Mexico's former opposition parties had specific social bases that would not, on their own, have catapulted either opposition party into power. In the 1990s, specific regional bases of support developed for the parties, reflecting their efforts to develop their organizations more locally. Nationally, this led to the emergence of two parallel two-party systems, PAN-PRI competition in the north and center-west and PRD-PRI competition in the south. In parallel, a proregime-antiregime cleavage came to dominate the Mexican party system, which, combined with local-level opposition efforts to oust the PRI, created new incentives for the opposition parties to abandon past emphases on ideological differences and to act like catch-all parties instead. The regime cleavage fostered the dealignment of the Mexican electorate, a process that promoted the development of catch-all parties. Movement within the parties to behave like catch-all parties has not come without internal tensions, but electoral dynamics prove powerful inducements to catch-all behavior.