Senate President Pro Tempore Daniel K. Inouye, the chamber's senior member and a hero of World War II, died Monday of respiratory failure, leaving what his colleagues said was a giant hole in the fabric of the chamber.
A Medal of Honor winner who served even as tens of thousands of other Japanese-Americans were placed in internment camps back home, Mr. Inouye, a Hawaii Democrat, was also the first Japanese-American elected to Congress.
President Obama, a native of Hawaii, said the country has "lost a true American hero."
"He worked to strengthen our military, forge bipartisan consensus and hold those of us in government accountable to the people we were elected to serve," the president said in statement. "But it was his incredible bravery during World War II ... that made Danny not just a colleague and a mentor, but someone revered by all of us lucky enough to know him."
Mr. Inouye, 88, lost an arm to a German hand grenade during a World War II battle in Italy. In 2000, he was one of 22 Asian-American World War II veterans who belatedly received the nation's top honor for bravery on the battlefield, the Medal of Honor.
He was elected to the House in 1959 — the same year Hawaii became a state. He has served in the Senate since January 1963, becoming the second-longest service senator in the chamber's history, trailing only Sen. Robert C. Byrd of West Virginia, who died in 2010 after 51 years in the chamber.
Mr. Inouye had already planned on running for re-election in 2016.
His last utterance, his office said, was "Aloha."
His death changed the line of presidential succession, since the chamber's president pro tempore is third behind the vice president and the speaker of the House. Soon after Mr. Inouye's death, the Senate by voice voted Sen. Patrick J. Leahy, Vermont Democrat, now the chamber's most senior member, to the pro tempore post.
Mr. Leahy this week also began managing a key spending bill, which would have been Mr. Inouye's job as chairman of the Appropriations Committee. But Mr. Inouye had been hospitalized for respiratory problems.
Senators gathered on the chamber floor late Monday afternoon to deliver testimonials.
"I have never known any like Dan Inouye. No one else has," said Senate Majority Leader Harry Reid, Nevada Democrat, as he announced the news on the Senate floor.
Hawaii's other Democratic senator, Daniel K. Akaka, who is retiring next month, called Mr. Inouye a "true patriot" and his state's "greatest leader."
"His legacy is not only the loving family that he leaves behind, it can be seen in every mile of every road in Hawaii, in every major preserve, in every facility that makes Hawaii a sacred place," Mr. Akaka said. "Every child born in Hawaii will learn of Dan Inouye — the man who changed the islands forever."
Hawaii Gov. Neil Abercrombie will appoint a replacement, choosing from a list of three candidates selected by the state Democratic Party. Whoever is appointed will serve until a special election in 2014.
"We're preparing to say goodbye," Mr. Abercrombie said. "Everything else will take place in good time."
Once a regular smoker, Mr. Inouye had a portion of a lung removed in the 1960s after a misdiagnosis for cancer. Last week, he issued a statement while hospitalized, saying "for the most part, I am OK."
Mr. Inouye had wanted to become a surgeon but changed his plans after the loss of his arm and enrolled in law school after the war.
A quiet but powerful lawmaker, Mr. Inouye ran for Senate majority leader several times without success. He gained power as a member of the Senate Appropriations Committee and chairman of the defense appropriations subcommittee before Republicans took control of the Senate in 1994.
When the Democrats regained control in the 2006 elections, Mr. Inouye became chairman of the Senate Commerce Committee. He left that post two years later to become chairman of the powerful Senate Appropriations Committee.
Mr. Inouye also chaired the Senate Indian Affairs Committee for many years. He was made an honorary member of the Navajo nation and given the name "The Leader Who Has Returned With a Plan."
• This story was based in part on wire service reports.
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