cordon welcoming Minister of Defense Ryamizard Ryacudu of Indonesia, to the Pentagon at 2:30 p.m. EDT on the steps of the River Entrance. All journalists desiring to cover the cordon must obtain a wristband from security screening. Journalists without a Pentagon facility access card must go through security screening at the base of the River Entrance Pedestrian Bridge, and will be escorted to the cordon from there. Security screening will begin at approximately 1:45 p.m. EDT; have proof of affiliation and two forms of photo identification. Journalists with a Pentagon facility access card, and whom have entered the building prior to 1:30 p.m. EDT, may go through security screening at the River Entrance to obtain their wristband. All journalists wishing to cover the honor cordon, including those with a Pentagon facility access card, must be in place no later than 2:15 p.m. EDT. Once security screening has been initiated at the base of the bridge, all journalists entering the building via the bridge for any reason, including those with a Pen
POOL
Secretary of Defense James N. Mattis and Chairman of the Joint Chiefs of Staff Marine Gen. Joseph F. Dunford Jr. will brief the media on Tuesday, Aug. 28, at 10 a.m. EDT, in the Pentagon Briefing Room (2E973). DAILY SCHEDULE: Secretary of Defense James N. Mattis and Chairman of the Joint Chiefs of Staff Marine Gen. Joseph F. Dunford Jr. brief the media at 10 a.m. EDT in the Pentagon Briefing Room (2E973). Both U.S. and foreign journalists without a Pentagon building pass must be pre-registered in the new Pentagon Visitor Management System to attend this briefing; plan on being escorted from the River Entrance Pedestrian Bridge or the Pentagon Metro Entrance Facility only. Please arrive no later than 45 minutes before the briefing; have proof of affiliation and photo identification. Please call 703-697-5131 for any questions and escort into the building. The briefing will also be streamed live on www.defense.gov/live. Secretary of Defense James N. Mattis hosts an enhanced honor cordon welcoming Minister of Defense Ryamizard Ryacudu of Indonesia, to the Pentagon at 2:30 p.m. EDT on the steps of the River Entrance. All journalists desiring to cover the cordon must obtain a wristband from security screening. Journalists without a Pentagon facility access card must go through security screening at the base of the River Entrance Pedestrian Bridge, and will be escorted to the cordon from there. Security screening will begin at approximately 1:45 p.m. EDT; have proof of affiliation and two forms of photo identification. Journalists with a Pentagon facility access card, and whom have entered the building prior to 1:30 p.m. EDT, may go through security screening at the River Entrance to obtain their wristband. All journalists wishing to cover the honor cordon, including those with a Pentagon facility access card, must be in place no later than 2:15 p.m. EDT. Once security screening has been initiated at the base of the bridge, all journalists entering the building via the bridge for any reason, including those with a Pen
Now playing
00:58
Mattis on McCain: US has lost a great patriot
Now playing
03:16
Vietnamese pay tribute to Sen. John McCain
Getty Images
Now playing
02:14
Daughter, Trump react to McCain's death

Story highlights

The Arizona Republican underwent brain surgery earlier this month

Senate Majority Leader Mitch McConnell needs at least 50 votes to advance the bill

(CNN) —  

Sen. John McCain, recently diagnosed with an aggressive form of brain cancer, will make a dramatic return to the Senate Tuesday to cast a critical vote on health care legislation.

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.

CNN’s Jeremy Herb contributed to this report.