Republicans expressed cautious optimism Monday that Sen. John McCain John Sidney McCainGOP lobbyists worry Trump lags in K Street fundraising Mark Kelly kicks off Senate bid: ‘A mission to lift up hardworking Arizonans’ Gabbard hits back at Meghan McCain after fight over Assad MORE (R-Ariz.), who was diagnosed with brain cancer last week, will return to the Senate in time for a key ObamaCare repeal vote on Tuesday.

The final decision rests with McCain’s doctors.

The 80-year-old lawmaker underwent an emergency craniotomy last week to remove a blood clot above his left eye. He was later diagnosed with a glioblastoma, an aggressive form of cancer.

Senate aides reported last week that McCain was alert and talking to staff shortly after the operation and eager to return to Washington.

But medical experts at the Mayo Clinic Hospital in Phoenix may not like the idea of him jumping back to work so soon.

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Senate Republican Whip John Cornyn John CornynGOP lawmaker says panel to investigate drug company gaming of patent system Senators grill drug execs over high prices Cornyn less popular than Cruz in Texas: poll MORE (Texas) told reporters Monday that McCain was trying to get approval from his doctors to return Tuesday.

But Sen. John Thune John Randolph ThuneWhite House pleads with Senate GOP on emergency declaration Overnight Energy: McConnell plans Green New Deal vote before August recess | EPA official grilled over enforcement numbers | Green group challenges Trump over Utah pipelines McConnell plans vote on Green New Deal before August recess MORE (S.D.), the chamber’s third-ranking Republican, said "I don't expect it" when asked if McCain would be there Tuesday.

McCain’s office declined to confirm his schedule.

Two Senate aides said they expect he will be present Tuesday afternoon when lawmakers vote on a motion to proceed to the House-passed American Health Care Act.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellWhite House pleads with Senate GOP on emergency declaration Senate Dems seek to turn tables on GOP in climate change fight Pence meets with Senate GOP for 'robust' discussion on Trump declaration MORE (R-Ky.) has said he will offer an amendment to replace it with a bill that repeals as much of ObamaCare as possible, setting up a two-year transition to craft replacement legislation.

If that amendment fails, the Senate is likely to move to its own version of legislation to repeal and replace the pillars of ObamaCare, the Better Care Reconciliation Act. That measure, however, did not appear to have enough votes to pass as of Monday afternoon.

The legislation will move under special budgetary rules setting up 20 hours of debate and rapid-fire votes on an indefinite number of amendments, a grueling schedule even for a lawmaker in fine health.

—Peter Sullivan contributed. Updated at 5:48 p.m.