MEXICO CITY — The leading candidate for governor of the northern Mexican state of Tamaulipas, Rodolfo Torre Cantu, was gunned down Monday in one of the highest profile assassinations since a presidential candidate was murdered in 1994.

Gunmen opened fire on Torre Cantu’s campaign van days before the July 4 election in 14 states and one month after a mayoral candidate was killed in the same state.

“Today has proven that organized crime is a permanent threat and that we should close ranks to confront it and prevent it from repeating acts such as the cowardly assassination that shocked the country today,” President Felipe Calderon said in a televised address.

Mr. Calderon added that the crime syndicates want "to interfere in the decisions of citizens and in electoral processes." He made the statements following a meeting with top Mexican security officials, adding that political parties should work together to bring the culprits to justice.

The murders are the first time in recent memory that Mexico's drug cartels have allegedly targeted electoral candidates. (Presidential candidate Luis Donaldo Colosio’s death in 1994 was considered more politically motivated.)

At least four others, including local Tamaulipas lawmaker Enrique Blackmore, were reportedly murdered along with Torre Cantu - a 46-year-old doctor who had stepped down as federal Congressman of the opposition Institutional Revolutionary Party (PRI) to run for governor.

The timing of the assassination sends a message to candidates of all parties that they may be called upon to collaborate with the traffickers, or perish, as currently happens with law enforcement officials, say analysts.

“The cartels don't seek a failed state. Rather they want ‘dual sovereignty’ - that is, to pay off public officials in return for their closing their eyes to criminality,” says George W. Grayson, a Mexico counternarcotics expert and professor of government at the College of William & Mary in Williamsburg, Va.

Caught between two cartels
The murder comes as the Zeta and Gulf cartels battle for territory in Tamaulipas. While the PRI in Tamaulipas had enjoyed relative calm during the Gulf cartel reign, bloodshed has escalated with the rise of the Zetas, says Grayson, concluding that the Zetas likely killed Torre Cantu to prove they are the most “savage” crime organization in town.

The assassination will likely hurt voter turnout by generating fear in the July 4 race, during which 12 governorships and municipal posts in 14 states will be decided, says Aldo Muñoz, a political scientist at Mexico State’s Autonomous University.

The May 13 murder of Jose Mario Guajardo, a candidate for mayor of Valle Hermoso in Tamaulipas, near the border with Brownsville, Texas, reportedly scared off other would-be candidates in the state.

Candidates will be affected more severely by this latest killing - increasing security or refusing to run - as they recognize that “whoever wants to be governor or mayor of an influential territory may have to negotiate with organized crime or risk losing their lives,” Muñoz says.