Arizona’s Sen. John McCain, who is battling an aggressive brain tumor, had surgery over the weekend in Phoenix, his office said Monday.

“On Sunday, Senator McCain was admitted to Mayo Clinic in Phoenix, Arizona, and underwent surgery to treat an intestinal infection related to diverticulitis. He is in stable condition," according to a statement from his office.

“Over the last few months, Senator McCain has been participating in physical therapy at his home in Cornville, Arizona, as he recovers from the side effects of cancer treatment. He has remained engaged on his work as Chairman of the Senate Armed Services Committee, and has enjoyed frequent visits from his family, friends, staff and Senate colleagues.

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“Senator McCain and his family are grateful to the senator’s excellent care team, and appreciate the support and prayers they continue to receive from people all over the country.”

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Cindy McCain tweeted Monday afternoon that her husband is "doing well after his surgery."

According to Mayo Clinic's website, "[diverticulitis] can cause severe abdominal pain, fever, nausea and a marked change in your bowel habits. "

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The condition is the result of inflammation or infection of bulging pouches that can form in the lining of the digestive systems. Called diverticula, these pouches "are common, especially after age 40, and seldom cause problems."

McCain has been outspoken in his criticism of President Donald Trump.

"An American president does not lead the free world by congratulating dictators on winning sham elections," he said in a statement late last month after the president congratulated Russian President Vladimir Putin for winning re-election.

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[READ MORE: Sen. John McCain has strong rebuke for President Trump]

McCain, 81, has previously called Putin a murderer and a thug. He's also pressed the Trump administration to respond aggressively to Russia's interference in the 2016 U.S. presidential election.

Last summer, McCain was diagnosed with brain cancer -- specifically a glioblastoma -- after undergoing surgery to remove a blood clot above his left eye.

[READ MORE: Sen. John McCain diagnosed with brain cancer]

Doctors say a glioblastoma is a particularly aggressive type of brain tumor that digs tentacle-like roots into normal brain tissue.

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Publisher Simon & Schuster announced in late March that the senator is writing "his most personal book in years."

[READ MORE: McCain penning 'no-holds-barred' memoir]

In "The Restless Wave: Good Times, Just Causes, Great Fights, and Other Appreciations," the Arizona Republican will offer "his no-holds-barred opinions on the current developments coming out of Washington," the publisher said. The book is slated for release next month.

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