CIUDAD VICTORIA, Mexico (AP) — Mexican politicians vowed to press on with elections despite the assassination of the leading candidate for governor of a border state, urging frightened citizens to vote and show they are not intimidated by drug cartels threatening the country’s democracy.

Gunmen ambushed Rodolfo Torre’s campaign caravan Monday less than a week before he was expected to win the governor’s race in Tamaulipas, a state torn by a turf battle between two rival drug cartels. Four other people were killed: three of the candidate’s bodyguards and a state lawmaker.

President Felipe Calderon called the attack an attempt drug gangs to sway Sunday’s elections for governors and mayors in 12 states. He warned that cartels want “to interfere in the decisions of citizens and in electoral processes.”

“Organized crime will never meet its objectives. It will not succeed in shaking our faith in democracy or undermine our confidence in the future of Mexico,” Calderon said in a televised speech.

But the attack emptied streets in Ciudad Victoria, the Tamaulipas state capital where Torre was killed. Heavily armed federal and state police patrolled in caravans. Some parents rushed to pick up their children from schools.

“I am not going to vote because there is a lot of fear. The tension is very strong,” said Maria Pilar Villegas, a convenience store clerk who said she was on the phone with her sister when she saw the news of the assassination on television. “I got chills when I saw the TV.”

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.

Torre’s death was the biggest setback yet for the elections. 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.

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.

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 PAN and the PRD said they would suspend campaigning by their own gubernatorial candidates in Tamaulipas.

Elsewhere in Mexico, campaigning continued, with candidates urging voters not to fear.

“They are not going to intimidate us. We are going to continue until Sunday with the same intensity,” said Jose Francisco Olvera, the PRI candidate for the central state of Hidalgo.

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.

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.

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.

George Grayson, a Mexico expert at the College of William & Mary in Virginia, said Torre’s assassination would keep many voters home, but he expected the situation would only benefit the PRI.

“The … climate of fear will dampen voter turnout on Sunday, which will help the PRI because they have the best political machine,” he said.

Gregorio Linares, a waiter an at upscale restaurant where Torre and other politicians were frequent customers, said the city was growing accustomed to violence. Two months earlier, a shootout between soldiers and gunmen had erupted near the restaurant. But he said Torre’s death would only encourage him to vote for the PRI’s new candidate.

“It’s our duty,” he said.