In September 1932, Colorado's governor appointed Walker to fill a vacancy in the United States Senate. He ran in the special election to complete the term, but was defeated by Republican nominee Karl C. Schuyler, who was sworn in to office in December. He continued to own and publish the "Daily Sentinel" and was a delegate to the Democratic National Conventions in 1936, 1940, 1944, 1948, and 1952. In 1936, he was a presidential elector for the ticket of Franklin D. Roosevelt and John Nance Garner.