WASHINGTON – Sen. John McCain will miss the tax reform vote in the Senate this week as he heads back home to Arizona to recover from the side effects of ​his latest round of cancer ​treatments.

McCain, 81, is fighting a​n aggressive form of brain cancer and will be resting in Arizona when President Trump’s signature tax reform legislation gets a vote in the upper chamber ​– as soon as Tuesday, according to CBS News.

“As anyone knows whose family has battled cancer or any significant disease, that oftentimes there are side effects to treatment that you have,” Ben Domenech, McCain’s son-in-law, said Sunday on “Face the Nation.”

The former Republican presidential nominee and ​hero ​Vietnam POW remains in “good spirits” after being hospitalized this week at Walter Reed National Military Medical Center near Washington from side effects from chemotherapy.

“He is doing well. He’s in good spirits,” said ​Domenech, who is married to Meghan McCain. “And he’s looking forward to heading back home to Arizona for the holidays. And he remains one of the toughest men on the face of the earth.”

President Trump said Sunday he spoke to McCain’s wife, Cindy.

“I wished her well. I wish John well,” Trump told reporters as he returned to the White House from Camp David. “They’ve headed back (to Arizona). But I understand he’ll come if we ever needed his vote, which hopefully we won’t. But the word is John will come back if we need his vote. It’s too bad. He’s going through very tough time, there’s no question about it. But he will come back if we need his vote.”

Meghan McCain said her dad is looking forward to being home and she urged donations to two charities that combat brain cancer.

“Thank you to everyone for their kind words. My father is doing well and we are all looking forward to spending Christmas together in Arizona,” she said.

“If you’re feeling charitable this Christmas @HeadfortheCure or @NBTStweets to help find a cure for brain cancer is what I recommend,” she said.

Earlier Sunday, Senate Majority Whip Sen. John Cornyn (R-Texas) said he wouldn’t speculate on his colleague’s health.

“We hope he comes back,” Cornyn told ABC’s “This Week.” “But I’m confident we’ll pass this bill, probably on Tuesday.”

Republicans have a slim 52-48 seat majority in the Senate and no GOP member has announced a “no” vote.

In addition to McCain, Mississippi Sen. Thad Cochran has missed votes recently because of health issues. If both men are absent, the majority threshold to pass the legislation is reduced from 51 to 49 votes, with Vice President Mike Pence available in the case of a tie.

The Senate secured the vote of one holdout, Sen. Marco Rubio (R-Fla.), last week by increasing the child tax credit. And Sen. Bob Corker (R-Tenn.) said Friday he’d support the “imperfect” bill because “we are better off with it.”

Just to be safe, Pence delayed his planned trip to the Middle East to be available on the Senate floor to deliver Trump his first major policy victory.

The House will take up the legislation first on Tuesday, where it’s expected to pass and then head quickly to the Senate.