MEXICO CITY (Reuters) - Mexico's Senate on Tuesday approved a wide-reaching but watered down labor reform bill in the biggest shake-up of the country's job market in more than four decades.

The bill's approval comes after a protracted tussle between outgoing President Felipe Calderon's National Action Party and pro-union hardliners within the Institutional Revolutionary Party (PRI) of president-elect Enrique Pena Nieto. The PRI has traditionally relied on union support.

The bill contains a raft of measures, including changes that would make it easier for firms to hire and fire workers and shorten labor disputes. However, parts of the bill that sought to make unions more transparent were cut back.

(Reporting by Miguel Gutierrez; Editing by Eric Beech)