MEXICO CITY -- 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.
The assailants ambushed Rodolfo Torre's vehicle as he headed to the airport in 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 the 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.
Calderon's government did not say which gang was suspected in Torre's assassination or why he would be targeted.
Tamaulipas, which borders Texas, has become a battleground between the Gulf cartel and its former ally, the Zetas gang of hit men. Gangs have staged bold attacks on security forces, ambushing military patrols and setting up blockades near army garrisons.
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.
Leaders of the PAN and the leftist Democratic Revolution Party, or PRD, had said they could not find anyone to run for mayor in some towns in Tamaulipas because of drug gang intimidation. PAN and PRD leaders have insinuated that the PRI has ties to drug gangs in the state, noting that the party has had no trouble fielding candidates in towns where other politicians are too scared to run.
The PRI, which has long governed Tamaulipas, has dismissed such talk as tired campaign tactics.
Tamaulipas Gov. Eugenio Hernandez said he didn't know of any threats against Torre, a doctor who had served as the state's health secretary. Hernandez said Torre didn't express any fear when the two met Sunday to watch the World Cup game between Mexico and Argentina.
"We couldn't see this attack coming at all," Hernandez said in an interview with Milenio television. "We are in a situation of a lot of pain."
Torre, 46, was heading from Ciudad Victoria to the border city of Matamoros to accompany the PRI's mayoral candidates in the closing of their campaigns Monday.
Television footage from the scene showed several vehicles and sheet-covered bodies along the side of the highway.
Jorge Luis Navarro, president of the Tamaulipas state election institute, said the vote would go forward.
PRI national leader Beatriz Paredes urged supporters to go the polls. "Nothing is going to intimidate us," she said in a statement. There was no announcement on who the PRI candidate would be.
The PRI, which ruled Mexico for 71 years until losing the presidency in 2000, is hoping that a strong showing in Sunday's elections will put it on the path to regain the presidency in 2012. Polls had indicated that Torre would easily win the election in Tamaulipas.
George Grayson, a Mexico expert at the College of William & Mary in Virginia, said the assassination would almost certainly keep many voters home, but he expected the situation would only benefit the PRI.
"The execution and the ... climate of fear will dampen voter turnout on Sunday, which will help the PRI because they have the best political machine," he said.
The conservative PAN has formed uncomfortable alliances with the PRD to oust the PRI from several states, though not in Tamaulipas.
That alliance, however, was sorely tested by the worst corruption scandal of the election.
Cancun mayor Gregorio Sanchez, of the PRD, was arrested last month for allegedly protecting two brutal drug cartels, forcing him to drop his campaign for governor of Quintana Roo state. His leftist party has dismissed the allegations as a political ploy by Calderon's government.
Despite the controversy, the PRD and the PAN, are going ahead with joint candidates in several states. They are fielding separate candidates in Quintana Roo.
In the hours after Torre's assassination, the parties made a point of leaving politicking aside and presenting a unified front against crime.
The PAN and the PRD said they would suspend campaigning by their own gubernatorial candidates in Tamaulipas.
Calderon said "the fight for security and justice and against crime must be above parties and political differences."
Hernandez, the governor, said Mexican must support Calderon's battle against drug gangs and said he would ask the federal government to reinforce security in Tamaulipas.
Drug gang violence has rocketed since Calderon deployed thousands of troops and federal police across the country in 2006 to wage an all-out battle against cartels. Some 23,000 people have been killed in drug-related violence.