In the 1966 U.S. Senate election, Duncan was the Democratic candidate for the seat vacated by retiring Senator Maurine B. Neuberger. While Duncan strongly supported President Johnson's Vietnam War policies, his Republican opponent, Governor Mark Hatfield, was an outspoken critic. 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, returning to the practice of law.