Obama, who ran against McCain in the 2008 presidential election, called him “an American hero & one off the bravest fighters I’ve ever known.” In a Twitter message, Obama added: “Cancer doesn’t know what it’s up against. Give it hell, John.”

The Mayo Clinic Hospital in Phoenix said tests revealed a primary brain tumor known as a glioblastoma associated with a blood clot above McCain’s left eye that was removed last week.

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“The Senator and his family are reviewing further treatment options with his Mayo Clinic care team,” said the hospital in a statement. “Treatment options may include a combination of chemotherapy and radiation.”

McCain’s type of cancer is particularly aggressive, with commonly poor prognoses, according to medical experts. The late senator Edward M. Kennedy (D-Mass.) survived less than 15 months after he was diagnosed with the same condition.

McCain’s family, fellow members of Congress, statesmen and supporters immediately took to Twitter to express shock, prayers and many appeals to his tough reputation.

His daughter, Meghan McCain, tweeted a statement saying “he is the toughest person I know. … Cancer may afflict him in many ways: But it will not make him surrender. Nothing ever has.”

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George H.W. Bush, who has experienced his own bouts of recent health scares, gave McCain a stirring tribute Wednesday night.

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“The Hanoi Hilton couldn’t break John McCain’s spirit many years ago, so Barbara and I know — with confidence — he and his family will meet this latest battle in his singular life of service with courage and determination,” he said.

Former president Bill Clinton, former vice president Joe Biden, former secretary of state Hillary Clinton and McCain’s former running mate, Sarah Palin, also expressed their support.

Sen. Jeff Flake (R) said that he spoke with his fellow senator from Arizona and that it was a “tough diagnosis, but even tougher man.” Flake joined former congresswoman Gabrielle Giffords (D), Rep. Andy Biggs (R), and Rep. Trent Franks (R), all Arizonans, in rallying behind McCain.

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“I have been amazed by his energy and vigor for serving his country over the years, and the sacrifices he made for this nation are unmatched,” Biggs said. “I hope to see him back on Capitol Hill soon, and I know that he will fight this diagnosis with every ounce of strength he has within him.”

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“John McCain is an American hero and possesses such great strength and courage,” Franks said. “He has exhibited those qualities his whole life and I have no doubt he will continue to inspire us all in this next fight.”

Politicians from both sides of the aisle offered their encouragement and prayers.

Sen. Christopher A. Coons (D-Del.) described a visit to Vietnam he made with McCain last month, remarking on the effort McCain has shown to strengthen relations with the country.

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“We visited the site where his plane was shot down and the prison where he was a POW,” Coons said. “Throughout our visit, I saw his humor and grace in recounting this very hard history, and in many meetings with senior Vietnamese officials, I saw the very high regard in which he is held throughout the country for his amazing work over 22 visits to Vietnam to reconcile our countries.”

Sean Sullivan contributed to this report, which has been updated.