Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) is receiving treatment at Walter Reed Medical Center in Washington, D.C., for side effects related to his cancer therapy, his office announced Wednesday.

"As ever, he remains grateful to his physicians for their excellent care, and his friends and supporters for their encouragement and good wishes. Senator McCain looks forward to returning to work as soon as possible," McCain's staff said in a statement.

McCain announced in July he had been diagnosed with glioblastoma, an aggressive form of brain cancer. He has been undergoing treatment since.

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In November, he suffered a minor tear in his right Achilles tendon as a side effect of the treatment.

The Senate is expected to vote early next week on a sweeping tax-reform bill.

Republicans in the chamber currently hold a narrow 52-48 seat majority, which is set to shrink to 51-49 with the swearing-in of Sen.-elect Doug Jones (D-Ala.), meaning they can ill-afford defections. Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) says Jones will not be seated until legislative work is done for the year.

McCain missed votes this week.

Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas) said he "definitely" expects McCain to be back in the Senate for next week's tax vote.

"He's just saving his energy and resting up for the big vote next week," he said, asked if McCain's absence would impact the tax vote.

The news of McCain's hospitalization came the same day Meghan McCain, the senator's daughter, became emotional while discussing his illness on "The View."

During an interview with Joe Biden, whose son, Beau, died of glioblastoma, McCain said the former vice president's new memoir was too distressing for her.

“I couldn’t get through your book, I tried,” she said. “I think about Beau almost every day, and I was told that this doesn’t get easier.”

-Jordain Carney contributed to this story which was updated at 6:17 p.m.