Twenty-nine states of Mexico were created as administrative divisions by the constitution of 1917, which grants them those powers not expressly vested in the federal government; Mexico's two remaining territories, Baja California Sur and Quintana Roo, achieved statehood on 9 October 1974, raising the total to 31. Each state has a constitution, a governor elected for six years, and a unicameral legislature, with representatives elected by district vote in proportion to population. An ordinary session of the legislature is held annually, and extraordinary sessions may be called by the governor or the permanent committee. Bills may be introduced by legislators, by the governor, by the state supreme court, and by municipalities (a unit comparable to a US county). In addition to the 31 states, there is also one federal district comprising Mexico City, whose governor serves as a member of the cabinet. Many state services are supported by federal subsidies.
The principal unit of state government is the municipality. Mexico's 2,378 municipalities are governed by municipal presidents and municipal councils. State governors generally select the nominees for the municipal elections. Municipal budgets are approved by the respective state governors. Until 1997, the president appointed the mayor of Mexico City. Political reforms allowed the first open elections in 1997. PRD candidate Cuauhtémoc Cardenas Solórzano was elected mayor. When he resigned to run for the presidency in 1999, Rosario Robles Berlanga became the first woman mayor of Mexico City. In 2000, PRD's Andrés Manuel López Obrador became the second democratically elected mayor of Mexico City. López Obrador's popularity as mayor has made him a prime candidate for the 2006 presidential election.