WASHINGTON/PHOENIX, Ariz. (Reuters) - U.S. Senator John McCain promised on Thursday he would return rapidly to Washington despite his newly diagnosed brain cancer, flashing the fighting spirit that has defined him since he was held in captivity as a Navy pilot during the Vietnam War.

McCain, a veteran senator and former Republican presidential candidate known as a strong and sometimes fiercely independent voice on defense and security issues, was found to have an aggressive form of brain tumor, glioblastoma, after surgery last week for a blood clot above his left eye.

The news, issued by his office late on Wednesday, drew a wave of support from across the political spectrum, and raised questions about how long McCain would be absent from the Senate, where Republicans have a narrow majority and are eager to notch up some legislative successes for President Donald Trump.

“I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I’ll be back soon, so stand-by!” McCain, 80, wrote on Twitter shortly before issuing a news release through his office related to Syria.

McCain was making phone calls from his home in Phoenix, Arizona, to stay abreast of congressional matters.

Republican Senator Lindsey Graham of South Carolina, one McCain’s closest friends in Congress, said the senator called him three times on Thursday morning. He wanted to discuss healthcare legislation that is at the center of the Senate’s attention, and a sprawling defense bill McCain would usher through as the Senate Armed Services Committee’s chairman, Graham said.

Graham told reporters McCain admonished him, “‘No more woe is me.’ He is yelling at me to buck up. I’m going to buck up.”

The son and grandson of admirals, McCain survived more than five years of captivity during the Vietnam War. As a lawmaker he has been a strong advocate for the U.S. military as well as being willing to cross the political aisle and work with Democrats. Among Republican lawmakers, he has been one of the most critical of Trump.

‘HUMAN SIDE TO POLITICIANS’

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Well-wishers included Democratic former President Barack Obama and Trump, who called McCain on Thursday to wish him well, the White House said.

Senator Dick Durbin, the No. 2 Senate Democrat, said the flood of “bipartisan respect and love for John McCain as he faces this cancer battle reminds us that, after all the meanness, there is a human side to politicians. Count this Democrat in John McCain’s corner.”

Glioblastoma multiforme is the most common and most deadly form of brain and nervous system tumor, typically killing half its victims within a year. Patients rarely survive more than three years. Democratic Senator Edward Kennedy died of the disease in 2009.

McCain’s doctors said he was recovering from surgery well, and praised his underlying health as excellent. His doctors told CNN on Wednesday that he had no sign of neurological impairment before or during his surgery. Treatment options include a combination of chemotherapy and radiation.

McCain’s illness has potential policy implications for healthcare legislation and other agenda items in Washington. Republicans hold a narrow 52-48 seat majority in the Senate, and McCain’s absence has made it more difficult to gather the 50 votes needed to advance a healthcare bill.

He has been recovering at his Arizona home since his initial surgery at the Mayo Clinic in Phoenix last week. It was not immediately clear when he would return to Washington.

McCain has previously battled back from injuries suffered during his time as a prisoner of war, and has had non-invasive melanomas removed at least three times.

McCain, who ran a failed White House race in 2008 against Obama, won a sixth six-year term in the U.S. Senate last November.

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If the senator were to retire or die before his term ends in January 2023, the state’s Republican governor would pick a Republican replacement, who would serve until a special election in November 2018. Whoever was elected would fill out McCain’s term.

Arizona’s other senator, Republican Jeff Flake, is up for re-election next year.

The state’s electorate is divided: one-third Republican voters, nearly one-third Democratic and one-third independent.

Matt Roberts, spokesman for the Arizona Secretary of State’s office, said registered independents would soon exceed Republicans. But Republicans tend to turn out to vote on Election Day in stronger numbers than their counterparts.

The Southwestern state is the birthplace of former Senator and 1964 presidential candidate Barry Goldwater, who some call the father of the Republican conservative movement.