Election campaigning in Mexico officially ended at midnight ahead of Sunday's vote, but violence continues, especially in four southern states. The combative teachers union and local organizations are calling for a boycott of Sunday's elections. In Guerrero, relatives of the 43 missing Ayotzinapa students and their classmates from the teachers' training college clashed with police in confrontations that left 12 students injured. On Oaxaca, teachers blocked the international airport. Electoral authorities announced that alternative polling stations on Guerrero and Oaxaca would be opened Sunday as protesters vowed to prevent the elections from taking place. Thus far, 21 election-related deaths have been reported across Mexico. The resentment of political parties and the governing elite runs deep and coupled with rejection of recently enacted neo-liberal reforms, could mean an escalation of tensions between now and election day. Clayton Conn reports from Oaxaca. teleSUR