Robert Blackford Duncan (born 1920) is an American politician from the state of Oregon. A Democrat, he served multiple terms in the Oregon Legislative Assembly and as a U.S. congressman from Oregon. In the Oregon House he served as speaker for four years, and in the U.S. House he represented two different districts. The Illinois native and World War II veteran twice ran unsuccessful campaigns to be elected to the U.S. Senate.
During World War II, he served in the United States Merchant Marine and United States Naval Air Force from 1942 to 1945. In 1948, Duncan received his law degree (LL.B) from the University of Michigan and passed the bar in October of that year. After graduation Duncan and his family moved from Michigan to Portland and then to Medford in Southern Oregon, where he moved to join the law practice of William M. McAllister.
In 1966, he was the Democratic candidate for United States Senate for retiring Senator Maurine B. Neuberger's seat. While Duncan strongly supported President Johnson's Vietnam war policies, his Republican opponent, Mark O. Hatfield, was skeptical of the administration's conduct of the war. This factor, along with Hatfield's statewide popularity as Governor of Oregon, gave Hatfield a narrow victory. In 1967, Duncan moved to Portland where he lived until 1974, returning to the practice of law.
In 1968, Duncan challenged incumbent Senator Wayne Morse in the Democratic Senate primary. Again, Duncan's war views played a role. Though Duncan was initially far ahead of the anti-war maverick Morse, Morse closed the gap at the end and won a narrow victory, aided by the beginning of the Paris Peace Accords, which brought the possibility of the end of the war. Morse went on to lose in the general election to Bob Packwood.
Duncan returned to his Portland law practice, but after Edith Green retired from Congress, Duncan was elected to her seat in 1974 and returned to the House, this time representing the 3rd district in Portland. He served another three terms, and was upset in the 1980 Democratic primary by eventual winner Ron Wyden.