Sen. Daniel K. Inouye, D-Hawaii, a member of the Watergate investigating committee, questions witness James McCord during the hearing on Capitol Hill in Washington, on May 19, 1973. Inouye, the influential Democrat who broke racial barriers on Capitol Hill and played key roles in congressional investigations of the Watergate and Iran-Contra scandals, died of respiratory complications, on Dec. 17. He was 88.

Daniel Inouye, a World War II combat veteran and the most senior senator in the U.S. Senate, died Monday of respiratory complications. He was 88.

His last words were "Aloha," Hawaiian for hello and goodbye.

Inouye, a Democrat from Hawaii, was hospitalized a week and a half ago at Walter Reed National Military Medical Center, where he said he was working with doctors to regulate his oxygen intake. Around the Capitol, Inouye had been seen with a portable oxygen supply.

He is survived by his wife, Irene Hirano, and son, Daniel "Kenny" Inouye. Kenny is his son with Margaret Shinobu Awamura, to whom he was married for 56 years until her death in 2006.

Inouye had served in the Senate for 49 years, since 1963. At the time of his death, he was the longest-living serving member of the Senate. The late Sen. Robert Byrd of West Virginia is the only senator who has served longer, for 51 years.

Hawaii became a state in 1959, and Inouye was the state's first Congressman. He also became the country's first Japanese-American Congressman.

He was also hospitalized on Nov. 15 after falling and cutting the back of his head. A statement released by his office spoke to the senator’s apparent dislike of being hospitalized: “The U.S. Army Captain and World War II combat veteran wanted to put a bandage on and come to work but his family insisted he get it checked out.”

Medal of Honor recipient Dan Inouye became the longest-serving senator, having served nine terms after first being elected in 1962. NBC's Brian Williams reports.

He was hospitalized the day before Pearl Harbor Day. Although ailing, he honored the day as he does every year, this time through a press release remembering his time as a Japanese-American teenager in Hawaii. He wrote:

In 1941, the date December 7th was a day that evoked anger, fierce patriotism and dangerous racism. Soon after that day, I suddenly found myself, pursuant to a decision by the government and along with thousands of Japanese Americans declared 4C, enemy aliens. It was a difficult time. I was 17.

Born to working class parents, Hyotaro, a jewelry clerk, and Kame, a homemaker, Inouye dreamed of being a doctor, according to the Washington Post, plans that were sidelined by the war. He was a second-generation Japanese-American, or nisei, and he wrote that it pained him that those who dropped bombs on Hawaii looked like him.

Inouye was 17 when he enlisted in the U.S. Army and served with the 442 Regimental Combat Team, according to a statement on his website. He lost part of his right arm while he was charging a series of machine gun nests in San Terenzo, Italy.

"I looked at it, stunned and disbelieving. It dangled there by a few bloody shreds of tissue, my grenade still clenched in a fist that suddenly didn't belong to me anymore," Inouye wrote in his 1967 autobiography, "Journey to Washington," according to the Honolulu Star-Advertiser.

After the war, he was nominated for the Medal of Honor but did not receive it. President Bill Clinton later bestowed the honor on him and 21 other Japanese-Americans for their courage during World War II, according to the Star-Advertiser.

He attended the University of Hawaii and received a law degree from Georgetown University.

As a lawmaker in D.C. in 1973, Inouye sat on the panel that investigated the Watergate scandal, according to the Post. He was apparently so frustrated by the testimony of a top White House aide that he whispered, “What a liar!” into a microphone that turned out to be hot.

Later, the aide’s lawyer referred to Inouye as, “that little Jap,” a comment that generated outrage, according to the Post.

Throughout his tenure, D.C., Inouye allied himself with the late Alaska Sen. Ted Stevens, a Republican, and proudly proclaimed himself the "No. 1 earmarks guy," in Congress, according to The Associated Press. He championed an older tradition of politics -- one that embraced bipartisanship and compromise.

Responding to his death Monday, U.S. Secretary of State Hillary Clinton released a statement: "No matter what barrier was in his way, Danny shattered it. ... Danny was an icon in his native state of Hawaii and a tireless advocate for the disenfranchised, minorities and women throughout the country. He spent his life working for a brighter future, and we are all better off for it."

Former Sen. Bob Dole wrote touchingly about a man he called one of the Senate's "giants."

"Never once do I recall his being critical of another colleague - Republican or Democrat," Dole wrote. "Danny and I saw service in World War II where he lost an arm and where I had other difficulties. 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."

NBC's Kelly O'Donnell contributed reporting.