Sen. John McCain (R-AZ) announced late Monday that he would be back at work Tuesday for a vital vote on healthcare reform, even as he recovers from a blood clot removal and a diagnosis of a brain tumor.

McCain, who announced last week that he had been diagnosed with a brain tumor, was set to cast a vital vote of support in a looming vote on the GOP healthcare reform bill, but the vote was initially postponed after he underwent surgery to remove a blood clot above his eye.

Look forward to returning to Senate tomorrow to continue work on health care reform, defense bill & #RussiaSanctions https://t.co/VQBtovnwF1 — John McCain (@SenJohnMcCain) July 25, 2017

It had been unclear when McCain was due to return, but Monday night his office announced that the 80-year-old veteran “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.”

While McCain’s office mentioned a number of bills, it was healthcare reform that is seen as the most significant. Sens. Rand Paul (R-KY) and Susan Collins (R-ME) had come out against the Senate’s version of the ObamaCare reform bill, leaving Senate Majority Leader Mitch McConnell (R-KY) with no wiggle room when it comes to Republican support — meaning the GOP’s entire congressional agenda has essentially been held up by McCain’s absence.

McCain has expressed concerns about the GOP healthcare bill but is expected to toe the party line and vote to at least begin debate on repealing ObamaCare. No Democrat is expected to vote for any bill that would replace, even in part, the Affordable Care Act.

President Trump leaned hard earlier Monday into senators not willing to vote to begin voting on an ObamaCare replacement, saying that “any senator who votes against starting debate is telling America that you are fine with the ObamaCare nightmare.”

Any senator who votes against starting debate is telling America that you are fine w/ the #OCareNightmare!

Remarks: https://t.co/tSvWaMdxBA pic.twitter.com/DI7e78hr6N — Donald J. Trump (@realDonaldTrump) July 24, 2017

The main dilemma that will face senators Tuesday is what exactly they will be voting on. The first vote taken Tuesday will be a procedural vote to debate the health care bill passed in the House in March. Then will follow a series of amendments. It would then be up to McConnell to either lead the Senate in a vote to repeal ObamaCare without replacing it or to try and pass the Senate health care bill.

However the vote plays out, McConnell will only have a wiggle room of two votes. McConnell pleaded with Republican colleagues on the Senate floor Monday to seize the chance to finally repeal President Obama’s unpopular signature law.

“Many of us have waited literally years for this moment to arrive, and at long last it has,” McConnell said. “I’ll keep my commitment to vote to move beyond the failures of ObamaCare. I would urge all of our colleagues to do the same.”

Adam Shaw is a Breitbart News politics reporter based in New York. Follow Adam on Twitter: @AdamShawNY