The mystery whether John McCain will be present during next week's historic tax bill vote is over: according to CBS, the Arizona Senator will not be on hand for the final vote on the Republican tax bill, expected early this week, and instead is returning to Arizona after spending several days in a Maryland hospital recovering from side effects from chemotherapy treatment for brain cancer. As CBS furter notes, McCain left Washington Sunday and is heading back to his home state to spend the holidays with his family. It is unclear when McCain might return to Washington, especially amid rumors that the senator's health has taken a turn for the worse.

Will McCain's absence be a dealbreaker to the GOP tax vote? Probably not: despite a razor-thin margin needed to pass the measure, McCain's presence will not likely be the determining factor in the vote. The reason is that two critical senators - Bob Corker of Tennessee and Marco Rubio of Florida - announced their support for the bill last week after initially saying they would oppose earlier versions. Unless, of course, one or both of them changes their opinion in the last minute, somethign which could happen following an IBT report this morning that according to John Cornyn, a tax cut potentially benefiting Bob Corker was part of the effort to secure votes for passage.

As note yesterday, Republican leaders aim to hold floor votes this week, with the House expected to take up the measure first on Tuesday.

McCain, 81, has been receiving treatment at Walter Reed National Military Medical Center in Bethesda, Maryland, since Wednesday. His son-in-law said on "Face the Nation" Sunday that McCain had been "in good spirits." More from CBS:

"I'm happy to say that he's doing well. The truth is that as anyone knows whose family has battled cancer or any significant disease that oftentimes there are side effects of treatment that you have. The senator has been through a round of chemo and he was hospitalized this week at Walter Reed," said Ben Domenech, a conservative writer and husband of Meghan McCain.

McCain was admitted to the hospital for what his office called "normal side effects" of his cancer treatment.

Sen. John Cornyn of Texas, the second most senior Republican, told reporters last week that he expected McCain to be back after "resting up." Sen. Lindsey Graham of South Carolina, a close friend of McCain, likewise said he had "not focused on the tax bill right now when it comes to Sen. McCain. I'm focused on his health."

McCain underwent surgery in mid-July to remove a blood clot from above his left eye after being diagnosed with glioblastoma, the same cancer that killed Massachusetts Sen. Ted Kennedy.

McCain's diagnosis, however did not prevent him from participating in another key vote - the repeal of the Affordable Care Act. McCain's vote was a critical factor in the failure of the Republicans' efforts to repeal the Affordable Care Act in July. McCain was one of only three GOP senators to vote against the measure, which came in dramatic fashion in the early morning hours. When reporters asked why he cast a "no" vote on , McCain's response was simple: "I thought it was the right thing to do." Shortly after the dramatic vote, he returned home to Arizona to begin radiation and chemotherapy treatments.

However, in recent weeks McCain's condition appears to have worsened. He suffered a minor tear in his right Achilles tendon, forcing him to wear a walking brace. He eventually began using a wheelchair with members of his staff assisting him. Some of the symptoms of glioblastoma, according to CBS News chief medical correspondent Dr. Jon LaPook, include headache, general malaise, visual problems and speech problems.

McCain is not the only senator who may be sidelined for the critical vite due to the health reasons: Sen. Thad Cochran, 80, of Mississippi, had a non-melanoma lesion removed from his nose earlier this week. A spokesman for Cochran told CBS News last week that the senator went through an outpatient procedure and "is doing well and is available for votes as needed."