WASHINGTON — With a razor-thin margin for success in the balance, Sen. John McCain, R-Ariz. — recently diagnosed with an aggressive brain tumor — will return to Washington Tuesday to take a critical vote on health care.

"Senator McCain looks forward to returning to the United States Senate tomorrow to continue to working on important legislation, including health care reform, the National Defense Authorization Act, and new sanctions on Russia, Iran and North Korea," McCain's office said in a statement Monday.

McCain's return would give Republicans a crucial vote in support of the motion to proceed to begin debate on health care legislation.

President Donald Trump on Tuesday morning tweeted his thanks to McCain for returning and said, "Brave — American hero!"

So great that John McCain is coming back to vote. Brave - American hero! Thank you John. — Donald J. Trump (@realDonaldTrump) July 25, 2017

Republican leaders are working to get the support of 50 of the Republican conference's 52 senators. McCain recently underwent surgery to remove a blood clot near his brain and was diagnosed with an aggressive brain tumor last week.

Still, less than 24 hours before an expected vote to begin debate on health care, Senate Republicans still haven’t been told exactly what kind of reform they will ultimately be voting on, creating even more uncertainty about the chances of gaining a majority of Republicans needed to proceed.

Tuesday's vote on a motion to proceed to debate on health care legislation will take place after a Republican policy luncheon where members could learn the details of what amendments might be included in the eventual legislation.

Related: Trump Says 'Obamacare Is Death'

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Republicans could be voting on the most recent version of the Better Care Reconciliation Act, an updated version of the bill that addresses the concerns of senators, a simple repeal of the Affordable Care Act or some combination of them.

Vice President Mike Pence spent about 30 minutes in Senate Majority Leader Mitch McConnell's office in the Capitol Monday night. "It's time to vote," Pence said as he left.

The meeting took place shortly after McConnell met with members of his leadership team and Sen. Rob Portman, R-Ohio, who has been unable to come out and say he'd support any of the health care proposals.

A group of moderates huddled in an unmarked office in the Russell Senate office building Monday night but Sens. Lisa Murkowski, R-Ala., and Cory Gardner, R-Colo., stayed for a short time. Murkowski left the meeting saying she was still undecided on how she'd vote.

As Senate Republican leaders continue to try to find at least 50 GOP votes, Trump gave his first extended remarks devoted to health care since his news conference in the Rose Garden after the House of Representatives passed its health care bill on May 4.

“Every Republican promised immediate relief from this law,” Trump said Monday at the White House, pressuring recalcitrant Republicans who have objected to part of the proposed bill. “We as a party must fulfill that solemn promise to the voters of this country to repeal and replace — what they’ve been saying for the last seven years, but so far Senate Republicans have not held up.”

Trump traveled to West Virginia Monday night to speak at the Boy Scouts Jamboree in a state that he won by more than 40 points but also home to Republican Sen. Shelley Moore Capito who has been reluctant to support the Senate bill.

Related: Trump Says 'Obamacare is Death'

This time Trump put pressure on his Health and Human Services Secretary Tom Price, whom he said, perhaps in jest, that he needs to get the job done.

"By the way, you gonna get the votes?" Trump asked Price, who was standing behind him. "You better. Otherwise I'll say, 'Tom, you're fired!'"

He added: "You better get Sen. Capito to vote for it."

Capito joined Trump behind stage at Monday night, but she’s also weighing the 28 percent of West Virginians who are on Medicaid, one of the highest percentages in the country. The number of uninsured in the Mountaineer state could rise 299 percent, according to one study by the left-leaning Center for Budget and Policy Priorities, many because of reductions to Medicaid.

The quest for 50 votes is elusive, especially if McCain is unable to return. Sen. Susan Collins, R-Maine, still appears to be solidly against the bill and so does Sen. Rand Paul, R-Ky., if there is no vote for a near-complete repeal of Obamacare. In an op-ed in The Hill on Monday, Paul harshly criticized the Senate process.

"'What bill?’ Good question. Remember that when you hear the clamoring of voices saying 'you have to vote to proceed to the bill' or worse 'you just have to be part of the team and vote for it!'" Paul wrote. "That's the kind of thinking that got us into this mess and I won’t be a part of it.'"

More than a handful of other senators have also expressed deep reservations and have not come out and said they’d support the motion to proceed.

Leadership is pushing Republicans to vote "yes" on the motion to proceed, insisting that they will be able to offer any amendment that pertains to health care that they want. For example, Sen. Mike Lee, R-Utah, can offer an amendment to reduce the cost of health insurance, Sen. Paul can offer an amendment to repeal Obamacare and Sen. Rob Portman, R-Ohio, can offer one to beef up Medicaid spending.

“I don't think anybody has determined what the exact sequence will be, but it's not like anybody is going to get any favorable treatment, everybody's going to get a chance to get a vote on everything they want to get a vote on. Including Democrats,” said Cornyn, the Republican whip.

One proponent of the motion to proceed, Sen. John Kennedy, R-La., said the Senate should "debate in front of God and country."