Gunmen killed a mayoral candidate in central Guanajuato state Friday, the third local candidate in Mexico's July 1 elections murdered in the past week and one of more than a dozen during the entire campaign.

Most of the candidates for mayorships or state assembly seats killed have been in the drug cartel-plagued southern state of Guerrero.

The Guanajuato state prosecutors' office confirmed that mayoral candidate Remedio Aguirre had been shot to death in the town of Apaseo El Alto. Aguirre was running for the leftist Morena party.

The party's candidate for governor, Ricardo Sheffield, said Morena's campaigns in Guanajuato would be suspended until officials guaranteed the safety of the contenders.

"What we have here in the state is an unbearable situation," Sheffield said.

Apaseo El Alto was once a town known mainly for its hand-crafted wood furniture. But in recent years it has been the scene of continuing violence involving fuel theft from government pipelines and train robberies.

Election violence has been a persistent problem in Mexico. In 2010, drug cartel gunmen killed the leading candidate for governor in Tamaulipas state.

But this year's presidential and local elections have been hit by mounting violence.

Last Friday, Morena's mayoral candidate in the city of Tenango del Aire, south of Mexico City was killed.

And a state assembly candidate of the ruling Institutional Revolutionary party was killed on Tuesday in Guerrero state.