“This was an act not only against a candidate of a political party but against democratic institutions, and it requires a united and firm response from all those who work for democracy,” a stern-faced President Felipe Calderón, who has found his presidency repeatedly bogged down by drug violence, said in a nationally televised address.

Gunmen with automatic weapons opened fire on the motorcade of the candidate, Rodolfo Torre Cantú, 46, as he headed to an event in the closing days of the campaign, the authorities said. The murders came during a rise in election-related violence in recent months, including the shooting deaths of a mayoral candidate and of an activist during a get-out-the-vote effort. Explosives have also been thrown at two separate campaign offices this month.

The Torre killing is arguably the highest-profile case of political violence since 1994, when a presidential candidate, Luis Donaldo Colosio, was assassinated. While it has helped unite political parties that had been in fierce competition, Mr. Torre’s death adds to a particularly dire month for Mr. Calderón, who unleashed a war on drug traffickers upon taking office in 2006 and has seen violence spike as a result.

The bloodiest 24-hour period of Mr. Calderón’s presidency occurred this month, when 85 people were killed, 10 of them federal police officers ambushed by traffickers. Also this month, the authorities found dozens of bodies, suspected trafficker victims, buried in a closed silver mine near Taxco, a popular tourist spot.

Statistics compiled by Mexican newspapers show that 2010 is on its way to becoming the deadliest year since Mr. Calderón took office. There have been more than 5,000 drug-related killings so far this year, exceeding the totals in 2007 and 2008 and nearing the worst year, 2009, when about 6,500 people died in drug-related violence.

Immediate suspicion in the Torre case also fell on drug traffickers. Mr. Torre’s state, Tamaulipas, which borders Texas on the gulf coast, has been the site of fierce fighting in recent months between rival drug organizations, the Zetas and their former allies, the Gulf Cartel.

Mr. Calderón has called on the country to stick with him through the violence, repeatedly describing it as a sign that traffickers were in their death throes and were striking back because of the sustained attack by the police and army.

Despite strong backing of Mr. Calderón by the Obama administration, many Mexicans have increasing doubts and his National Action Party has suffered at the polls in previous contests, in part because of weariness with the drug war.

“The argument whereby the government says that the more violence there is, the greater is their success, is absurd,” said Jorge Castañeda, a former foreign minister and a leading critic of Mr. Calderón’s approach.

Mr. Torre, 46, was a burly, gray-haired surgeon and federal congressman representing the Institutional Revolutionary Party, or P.R.I., which currently controls Tamaulipas. He was married and had three children.

Two campaign vehicles that had been carrying the candidate, aides and bodyguards to the airport outside the state capital, Ciudad Victoria, were found about 10:30 a.m. The windows had been shattered by bullets, the doors were open and bodies were sprawled in the roadway and on the shoulder in images broadcast on television. Mr. Torre, whose face was painted on the side of the bullet-ridden vehicles, was among the dead, the authorities said. Also killed were his campaign manager, Enrique Blackmore, a state legislator.

Although law enforcement officials were tight-lipped Monday on their investigation into the killings, politicians said they saw them as a brash message from organized crime.

“What organized crime wants is that we stay in our houses, that we are afraid, and they are saying, ‘We want to control the country,’” said Xóchitl Gálvez, a candidate for governor in Hidalgo State. She urged voters to turn out at the polls to show that the people, not criminals, still rule.

Ms. Gálvez told reporters that her campaign had received threats from people who identified themselves as members of the Zetas. As a result, Ms. Gálvez said on Monday that she was moving her children and other relatives into hiding to keep them save.

Even before the killing of Mr. Torre, those involved in a variety of the gubernatorial, mayoral and local legislative races to be held Sunday in 14 states said they had been forced to limit travel during evening hours and avoid some areas altogether.

In Tamaulipas, rival candidates suspended their campaigns on Monday to honor Mr. Torre. The Associated Press reported Monday that elections in the state will proceed on July 4, as scheduled.

The state has been one of the country’s most troubled regions recently. On June 3, in Ciudad Madero, a mayoral candidate and a candidate for the local congress were pinned down during an armed battle between rival drug gangs. A month before that, a mayoral candidate in Valle Hermoso was killed.

Some areas of the state are so dangerous that the president’s party announced last month that it would not campaign in three small border towns because of the risk to its candidates. An opposition party candidate announced in April that he was pulling out of his race, one day after his house was burned down.

On Mr. Torre’s campaign page on Facebook, supporters expressed shock and indignation. “I feel such anger and impotence,” wrote one backer. “How long are these things going to happen?”

Over the weekend, another killing left Mexicans, weary of the drug wars, shaking their heads. A popular singer who often sang of the drug trade, Sergio Vega, known as El Shaka, was killed Saturday night in Sinaloa State. Hours before his killing he had denied false reports that he had been killed.

"It’s happened to me for years now, someone tells a radio station or a newspaper I’ve been killed, or suffered an accident," Mr. Vega told an entertainment Web site. "And then I have to call my dear mother, who has heart trouble, to reassure her.”

The latest report of his death, however, was accurate.