Mayoral candidates were also assassinated in Tamaulipas and in Chihuahua, a state in northwestern Mexico.

Many candidates have already stopped campaigning in Tamaulipas because of security concerns.

Franc Contreras, Al Jazeera's correspondent in Mexico City, reported that in the state of Sinaloa, "where a drug cartel holds power, a citizen observer who is part of an important electoral commission was murdered on Friday".

Call for unity

Mexico's interior ministry has offered armoured cars and bodyguards to candidates in advance of the July 4 poll, and Calderon called on Mexico's political parties to "unite against organised crime".

Sunday's vote pits Calderon's National Action Party (PAN) against the Institutional Revolutionary Party (PRI), which ruled Mexico for decades before Calderon took office in 2006.

Al Jazeera's Mariana Sanchez reports on the shootout in Sonora that left 21 people dead

Many analysts expect the PRI to post big gains because of public anger over the rising drug violence.

"The political cost will be for the government's party," Javier Oliva, a national security analyst at Mexico's National Autonomous University, said.

"The public security policies are not showing results."

More than 25,000 people, mainly traffickers and police, have been killed in drug-related violence since Calderon launched an army-led offensive against drug gangs shortly after taking office.               

Drug violence was much lower during the PRI's rule, partly because of a murky series of deals between politicians and the leaders of drug cartels.

Several other politicians and law-enforcement officials have been targeted in recent days.

Drug gang battle

In the northern state of Sonora, 21 people were killed on Thursday in a gun battle between suspected rival drug gangs.

Prosecutors say the fighting occurred in a sparsely populated area about 20km from the US border.

The previous night, Sandra Salas Garcia, an assistant attorney-general in Chihuahua, was gunned down by a group of armed men in Ciudad Juarez, one of the cities hit hardest by escalating drug violence.

Garcia had been working on a special unit charged with investigating the drug trade. 

Also in Ciudad Juarez, unidentified men left a severed head on Thursday outside the house of Hector Murgia, the PRI candidate favoured to win the city's mayoral race.

Al Jazeera's Contreras said that the "widespread violence is having an effect on voters.

"I should say that voter turnout is usually low for the local level elections in any case, but with the violence in some of these regions, it is expected to be even lower."