In 1934, he became the CCF's first national secretary. In the 1935 federal election, Coldwell was elected to the House of Commons as Member of Parliament (MP) for the riding of Rosetown-Biggar. Coldwell also served as the CCF's national chairman from 1938 to 1942. He split with CCF leader J. S. Woodsworth when World War II broke out in 1939. Woodsworth, a pacifist, opposed the war effort, while Coldwell and the rest of the CCF caucus supported the war, and Coldwell's view was the party's official position.