Story highlights During WW II, Inouye was hospitalized with another young soldier, future Sen. Bob Dole

Daniel Inouye died of respiratory complications, his wife and son at his side

Inouye represented Hawaii in the Senate for five decades

His last word was "Aloha," his office says

Daniel Inouye, a World War II veteran who received the Medal of Honor and represented Hawaii in the Senate for five decades, has died, his office announced Monday. He was 88.

He died of respiratory complications Monday evening shortly at Walter Reed National Military Medical Center, with his wife and son at his side.

Inouye was hospitalized last week and had undergone procedures to regulate his oxygen intake.

He won his ninth consecutive term in 2010 and was the second-longest-serving senator in the chamber's history, trailing only Sen. Robert Byrd of West Virginia. Inouye was a senator for all but three of Hawaii's 53 years as a state and had served as its first House member before that.

Senators of both parties took to the chamber floor Monday to mourn his death, and President Barack Obama described Inouye as "a true American hero."

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"In Washington, 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," Obama said in a statement. "But it was his incredible bravery during World War II -- including one heroic effort that cost him his arm but earned him the Medal of Honor -- that made Danny not just a colleague and a mentor, but someone revered by all of us lucky enough to know him. Our thoughts and prayers are with the Inouye family."

Vice President Joe Biden, who served alongside Inouye in the Senate, recalled his "moral bravery" in the body. "He was one of the most honorable men I ever met in my life, and one of the best friends you could hope for. He was honest, and fiercely loyal, and I trusted him absolutely."

Fellow Hawaiian Daniel Akaka choked up as he said, "It is very difficult for me to rise today with a heavy heart to bid aloha, aloha to my good friend, colleague and brother.

"Tomorrow will be the first day since Hawaii became a state in 1959 that Dan Inouye will not be representing us in Congress. Every child born in Hawaii will learn of Dan Inouye, a man who changed the islands forever."

Inouye enlisted in the U.S. Army shortly after the attack on Pearl Harbor. In World War II, Inouye lost an arm charging machine gun nests in San Terenzo, Italy, earning him the Medal of Honor.

He was of Japanese heritage and served in an Army unit made up of Japanese-Americans. His battalion was the most decorated unit of the war. Meanwhile, stateside, many Japanese-Americans were held during the war by the government in internment camps based on their race.

Sen. John McCain, R-Arizona, remembered his late colleague as a "unique, brave legislator" and as a fellow veteran who was injured in war.

Inouye's unit was in "many of the most gruesome and difficult bloodlettings of the entire conflict," McCain said. "In fierce combat, Dan Inouye was gravely wounded on the battlefield. He was brought home ... as we all know, (he) lost his arm."

What happened after that battlefield injury would lead to a long-standing friendship: Inouye was hospitalized alongside the second lieutenant who would also become a U.S. senator, Bob Dole.

"Danny and I saw service in World War II where he lost an arm and where I had other difficulties," Dole said in a statement. " When we left the hospital, we eventually became United States senators and he was always telling his friends that I talked him into it. I don't recall it, but if Danny said it was true, that was good enough for me."

Dole said his friend reached out to all, regardless of the party.

"Never once do I recall his being critical of another colleague -- Republican or Democrat," he said.

Inouye served as chairman of the Senate Committee on Appropriations and the Senate Commerce Committee, and was the first chairman of the Senate Select Committee on Intelligence.

As head of the Appropriations Committee, he defended the practice of earmarking funds for pet projects and brought millions to his home state.

He gained national notoriety as a member of the Senate panel investigating the Watergate scandal in the 1970s.

At the time of his death, Inouye was president pro tempore of the Senate and had held that position since the death of West Virginia's Robert Byrd in 2010. In that position, Inouye was third in the line of succession to the U.S. presidency, behind Vice President Biden and House Speaker John Boehner.

Late Monday, the Senate approved a resolution giving Senate Democratic Leader Harry Reid the authority to swear in Sen. Patrick Leahy, D-Vermont, now the chamber's most senior Democrat, as the new Senate president pro tempore.

Reid said in remembrance of Inouye, "If there was ever a patriot, Dan Inouye was that patriot." Senate Republican Leader Mitch McConnell described him as someone who "rarely called attention to himself but who lived a remarkable American life filled with dignity and grace of the true hero that he was."

Inouye graduated from the University of Hawaii and the George Washington University School of Law.

Asked recently how he would want to be remembered, the senator said: "I represented the people of Hawaii and this nation honestly and to the best of my ability. I think I did OK," according to his office.

His last word was "Aloha," it said.