Complex sentence: Ortega vowed that he would govern "desde abajo" (from below); in other words due to his widespread control of institutions and Sandinista individuals in all government agencies, he would still be able to maintain control and govern even without being president.
Split into simpler sentences: Ortega vowed that he would govern "desde abajo" (from below). Given his party's widespread control of institutions and many Sandinista individuals in all government agencies, he would maintain control and govern even without being president.