Mexico City (CNN) -- Mexico's leading opposition party appeared poised Monday to capture most of the 12 governorships at stake in Sunday's elections, preliminary results show.
Candidates from coalitions led by the Institutional Revolutionary Party, known by its Spanish acronym PRI, led in nine of the 12 governor's races, according to incomplete results published by the government-run Notimex news agency.
President Felipe Calderon's PAN party led in three races, Notimex said.
"Without doubt, the PRI is the premiere political force in the country," said the party's president, Beatriz Paredes, at a news conference Sunday.
The PRI led in the states of Chihuahua, Durango, Hidalgo, Quintana Roo, Tamaulipas, Tlaxcala, Veracruz, Aguascalientes and Zacatecas, according to Notimex.
PAN, which is the Spanish acronym for National Action Party, led in the states of Oaxaca, Sinaloa and Puebla, Notimex said.
Despite the dominance of the PRI, the overall balance of governorships appeared likely to tip a bit more in the PAN's favor, as it stood to lose fewer of the seats it currently holds than would the PRI.
In all, six of the 12 governorships at stake Sunday stood to change hands from one party to another if the early returns were to hold.
The PRI would pick up governorships from the PAN in Aguascalientes and from a PAN-led coalition in Tlaxcala, while in Zacatecas it would replace the third major party in the nation, the Party of the Democratic Revolution.
The PAN would pick up PRI-held governorships in all three of the states where it holds the lead.
In Tamaulipas, the brother of slain PRI gubernatorial candidate Rodolfo Torre Cantu was comfortably ahead and was expected to win. The PRI picked Egidio Torre Cantu to take his brother's place after the candidate's assassination Monday. The slain Torre Cantu was the front-runner in the race before the ambush that killed him and three others.
In Oaxaca, a PAN victory would end an 80-year-rule by the PRI.
Final election results were expected later this week.
In addition to the governorships, many municipal seats were at stake Sunday.
The elections were being watched closely as a possible indicator of the political winds in Mexico two years before presidential elections.
Calderon, who narrowly won the presidency in a disputed vote in 2006, has been mired in a controversial war on drug cartels that has killed more than 22,000 people since he came into office. The Mexican economy also has faltered due to the global economic downturn.
Calderon's PAN party did not fare well in midterm elections last year.
The PRI held the Mexican presidency for 71 years until the PAN captured the top spot in 2000.