However, one of them could also be the oil reserves that exist in the country and that were nationalized by the Cárdenas government. Several empirical studies point to a correlation between the existence of natural resources and the difficulty of turning into a democracy. The first cracks in the system, even though they were merely symbolic, were the 1970s reforms to the electoral system and the composition of the Congress of the Union which for the first time incorporated proportional representation seats allowing opposition parties to obtain seats, though limited in number, in the Chamber of Deputies. As minority parties became involved in the system, they gradually demanded more changes, and a full democratic representation. Even though in the 1960s, a couple (of more than two thousand) municipalities were governed by opposition parties, the first state government to be won by an opposition party was Baja California, in 1989. Historically, there were important high-profile defections from the Institutional Revolutionary Party, like the ones of Juan Andreu Almazán (1940), Ezequiel Padilla (1946), Miguel Henríquez Guzmán (1952), and Cuahtémoc Cárdenas (1988), son of President Lázaro Cárdenas. These departures happened mainly because they opposed the presidential candidate nominations; however, only Cárdenas departure in 1988 resulted in the establishment of another political party (Party of the Democratic Revolution). The presidential elections held in 1988 marked a watershed in Mexican politics, as they were the first serious threat to the party in power by an opposition candidate: Cuauhtémoc Cárdenas, who was nominated by a broad coalition of leftist parties. He officially received 31.1 percent of the vote, against 50.4 percent for Carlos Salinas de Gortari, the PRI candidate, and 17 percent for Manuel Clouthier of the National Action Party (PAN). It was believed by some that Cardenas had won the election, but that the then government-controlled electoral commission had altered the results after the infamous "the system crashed" ("se cayó el sistema", as it was reported). In the concurrent elections, the PRI came within 11 seats of losing the majority of Chamber of Deputies, and opposition parties captured 4 of the 64 Senate seats—the first time that the PRI had failed to hold every seat in the Senate. Capitalizing on the popularity of President Salinas, however, the PRI rebounded in the mid-term congressional elections of 1991, winning 320 seats.