Mexico's Institutional Revolutionary Party, or PRI, has posted strong showings in nine of 12 elections for state governors on Sunday.
PRI ruled Mexico for 71 years before being ousted from power a decade ago by the conservative National Action Party of current President Felipe Calderon.
Mr. Calderon's standing with voters has been hurt by the drug-related violence that has killed about 23,000 people during his term. Mexico has a presidential election in 2012, so the strong PRI showing is a concern for Mr. Calderon.
His National Action Party or PAN joined with leftist parties and is running strongly in the race for governor in three states that are traditional strongholds of the PRI, Oaxaca Puebla, and Sinaloa.
The elections took place six days after the PRI candidate for governor from the northern state of Tamaulipas, Rodolfo Torre, was assassinated while campaigning in the state. His brother is running in his place.
Some information for this report was provided by AP, AFP and Reuters.