* Embarrassing incidents on eve of election highlight chaos
* Candidates have shied away from drug war issue
* More than 55,000 killed in drug war under Calderon
By Ioan Grillo
MEXICO CITY, June 27 (Reuters) - Gunmen kill three policemen near a fast-food court at Mexico City's international airport as panicked travelers hit the deck. Hours later the government says the killers are fellow officers on a drug cartel payroll, and that they escaped.
Mexican marines announce they have captured a son of the country's most wanted trafficker, Joaquin "Shorty" Guzman and the U.S. government applauds the arrest. The next day, the attorney general has to admit they have the wrong man.
Meanwhile, drug cartel gunmen commit brutal massacres across the country, dumping headless, mutilated corpses on city streets and major highways, and rarely getting caught.
The relentless carnage and series of humiliating episodes in the final days before Mexico's presidential election on Sunday have further tainted outgoing President Felipe Calderon's offensive against the drug cartels.
More than 55,000 people have been killed in drug-related violence since Calderon took office and sent the army to hunt down drug lords in a crackdown that has defined his presidency.
In view of the immense problems Calderon has faced since December 2006, his successor almost certainly will focus on damage limitation rather than stake his or her reputation on what many consider an unwinnable campaign against the gangs.
While the drug war is a central challenge to whoever takes over, the candidates have shied away from it in their campaigns. None of them touched on the issue of drug seizures and the shooting of cartel kingpins during two long televised debates.
"Whoever wins the election, the new president will try to avoid being in the same position as Calderon," said Jose Reveles, an author on Mexico's drug cartels.
"Calderon has shown that it is not good politics in Mexico right now to make a drug war your central policy."
Calderon is unable to seek re-election but polls show that Josefina Vazquez Mota, the ruling National Action Party's (PAN) candidate, is trailing in third place with dissatisfaction over the drug war cited as a key reason for her poor showing.
Leading the race by double digits is Enrique Pena Nieto of the opposition Institutional Revolutionary Party, or PRI, which ruled Mexico for 71 years until it was voted out in 2000.
Throughout those decades of PRI rule, Mexico suffered from endemic corruption and bouts of political turmoil but nothing like the current levels of drug violence.
Pena Nieto says his main priority on security issues if he wins on Sunday will be to reduce the rates of homicides, kidnapping and extortion. Seizing narcotics and taking down drug bosses will still be a goal, but not the dominant one.
"I reaffirm the Mexican state's obligation to combat drug trafficking," he told Reuters during the campaign. "But now we have another matter which for me takes higher priority, that of the violence. I would focus efforts on reducing the violence."
Pena Nieto is focusing his campaign on economic growth and leftist candidate Andres Manuel Lopez Obrador, running in second place in most polls, is promising above all to fight poverty.
Vazquez Mota also talks about the economy and education much more than drug policies and has tried to distance herself from Calderon's government, running under the slogan "Different."
Calderon launched his offensive against the cartels within 10 days of taking office in late 2006, sending 6,500 police and soldiers to his home state of Michoacan, where thugs had recently rolled five severed heads onto a nightclub dance floor.
Within three months, the crackdown expanded to include 50,000 troops across the country and Calderon broke tradition to don an army uniform and cheer them on in a Michoacan base.
He argues it has been a success as s ecurity forces have shot dead or arrested 22 from a list of 37 most wanted kingpins. They have also made record seizures of cocaine, crystal meth and drug money. But the tactic of "decapitating" cartels - or taking out their leaders - triggered turf wars and inflamed the violence.
Many of the cartels have been able to bounce back with new leaders and they appear to be trafficking just as many tons of narcotics to American users. For example, the U.S. border patrol seized 2.5 million pounds (1.2 million kg) of marijuana coming from Mexico in 2011, compared to 1.8 million pounds (816,400 kg) in 2007.
The violence has scared away some tourists and investors, shaving about a percentage point off annual economic growth, according to Finance Ministry estimates.
The war also has highlighted corruption inside the security forces. While the shootout at the Mexico City airport on Monday was a shocking example, Mexicans already knew that many police officers do dirty work for the cartels, from killing other police to torturing and butchering rival gang members.
Minerva Bautista, who served as secretary for public security f or Michoacan, survived an attack in which assassins sprayed 2,700 bullets at her armored car in 2010, injuring a driver and bodyguard and scattering shrapnel over the seats.
One of her police commanders was arrested and confessed to working with cartel members on the hit.
"Calderon had a very visible strategy that in the beginning got easy applause," said Bautista, a member of the leftist Party of the Democratic Revolution. "But after six years and with the number of dead we have realized that it is not the right path, it hasn't given the results and hasn't tackled the causes."
Even when corruption isn't the problem, security forces are often embarrassingly inept. Last week, m arines were apparently so eager for good publicity that they paraded Guzman's alleged son in front of reporters before confirming his identity. They now say he was a lower ranking cartel member.
Pena Nieto is calling for a major overhaul of the police and has hired as an adviser the former head of Colombia's National Police, Oscar Naranjo. Under him, the Colombian force was widely praised for reducing murders and kidnappings, although Colombia is still believed to be the world's No. 1 cocaine producer.
A less openly confrontational approach could dampen the violence although Pena Nieto has promised to keep the army in certain states because of the threat of armed groups.
The PRI has come under fire recently after several senior members were accused of working with drug traffickers, including Tomas Yarrington, a former state governor who faces money laundering charges in the United States.
Amid the scandals, some rival politicians have claimed the PRI might make pacts with cartels rather than fight them.
Pena Nieto categorically denies the allegation.
"The law is enforced," he said last week. "It is never negotiated."