Mexico's main leftist party appeared Monday to have retained the governorship in the southern state of Guerrero, thwarting the former ruling party at the start of a pivotal election cycle.

Angel Aguirre of the leftist Democratic Revolution Party, or PRD, finished 13 points ahead of Manuel Anorve in the race for governor, with votes tallied from all but a handful of polling sites.

Both candidates claimed victory soon after polls closed Sunday, capping a bitter campaign marred by mutual accusations of vote fraud, dirty tricks and strong-arm tactics. Campaigning took place as Guerrero, home of the Acapulco beach resort area, has been buffeted by a wave of drug violence.

Anorve, of the once-dominant Institutional Revolutionary Party, or PRI, on Monday accused his rivals of waging a "dirty war" and said his party would contest the results.

By winning in Guerrero, the fractious PRD managed to keep political control of the first of three states with PRD governors that are scheduled for elections this year. One of those, Baja California Sur, holds an election Sunday.

The outcome in Guerrero, the first of six states to elect governors in 2011, puts a dent in the PRI's hopes of creating an air of inevitability as it seeks to retake the presidency next year.

The centrist PRI, toppled in 2000 after seven decades of top-to-bottom control, has stormed back by regaining control of the lower Chamber of Deputies and snaring six governorships from rivals since 2007.

The PRI's most likely presidential candidate, Gov. Enrique Peña Nieto of the central state of Mexico, leads polls in hypothetical matchups against opponents from the PRD and the conservative National Action Party, or PAN, of President Felipe Calderon.

Guerrero's bitter race scrambled party lines. Aguirre, most recently a federal senator, was considered an old-school "dinosaur" of the PRI until he broke off to run on behalf of the PRD-led coalition against Anorve, a mayor of Acapulco who had been an ally.

Aguirre dominated in the two largest cities, Acapulco and Chilpancingo, the state capital, and ran strong across much of the rural and impoverished state, according to preliminary results from Guerrero's electoral institute.

George Grayson, a Mexico expert at the College of William and Mary in Williamsburg., Va., said voters punished Anorve for a poor job as mayor of Acapulco, which is now a hotbed of drug violence. Grayson said the results showed individual candidates matter more than party tags.

The campaign in Guerrero was a proxy for wider jockeying, with campaign appearances by heavyweights from the PRI and PRD.

Front-page photographs in Mexico's main newspapers Monday showed an exultant Aguirre embracing Mexico City Mayor Marcelo Ebrard, who wants to be the PRD's candidate for president and campaigned often in Guerrero. Anorve accused the Mexico City government of spending public funds to buy votes for Aguirre.

Calderon's rightist PAN trailed by a large margin and, in the final days of the campaign, threw its support to the leftist PRD in hopes of frustrating the PRI.

PRI national leader Beatriz Paredes complained that Anorve was pitted against an "unnatural" alliance of ideological opposites.

In gubernatorial elections last year, the PAN and PRD put aside vast ideological differences to form alliances that defeated PRI candidates in three states. That tactic has also been discussed as a possible way to beat the PRI in Mexico state, which picks a new governor in July, and thus slow Peña Nieto's presidential plans.

Party lines have also been blurred in the gubernatorial race in Baja California Sur, home to the Los Cabos resort area. Recent polls show a hefty lead for Marcos Covarrubias, an ex-congressman who defected from the PRD and is now running under the banner of the PAN.