Christian Carlos Hernan Castillo is an Argentinian activist, politician, sociologist and university teacher. He is a founding member of the Socialist Workers' Party (Argentina) (PTS). In the 2013 elections he was voted in as a deputy for the Buenos Aires Province for the Workers Left Front; he resigned to his seat on June 10, 2015 to leave his place to the next candidate based on the banking rotation between the different parties that make up the Front. He was pre-candidate to Governor of the Buenos Aires Province during the 2015 primary elections, losing to Néstor Pitrola.

The Workers' Left Front – Unity is an alliance of initially three Trotskyist parties in Argentina formed to participate in elections in 2011, announced at a press conference in April that year. They are the Workers' Party (PO), the United Socialist Workers' Party (PSTU), the Socialist Workers' Party (PTS), and Socialist Left (IS). In 2019, the Workers' Socialist Movement (MST) joined the alliance.