Dow was elected to Congress in 1964, defeating 18-year incumbent Katharine St. George. He was reelected in 1966, and served from January 3, 1965, to January 3, 1969, and was described as an early opponent of the Vietnam War. He was a delegate to the 1968 Democratic National Convention. He unsuccessfully ran for re-election to the U.S. House in 1968, losing to Republican Martin B. McKneally. From 1968 to 1970, Dow worked on the staff of the U.S. House. In 1970, he defeated McKneally and served in Congress a second time from January 3, 1971, until January 3, 1973.