Senator Daniel K. Inouye of Hawaii, who went to Washington at the birth of his state in 1959, dominated public life in the Hawaiian islands for more than 50 years and became a quiet voice of national conscience during the Watergate scandal and the Iran-contra affair, died on Monday in Bethesda, Md. He was 88.

A statement by his Washington office said he had died of respiratory complications at the Walter Reed National Military Medical Center. His last word was “aloha,” the statement said.

A hero of World War II who lost his right arm in combat in Europe, Mr. Inouye, a Democrat, served two terms in the House of Representatives early in his career and was first elected to the Senate in 1962. He was the first Japanese-American elected to both the House and the Senate.

After the death of his West Virginia colleague Robert C. Byrd in June 2010, Mr. Inouye became the Senate’s senior member, with a tenure nearing 48 years, and president pro tempore, making him third in the line of presidential succession, after the vice president and speaker of the House. Mr. Byrd’s death also made him the highest-ranking public official of Asian descent in United States history. Months later, he was elected by another overwhelming margin to his ninth consecutive six-year term.