In 1938, McClellan unsuccessfully challenged first-term incumbent Hattie Caraway for the Democratic nomination for the United States Senate. During the campaign, he criticized Caraway for her support for the 1937 Reorganization Act and accused her of having "improper influence" over federal employees in Arkansas. Nevertheless, he was defeated in the primary election by a margin of about 8,000 votes. He subsequently resumed the practice of law in Camden, where he joined the firm Gaughan, McClellan and Gaughan. He served as a delegate to the Democratic National Convention in 1940 (Chicago), 1944 (Chicago), and 1948 (Philadelphia).