Sen. John McCain is expected to return to the Senate Tuesday as lawmakers face a key vote on health care, just days after being diagnosed with an aggressive form of brain cancer.

"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 Monday evening.

McCain took to Twitter to also make the announcement saying he looks forward to returning to Senate:

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

The Arizona Republican's attendance in the upper chamber Tuesday could be key in determining whether Senate Republicans are able to open the debate and amendment process on health care reform.

Senators will first vote Tuesday to proceed to the American Health Care Act (AHCA), the health care legislation that narrowly passed the House in early May. To advance to the next step, that will require 51 votes. If that's successful, Senate Republican leaders then expect amendments to be offered to that bill, a Republican aide told CBS News. With McCain's scheduled return, Republicans can only afford two defections on some of the major proposals, assuming Democrats stick together in opposition.

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McCain, for his part, has advocated going back to the committee level and negotiating health care reform on a bipartisan basis.

The 80-year-old has been diagnosed with glioblastoma, an aggressive brain tumor, which was revealed to the public last week. It was discovered after he underwent a procedure to remove a blood clot above his left eye earlier this month.