In 1954, Duncan was nominated as a
write-in candidate for the
Oregon House of Representatives. The differences between Duncan and Hatfield on the war would produce one of the great splits in the modern Oregon Democratic Party. The state's senior U.S. Senator,
Wayne Morse—a staunch Democratic opponent of the Vietnam War—endorsed Hatfield over fellow Democrat Duncan, an act that infuriated Democratic Party regulars. This factor, along with Hatfield's statewide popularity, gave Hatfield a narrow victory. In 1967, Duncan moved to
Portland where he lived until 1974, Morse went on to narrowly lose in the general election to Republican state Representative
Bob Packwood, who favored continued funding of the war. Duncan returned to his Portland law practice. He ran once more for the Senate in 1972, again losing the Democratic nomination to Morse, this time by a wider margin. Morse then lost to Senator Hatfield. After
Edith Green retired from Congress, Duncan ran for her Portland-based seat from in
1974 and returned to the House. He served another three terms from 1975 to 1981. He lost in an upset in the 1980 Democratic primary to eventual winner
Ron Wyden. == Later years and family ==