UPDATE | McCain says he's feeling well, vows return in January

WASHINGTON (AP) — Republican Sen. John McCain today expressed thanks for the overwhelming support as he battles brain cancer, saying he is feeling well and looks forward to being back on the job in January.

The 81-year senator returned home to Arizona after being treated for a viral infection at Walter Reed Medical Center in Maryland. He had missed votes last week and will miss a crucial vote on the GOP tax bill this week.

McCain tweeted today: "Thanks to everyone for your support & words of encouragement! I'm feeling well & looking forward to returning to work after the holidays."

His wife, Cindy, tweeted earlier: "Nothing beats the feeling of being home in Arizona for the Holidays!" with a photo of a Christmas tree.

A statement from his office late Sunday said he would undergo physical therapy and rehabilitation at the Mayo Clinic in Arizona. Now in his sixth Senate term, McCain underwent surgery in mid-July to remove a 2-inch blood clot in his brain and was diagnosed with glioblastoma.

McCain's daughter, Meghan, tweeted on Sunday: "My father is doing well and we are all looking forward to spending Christmas together in Arizona."

In its statement on Sunday, McCain's office provided an assessment from Dr. Mark Gilbert, chief of neuro-oncology at the National Institutes of Health's National Cancer Institute.

"Senator McCain has responded well to treatment he received at Walter Reed Medical Center for a viral infection and continues to improve," Gilbert said. "An evaluation of his underlying cancer shows he is responding positively to ongoing treatment."