Final approval is expected as soon as Wednesday for the bill, up for consideration in Mexico's lower house. It is backed by President Enrique Pena Nieto and a centre-right coalition between Pena Nieto's Institutional Revolutionary Party (PRI) and the conservative National Action Party (PAN) and believed to have the votes to pass.
The union provision was added on Tuesday after debate in Mexico's Senate, which approved the proposal near midnight.
