Guerrero, home to tourist-haven Acapulco and a cartel battleground has seen corruption and political violence overshadow its race for governor.
The election is the first of six governor's races in Mexico this year that will set the stage for the 2012 presidential elections.
If the Institutional Revolutionary Party, or PRI, is able to retake Guerrero, it could be a sign of resurgence as it tries to regain the presidency, which it controlled for 71 years before losing it in 2000 to the National Action Party of current President Felipe Calderon.
The PRI lost control of Guerrero in 2005 to the leftist Democratic Revolutionary Party, known as the PRD.
Polls opened at 8 a.m. Sunday with no major incidents in Acapulco or elsewhere. More than 100 voters lined up at balloting stations in an area east of the resort town that has seen murders and armed attacks by drug gangs in recent months.
Private cars, taxis and trucks were transporting voters, while local police and members of the Mexican navy patrolled Acapulco neighborhoods that are frequented by tourists.
The Guerrero race has already demonstrated the far-reaching influence of the PRI, which ruled for decades through paternalism and strong-arm election tactics that many Mexicans considered a quasi-dictatorship.
The main candidates for Guerrero governor both have roots in the PRI. The party's candidate, Manuel Anorve, is facing his second cousin, Angel Aguirre, who split off from the PRI recently to run on a PRD ticket.
"This race pits two PRI dinosaurs against each other," George Grayson, a Mexico expert at the College of William and Mary in Williamsburg, Virginia, wrote in an analysis.
Grayson predicted that if Aguirre wins, he would still back Mexico state Gov. Enrique Pena Nieto, a favorite for the PRI presidential nomination next year.
"In his race to succeed (President Felipe) Calderon, Pena Nieto faces a win-win situation in Guerrero," Grayson wrote.
Still, the race between the two cousins has been bitter.
Aguirre's campaign accused PRI activists of badly beating one of his supporters, while the PRI claims two of its activists were the targets of political attacks.
The PRD demanded an investigation into Anorve's finances after the newspaper Reforma published allegations from a protected witness who said in court documents that the PRI candidate had received millions in cash from drug gangs.
Anorve angrily denied those allegations. The federal Attorney General's Office dismissed the significance of the documents, saying in a statement that secret witness testimony has no value unless backed by concrete evidence. The statement stressed that there is no criminal investigation against Anorve.
Meanwhile, the Guerrero election has been another demonstration of the PAN's failure to secure political dominance despite controlling the presidency since Vicente Fox's 2000 victory.
The government of Calderon, elected in 2006, is grappling with widespread frustration with Mexico's soaring drug-gang violence and an economy just starting to recovering from a severe recession.
Trailing in the polls, PAN candidate Marcos Parra dropped out of the Guerrero race at the last minute and threw his support behind Aguirre.
Such uncomfortable alliances between the conservative PAN and the leftist PRD helped defeat the PRI in three gubernatorial races last year.
Guerrero, a state of 3.3 million people, has been a hot spot of the bloodletting that has marred Calderon's presidency.
Earlier this month, the bodies of 15 men, all but one of them headless, were found on a street outside a shopping center in Acapulco, a coveted drug trafficking zone and the site of turf battles between the cartels.
There were 1,137 drug-related homicides in Guerrero in 2010, surpassing the 879 in 2009.
The Associated Press contributed to this report.