A Mexican congressional candidate was shot dead in a town bordering the capital Tuesday, becoming the fourth politician to be slain ahead of Sunday's midterm elections.

Miguel Angel Luna, a former mayor of Valle de Chalco in the state of Mexico southeast of Mexico City, was attacked by armed men at his campaign offices, according to a statement from his Democratic Revolution Party. Luna died shortly afterward at a hospital. An assistant was wounded.

Since March, two candidates running in mayoral races have been slain in the southern states of Michoacan and Guerrero and a third woman who was planning to run in Guerrero was killed.

Also in Guerrero, an armed group broke into electoral offices in the city of Tlapa and burned more than 116,000 ballots late Monday, the state electoral institute reported.

The attack followed similar ballot burnings and thefts in neighboring Oaxaca state.

Radical groups and unionized teachers have vowed to block Sunday's voting for the lower house of congress, nine governorships and hundreds of mayorships.

Authorities pledged to reprint or replace lost or stolen ballots.

Radical teachers oppose a 2013 education reform that requires competitive testing for teachers. They say those tests don't measure teaching skills or the special knowledge needed to teach in Indian and rural areas.