In October 1927, when a student, he joined the newly formed Iron Guard and became responsible for the Banat area. In the early 1930s, Sima participated in the Legion's "Excursions among the People", wherein Iron Guard members would promote their movement in rural areas among peasants, and had become a leading organizer for the Legion in Severin County by the 1930s. In 1935, he was promoted to the position of Legionary commander of the Timișoara region, and in the 1937 Romanian general election, he ran as a candidate in Severin County. Sima became commander of the Iron Guard in late 1938 after its founder and leader, Corneliu Zelea Codreanu, was imprisoned and later murdered. The Iron Guard had initially formed an interim leadership including Sima, Ion Belgae, Iordache Nicoara, Ion Antoniu, and Radu Mironovici in April 1938, but by August, Sima remained the only leader not imprisoned by the Romanian government, eventually allowing him to bypass the hierarchy of leadership previously established and become leader of the Iron Guard. Prior to his ascension to leadership, tension had built significantly both within the organization and country-wide following a series of assassinations of Iron Guard members, including of Codreanu (who was killed by order of King Carol II).