U.S. Sen. John McCain has been hospitalized at Walter Reed Medical Center where he's being treated for "normal side effects of his ongoing cancer therapy," according to a statement Wednesday from his office.

McCain, who missed votes on Monday and Tuesday, will return to work in the U.S. Senate "as soon as possible," the statement said.

His hospitalization comes amid speculation about his future, and as the House of Representatives and Senate attempt to hold final votes next week on a $1.5 trillion tax-cut bill. The GOP is trying to maintain a fragile coalition, especially in the Senate, that could give President Donald Trump his first significant legislative achievement.

It was not immediately clear how long McCain would be hospitalized.

Since his cancer diagnosis, McCain has been healthy enough to attend the wedding of his daughter, Meghan, in Cornville last month and to receive the Liberty Medal from the National Constitution Center in Philadelphia in October.

But the disease is clearly taking a toll. He has been in a walking boot for weeks after tearing a tendon in his leg, which was also ascribed to the effects of his treatment.

McCain on July 14 underwent an emergency craniotomy to address a blood clot that was later revealed to be associated with glioblastoma, a deadly kind of brain cancer.

McCain returned to Capitol Hill after Labor Day and has maintained a regular Senate work schedule as he continues chemotherapy and radiation treatment.

RELATED: 'The View': Joe Biden tells Meghan McCain, 'If anyone can make it, your dad can' | Meghan McCain has harsh words for Bannon after Moore loss

"I am more energetic, and more engaged, as a result of this because I know that I've got to do everything I can to serve this country while I can," said McCain in a CBS "60 Minutes" interview that aired in September.

U.S. Rep. Andy Biggs, R-Ariz., was among those quick to post sympathies online.

It also came as former Vice President Joe Biden tried to comfort Meghan McCain on her father's illness during an emotional appearance on ABC's "The View" that aired Wednesday.

For his part, McCain welcomed the compassion for his family, praising Biden in a tweet for "serving as an example & source of strength for my own family."

In the Wednesday statement about his current treatment, McCain's office said: "As ever, he remains grateful to his physicians for their excellent care, and his friends and supporters for their encouragement and good wishes."

McCain's latest illness also generated vitriol on social media, much of it targeted at his support for the GOP tax cuts that would remove the insurance mandate from the Affordable Care Act. Nonpartisan estimates suggest that move would lead about 13 million to lose insurance coverage and would raise premiums for millions more.

READ MORE:

McCain on health: Prognosis 'pretty good,' but 'every life has to end one way or another'

Sen. John McCain treated for cancer therapy side effects, tear in Achilles tendon