The mayor of La Piedad was handing out campaign fliers outside a fast-food restaurant when a black SUV pulled up, a hand holding a pistol appeared at its window, and he went down with a shot.

Ricardo Guzman, 45, died late Wednesday in an ambulance racing to the hospital, one of more than two dozen Mexican mayors who've been assassinated since 2006, the majority presumed victims of drug violence.

But Guzman's killing raised new questions about organized crime's impact on Mexico's democracy, specifically the Nov. 13 elections in the western state of Michoacan, where Guzman had been handing out campaign material for gubernatorial candidate Luisa Maria Calderon, President Felipe Calderon's sister.

Before Guzman's assassination, polling firm workers were kidnapped in August while trying to conduct surveys on the election, though they were later released unharmed. The three major political parties all say they have local candidates who have received some kind of pressure or threats in the Calderon family home state, where the president launched his drug war five years ago.

Michoacan "appears to be the state that is most infected with narco-politics," said political analyst and columnist Raymundo Riva Palacio. He noted that while mayoral candidates, and even one gubernatorial candidate, have been killed in other states, nowhere is the cartel pressure on candidates as systematic as in Michoacan.

The Calderons, like Guzman, are all in the conservative National Action Party, or PAN. When asked by local media Thursday if she thought a drug gang may have been involved in Guzman's death, Luisa Maria Calderon said, "Probably."

"I don't think he had any political enemies," she said.

"He stayed to defend his city from the incursion of organized groups, and his police force had suffered casualties," Calderon said. "He told me 'I'm going to stay in my city, to protect it.'"

Gunmen killed La Piedad police chief Jose Luis Guerrero in March, just a couple of months after he took the job. Shell casings from AK-47 assault rifles, the cartels' favored weapon, were found littered at the scene.

His successor, Miguel Angel Rosas Perez, was recruited from the better-trained federal police, but he, too, came under attack in July, when more than 40 armed men pulled up to his police station in a 10-vehicle convoy, sprayed his station with gunfire, and then lobbed hand grenades at it.

Though Rosas survived, at least six municipal police chiefs have been killed in Michoacan in 2011. Twenty-five mayors have been killed throughout Mexico since December 2006, when the drug war began.

The state Attorney General's Office said Guzman was hit in the torso and his right arm and was still alive as he was loaded into the ambulance.

It said investigators interviewed 11 witnesses who gave conflicting reports on the number of suspects who drove by in the black Jeep Liberty from which a shotgun emerged. Prosecutors had earlier said that between three and four men were in the SUV.

Michoacan prosecutor Jesus Montejano said the vehicle had plates from the neighboring state of Jalisco, which borders La Piedad. Montejano said they have surveillance videos of the area.

Drug cartels have "been very active" in the area around the city of about 100,000 on a key transit route where the territory of three gangs intersect: the Zetas, The Knights Templar and the Jalisco New Generation.

La Piedad has been hit deeply by the violence, said municipal policeman Jose Castro. The local force beefed up security Thursday for Guzman's funeral, which was attended by top PAN political figures.

Castro called the mayor "someone who really looked out for people, who was really dedicated to his work." He also said groups of drug cartel gunmen roam the outskirts of the township.

The Knights Templar, and their predecessor, La Familia, appear to be deeply involved in Michoacan politics, boosting their favored candidates by pressuring opponents to drop out of mayoral races, running for legislative seats themselves or through proxies and sponsoring public marches and protests, according to party leaders and state security officials.

The Knights Templar is a pseudo-religious gang specializing in methamphetamine production and smuggling, extortion and other crimes.

Victor Lopez Landeros, the spokesman for the Michoacan State Electoral Institute, says problems are limited to a few of the state's 113 townships, and expressed confidence the elections can be held normally. But leaders of the state's three main political parties _ the PAN, the Democratic Revolution and the Institutional Revolutionary Party, or PRI _ all say they have mayoral candidates who have dropped out of races. Though the candidates cite health and other reasons, party leaders suspect at least some resignations were the result of cartel pressure designed to favor one candidate.

"Organized crime is getting involved in discouraging (mayoral) candidates," said PRI gubernatorial candidate Fausto Vallejo. "And that is not only happening to the PRI, but in all of the three political parties."

Gunmen believed to be working for cartels also kidnapped workers carrying out opinion polls on the Michoacan elections in August. While all nine workers were later released, the kidnappings increased concerns about the overt or covert disruption of next Sunday's vote.

Labor Secretary Javier Lozano had few doubts about what the effects of Guzman's killing would be. In his Twitter account, Lozano wrote, "this cowardly crime seeks to discourage citizens from voting in the Nov. 13 elections."

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Associated Press writer Mark Stevenson reported this story from Mexico City and Gustavo Ruiz reported in Morelia.