"United, Mexicans can overcome and we will overcome a common enemy, which today threatens to destroy not only our tranquillity, but our democratic institutions," he said.

Al Jazeera's Mariana Sanchez, reporting from Cuidad Victoria, the state capital of Tamaulipas, said that many observers are comparing the current security crisis to Colombia in the 1980s and 1990s, when the drug cartels threatened the state with bombings and political killings

"The message these cartels are sending to the Mexican government is that they have the power to decide when and how to control towns, territories and and ultimately to decide who they want to kill," she said.

Jorge Luis Navarro, president of the Tamaulipas state election institute, confirmed the vote would go forward as scheduled on July 4.

Tensions high

Monday's attack emptied streets in Ciudad Victoria, and heavily armed federal and state police patrolled in caravans.

"I am not going to vote because there is a lot of fear. The tension is very strong"

Maria Pilar Villegas, convenience store clerk in Tamaulipas

"I am not going to vote because there is a lot of fear. The tension is very strong,'' Maria Pilar Villegas, a convenience store clerk told the Associated Press.

The PRI says it has chosen a new candidate for Tamaulipas, but by late Tuesday, had not announced the name.

Torre is the first candidate running in national polls to be assassinated in Mexico in recent memory. He is also the highest-ranking candidate killed since Luis Donaldo Colosio, also from the PRI, was shot dead while running for president in 1994.

The candidate's death is the biggest setback yet for the elections, and corruption
scandals, threats and attacks on politicians have raised fears that Mexico's powerful drug cartels are buying off candidates they support and intimidating those they oppose.

Tamaulipas, which borders Texas, is the birthplace of the Gulf cartel, and became a battleground between the cartel and the Zetas, its former ally.

According to Mexican and US officials, the Gulf cartel has enlisted the help of the Sinaloa and La Familia drug gangs to fight the Zetas, a brutal gang of assassins that has grown into a powerful cartel in its own right, extending its reach all the way into Central America.

The Zetas are also suspected in attacks on security forces in Tamaulipas, where armed men have ambushed military patrols and set up blockades near army garrisons.