Day was elected as a Republican to the Seventy-seventh and Seventy-eighth Congresses (January 3, 1941 – January 3, 1945). He was an unsuccessful candidate for re-election in 1944 to the Seventy-ninth Congress. During his term, Day opposed U.S. involvement in World War II, claiming it would entail "national suicide" and "economic slavery".[2] His reputation suffered when his name was linked to Nazi agent George Sylvester Viereck. Day published a book, We Must Save the Republic, through Flanders Hall, a small company with ties to registered Nazi agents. In an investigation of Viereck's links to Congress, Day was named as one of four federal politicians who had knowingly collaborated.[3]
Day resumed the practice of law in Evanston, where he died on January 5, 1950.[4] He was interred in Memorial Park, Skokie, Illinois.[citation needed]