Complex sentence: Doxey had the support of President Franklin D. Roosevelt and Mississippi's senior US Senator, Theodore G. Bilbo, but Eastland defeated him in the Democratic primary when winning the Democratic nomination was tantamount to election in Mississippi.
Split into simpler sentences: Doxey had the support of U.S. President Franklin D. Roosevelt and Mississippi's senior U.S. Senator, Theodore G. Bilbo, but Eastland defeated him in the Democratic primary. In those days, winning the Democratic nomination was tantamount to election in Mississippi.