MEXICO CITY—Mexican President-elect Andrés Manuel López Obrador is preparing legislation to overhaul the country’s collective-bargaining system to improve wages and help Mexico comply with labor-rights provisions of a recent trade agreement with the U.S., a top aide said Tuesday. Mr. López Obrador aims to effectively ban so-called protection agreements, or collective contracts signed by union leaders and employers without worker consent, by requiring unions to show they have the backing of at least 30% of workers before signing a contract, said Luisa María Alcalde, who has been tapped to be labor minister when Mr. López Obrador takes office Dec. 1.