Gunmen assassinated the front-running candidate for governor of a Mexican border state Monday in what President Felipe Calderon called an attempt by drug gangs to sway local and state elections this weekend.

Investigators work at the scene where Rodolfo Torre Cantu, candidate of the Institutional Revolutionary Party (PRI) for governor of the Mexican state of Tamaulipas, was killed with four aides in an ambush near the airport in Ciudad Victoria, Mexico, June 28, 2010. [Xinhua]

The assailants ambushed Rodolfo Torre's vehicle as he headed to a campaign event near Ciudad Victoria, capital of Tamaulipas, a state torn by a turf battle between two rival drug cartels. At least four other people traveling with him were killed.

"Today has proven that organized crime is a permanent threat and that we should close ranks to confront it and avoid more actions like the cowardly assassination that today has shaken the country," Calderon said in a televised speech. "We cannot and should not permit crime to impose its will or its perverse rules."

He warned that organized crime "wants to interfere in the decisions of citizens and in electoral processes."

Torre, of Institutional Revolutionary Party, or PRI, is the first gubernatorial candidate assassinated in Mexico in recent memory. He is the highest-ranking candidate killed since Luis Donaldo Colosio, also for the PRI, was gunned down while running for president in 1994.

The attack was the biggest setback yet for Sunday's elections in 12 states. Corruption scandals, threats and attacks on politicians have raised fears for months that Mexico's powerful drug cartels are buying off candidates they support and intimidating those they oppose.

Last month, gunmen killed Jose Guajardo Varela, a candidate for mayor of the Tamaulipas town of Valle Hermoso. Guajardo, of Calderon's National Action Party, or PAN, had received warnings to drop his campaign.

Several parties, including the PAN, had said they could not find anyone to run for mayor in some towns in Tamaulipas and other border states because of drug gang intimidation.