Mexico's main opposition party has swept to victory in a crucial state election, strengthening its bid to retake the presidency and handing a major defeat to the governing conservatives.
Initial results on Sunday suggested that the PRI also strengthened its grip on two other states, brightening the political prospects for its PRI's main presidential hopeful, Enrique Pena Nieto.
Exit polls showed the PRI increased its share of the vote in the state of Mexico, the country's most populous, while Calderon's National Action Party (PAN) had its worst showing in a generation in the region that encircles the nation's capital.
The winning PRI candidate, Eruviel Avila, a popular former mayor of the state of Mexico's largest municipality Ecatepec, was backed by Pena Nieto, who happens to be the state's outgoing governor.
With 80 per cent of the ballot boxes counted, Avila had a 62.1 per cent support, while Alejandro Encinas of the leftist Democratic Revolution Party (PRD) had 21.4 per cent.
And trailing in third place, Luis Felipe Bravo Mena of President Felipe Calderon's ruling National Action Party (PAN), had 12.6 per cent.
This would be nearly half the electoral support PAN secured in 2005, and its worst performance there since 1987.
Laura Carlsen, director of the Americas programme at the Centre for International Policy, told Al Jazeera that the state elections were "critical" when viewed in relation to the 2012 presidential polls.
"[The PRI] appears to have won by a landslide, and this means that they're very well positioned to go into these elections. It was critical because Mexico state is a very powerful state and it is a power centre for the PRI," she said.
"When the PAN [...] came into power [...] people expected changes and they didn't see it. And then that party has also brought the country to a critical state because of the drug war that was launched.
"That has led to 40,000 people dead in the country and a general situation of chaos, not only because of the violence but because of the erosion of political institutions."
"This is a historic win in the state of Mexico," Humberto Moreira, PRI's national chairman, said. He said the PRI had also successfully defended its governorships in the northern state of Coahuila state and the western state of Nayarit.
Initial exit polls showed the PRI had also increased its share of the vote in the two states. However, one media report said the PAN was claiming victory in Nayarit.
Both Coahuila and Nayarit have seen a sharp rise in drug-related murders and violence in the past year, and PAN has taken a hit in public opinion for it.
The PRI ruled Mexico for more than 70 years until it was ousted by the PAN in 2000.
Although its control of Mexico was dogged by allegations of corruption and vote-rigging, analysts say the PRI has a strong chance of returning to power in 2012.
The centrist PRI has never lost an election in the state that is home to over 15 million people, where Pena Nieto was elected with just under 48 per cent of the vote in 2005.
Carlsen, of the Centre for International Policy, said that there was "mounting evidence of the scandalous use of public funds in this Mexico state election, as well as vote buying", pointing the finger at the PRI.
"One of the other critical things is that the Mexican people and the Mexican electoral institutions will have to be repaired ... for the next elections, because another thing that we saw clearly [in these elections] was that the electoral institutions ... were unable to stop some of the electoral law violations that the PRI put into place: by using excessive propaganda, by going over the spending limits," she said.
More than 40,000 people have been killed in drug-related violence in Mexico since 2007.
On the eve of the election, five dismembered bodies were found and two people died in a violent shootout in the State of Mexico, local media reported.
Flooding caused by heavy rains had also caused the emergency relocation of more than 170 voting booths in two of the state's largest cities.