More than a dozen Mexican states have held elections after campaigning besieged by assassinations and scandals that displayed the power of drug cartels and posed the biggest challenge yet to president Felipe Calderon.
With public discontent rising over the violence set off by Mr Calderon's offensive against drug gangs, the opposition party that ruled Mexico for 71 years hoped to capitalize on the frustrations and gain momentum from the votes for governors, mayors and other posts in its bid to regain the presidency in two years.
The first exit polls released by TV Azteca and Televisa said the PRI, which long held on to power through a system of largess and corruption that many considered a quasi-dictatorship, won governship races in eight states.
Those included the border state of Tamaulipas, where PRI candidate Rodolfo Torre was killed Monday by suspected drug cartel gunmen. His brother, Egidio, was picked to run in his place.
But the Azteca poll pointed to a PRI loss in Puebla and both polls indicated a PRI defeat in Oaxaca, two of several states where Calderon's conservative National Action Party formed alliances with leftist parties in an effort to thwart PRI's resurgence.
PRI and PAN leaders both claimed victory in Oaxaca, raising the possibility of unrest in a highly volatile state where a five-month uprising erupted in 2006 over allegations that outgoing Gov. Ulises Ruiz stole his election victory. The PRI has governed Oaxaca for 80 years.
"This theatre is for them to say ... that we committed fraud," said PRI official Carlos Jimenez. "This is part of a strategy. They know they lost."
According to the Azteca poll, alliance candidate Gabino Cue won 54% of the vote, compared to 40% for PRI candidate Eviel Perez. The Televisa poll had Cue winning with 53%.
In Tamaulipas, the PRI, held up Torre's assassination as evidence Mr Calderon has failed to bring security despite the presence of tens of thousands of troops 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 the northern state of Sinaloa, the cradle of the cartel by the same name.