Although he was not from the state of Sonora, whose generals had dominated Mexican politics in the 1920s, Cárdenas was loyal to Sonoran general and former president Plutarco Elías Calles (1924–1928). Calles had founded the National Revolutionary Party (PNR) in the wake of the assassination of Sonoran general Alvaro Obregón, who served as president (1920–1924) and was president-elect in 1928. Cárdenas was Calles's hand-picked candidate in 1934 to run for the presidency. While Calles did not hold the title of president, he had remained the power behind the presidency, and expected to maintain that role when Cárdenas took office. However, Cárdenas out-maneuvered him politically and eventually forced the former president into exile, establishing Cárdenas's legitimacy and power in his own right during his remaining time in office. In 1938, Cárdenas transformed the structure of the party Calles founded, creating the "Partido de la Revolución Mexicana" (PRM), based on sectoral representation of peasants via peasant leagues, unionized workers, professionals, and the Mexican army. Cárdenas's incorporation of the army into the party structure was a deliberate move to diminish the power of the military and prevent their traditional intervention in politics through coups d'état. An important political achievement of Cárdenas was his complete surrender of power in December 1940 to his elected successor, Manuel Ávila Camacho, who was a political moderate without a distinguished military record.