A victory in the southern state of Oaxaca would be a much needed boost for Calderon after a campaign for local elections in more than a dozen states that was besieged by assassinations and scandals that displayed the power of drug cartels and faced his government with its most serious political challenge.
The vast majority of citizens didn't show up to vote in the northern state of Tamaulipas, where the leading gubernatorial candidate was assasinated a week ago by suspected cartel gunmen.
Impoverished and volatile Oaxaca is one of several states in which Calderon's conservative National Action Party formed alliances with leftist parties seeking to thwart a resurgence of the Institutional Revolutionary Party, which ruled Mexico for 71 years and still controls many state governments.
The PRI had hoped for significant gains in Sunday's elections to pick up momentum for its bid to regain the presidency in 2012, trying to capitalize on growing frustration with surging drug gang violence. But exit polls released by TV Azteca and Televisa indicated the PRI would not significantly improve on the nine governorships it already held among the dozen seats up for grabs.
The polls and preliminary official results pointed to a PRI defeat in Oaxaca, a heavily indigenous state that it had ruled for 80 years. The PAN and its leftist allies were also in a tight race in the PRI bastion of Sinaloa, a violent northern state that is the birthplace of the powerful drug cartel of the same name.
The PRI gubernatorial candidate in Sinaloa, Jesus Vizcarra, had long faced allegations of ties to the cartel led by Joaquin "El Chapo" Guzman, Mexico's most-wanted drug lord.
The newspaper Reforma recently published a photograph of Vizcarra attending a party many years ago with El Chapo's second-in-command, Ismael "El Mayo" Zambada. Vizcarra, the mayor of state capital Culiacan and a distant relative of slain drug trafficker Ines Calderon, dodged questions about whether Zambada is the godfather of one of his children, saying only that he had never committed a crime.
With about 20 percent of the vote counted, preliminary official results showed alliance candidate Mario Lopez with 52 percent of the vote, compared to 46 percent for Vizcarra. The exit polls said the race was too close to call.
The victory over the PRI in Oaxaca was highly symbolic. A five-month uprising erupted in 2006 over allegations that outgoing Gov. Ulises Ruiz, who was not seeking re-election, stole his election victory. Critics accused Ruiz of strong-arm politics that exemplified the coercion and corruption that the PRI used to govern Mexico for seven decades.
"After 80 years, Oaxaca will have an opposition government and this is a great opportunity," alliance candidate Gabino Cue told Televisa.
According to the Azteca poll, Cue won 54 percent of the vote, compared to 40 percent for PRI candidate Eviel Perez. The Televisa poll had Cue winning with 53 percent and Perez garnering 45 percent. The official count had Cue leading with 50 percent, compared to 42 percent for Perez, with 17 percent of the vote counted.
"I think democratic transition is good, and that the PRI leaves Oaxaca because we've had many years of misery," said Israel Cruz, 31, who voted for Cue.
The exit polls said the PRI won in at least nine states, including three that it wrested back from the PAN or the leftist Democratic Revolution Party.
The polls indicated the PRI easily held onto the border state of Tamaulipas, where PRI candidate Rodolfo Torre was assassinated Monday.
His brother, Egidio, was picked to run in his place. He voted at an elementary school in Ciuadad Victoria wearing a bulletproof vest and escorted by federal police in two trucks.
The PRI held up Torre's assassination as evidence Calderon has failed to bring security despite the presence of tens of thousands of troops and federal police in drug trafficking hot spots.
PAN Leaders, in turn, insinuated the PRI protects drug traffickers in Tamaulipas, the birthplace of the Gulf cartel, and in Sinaloa.
A new scandal enveloped outgoing Tamaulipas Gov. Eugenio Hernandez: On Sunday, federal prosecutors said they were questioning one of his bodyguards, Ismael Ortega Galicia, after the newspaper Reforma reported that the U.S. Treasury Department has listed him as a key member of the Gulf or Zeta drug gangs. The former allies split this year and are fighting for turf in Tamaulipas.
Tamaulipas Public Safety director Jose Soberon said Mexican federal prosecutors had previously investigated Ortega and found no evidence against him. Officials at the federal Attorney General's Office said they had no immediate information on that claim. Soberon also said Ortega had traveled to the U.S. several times with the governor and had never been detained.
Fear discouraged many people from voting in Tamaulipas, where extorsion and abductions are rampant and armed men openly drive on highways with the acronym of the Gulf cartel stamped on their SUVs.
Just 20 percent of voters cast ballots, according to the state election institution -- a dramatic drop from the 50 percent turnout in the last state elections in 2007.
Dozens of poll workers quit in fear over the past week. One man, an orange farmer, said his brother-in-law was kidnapped early Sunday before he was to preside over a voting station in a village outside Ciudad Victoria.
"We still don't know if he was kidnapped because of the elections or because they will ask for money," said the farmer, who asked not be quoted by name out of fear for his own safety. "Here the government is part of the problem."
In the resort city of Cancun, voters passed by banners reminding them of a scandal that shook the race for governor of Quintana Roo state: Gregorio Sanchez, candidate for the leftist Democratic Revolutionary Party, was arrested last month for allegedly protecting two drug cartels as mayor of Cancun, allegations he has demised as politically motivated.
The banners read: "Greg's Cartel. Enough of criminals in government."
The PRI won easily in that state, exit polls indicated.