"An evaluation of his underlying cancer shows he is responding positively to ongoing treatment", Gilbert said of McCain.
Earlier in the day, President Donald Trump told reporters he had spoken to McCain's wife, Cindy.
McCain tweeted on Monday: "Thanks to everyone for your support & words of encouragement!" He was diagnosed with the tumor following surgery to remove a blood clot in July. But he said he's "encouraged that John's going to be back in January and be good to go".
If the count is deadlocked, Vice President Mike Pence would cast the tie-breaking vote in favor of Republicans.
CNN also reported that McCain would miss the vote, which is expected to be held in the next few days.
Christian Benteke finds redemption as Eagles soar in Leicester
Palace have 17 points from 18 games, which is a remarkable turnaround considering they had none, and no goals, after seven. Christian Benteke scored his first goal since May to help Palace to their first away victory of the season on Saturday.
Mr. McCain's office said in a statement on Sunday night that the senator, who had been hospitalized recently in the Washington area, would undergo physical therapy and rehabilitation at the Mayo Clinic in Arizona and "looks forward to returning to Washington in January".
The 81-year-old McCain has been hospitalized at Walter Reed Medical Center in Maryland.
The House and Senate are scheduled to vote this week on Trump's top legislative accomplishment, a sweeping tax package. 'My father is doing well and we are all looking forward to spending Christmas together in Arizona, ' she continued. Trump added that the senator would return to Congress if he's needed for a vote on the tax bill. The bill has already passed the House of Representatives and Senate, but differences in each version needed to be reconciled before a final vote could take place. Marco Rubio of Florida and Bob Corker of Tennessee last week and should have enough votes for the GOP measure.
McCain's daughter, Meghan McCain, confirmed that the senator would be in Arizona for Christmas.
On Sunday morning, McCain's son-in-law Ben Domenech, a conservative writer, said on Face the Nation, "I'm happy to say that he's doing well", but acknowledged that McCain was dealing with symptoms from his treatment. "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".
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