When President Obama arrives in Colombia for a hemispheric summit this weekend, he will hear Latin American leaders say that the U.S.-orchestrated war on drugs, which criminalizes drug use and employs military tactics to fight gangs, is failing and that broad changes need to be considered.

Latin American leaders say they have not developed an alternative model to the approach favored by successive American administrations since Richard Nixon was in office. But the Colombian government says a range of options — including decriminalizing possession of drugs, legalizing marijuana use and regulating markets — will be debated at the Summit of the Americas in the coastal city of Cartagena.

Faced with violence that has left 50,000 people dead in Mexico and created war zones in Central America, regional leaders have for months been openly discussing what they view as the shortcomings of the U.S. approach. But the summit marks the first opportunity for many of them to directly share their grievances with Obama.

Those leaders who have most forcefully offered new proposals, or developed carefully argued critiques of American policy, are among Washington’s closest allies. They include Colombian President Juan Manuel Santos, a former defense minister who marshaled U.S. aid to weaken drug syndicates; Guatemalan President Otto Perez, a former military man who has long battled drug gangs; and Mexican President Felipe Calderon, whose nation has been engaged in an all-out war with cartels.

“There’s probably been no person who has fought the drug cartels and drug trafficking as I have,” Santos said in an interview last week with The Washington Post. “But at the same time, we must be very frank: After 40 years of pedaling and pedaling very hard, sometimes you look to your left, you look to your right and you are almost in the same position.

“And so you have to ask yourself: Are we doing the correct thing?”

Perez, whose small country has been engulfed by violence that his security forces can barely contain, has been the most forceful and surprising proponent of far-reaching policy changes. The military and police under his command have continued to battle traffickers, he said in an interview from Guatemala City. But he said they have little to show for their effort.

“The strategy that we have followed these 30 or 40 years has practically failed, and we have to recognize it,” he said.

In Washington, the White House Office of National Drug Control Policy, which oversees anti-drug policies for the Obama administration, declined to comment on the debate. But during a two-day visit to Central America and Mexico last month, Vice President Biden laid out the U.S. position, saying, “There are more problems with legalization than non-legalization.”

“It’s worth discussing,” he told reporters, “but there’s no possibility the Obama-Biden administration will change its policy on legalization.”

U.S. statistics signal some progress, such as a 40 percent drop in cocaine use in the United States since 2006 and a 68 percent plunge over the same period in the number of people testing positive for cocaine in the workplace.

And in Colombia, where the United States has been heavily involved in upgrading the military and in funding aerial fumigation of drug crops, the amount of land dedicated to growing the plant used to make cocaine dropped by nearly two-thirds from 2000 to 2010. Estimated potential production of cocaine, meanwhile, tumbled from 700 metric tons in 2001 to 270 metric tons in 2010, although it picked up in Bolivia and Peru, according to U.S. statistics.

Latin American leaders, however, point out that the United States remains the world’s largest cocaine market and that there have been record levels of violence from Venezuela to El Salvador, Guatemala and Mexico.

Cesar Gaviria, a former Colombian president who has been a forceful critic of the U.S. policy, said American officials acknowledge the failure of the policy behind closed doors and do little to defend it publicly. He said it is simply a policy on automatic pilot.

“You reach the conclusion that all this killing in Mexico and Central America has been in the name of a failed policy that the United States does not believe in or vigorously defend,” said Gaviria, speaking in his Bogota office.

Much of the momentum for a shift began after Gaviria, former Mexican president Ernesto Zedillo and former Brazilian president Fernando Henrique Cardoso issued a report in 2009 calling for drug policy reform. They have been joined by a range of intellectuals, among them Mexican novelist Carlos Fuentes, and retired officials, including former U.S. secretary of state George Shultz.

What they and many current presidents in Latin America propose is not a wide-open policy of legalization but a softening of the laws.

Decriminalizing drug possession would free billions of dollars spent in the criminal justice system, advocates say, while vastly improving drug treatment. Heavy drug users, who drive the illicit trade, could be weaned off drugs through maintenance models that provide drugs legally but under close supervision.

Legalizing marijuana, which advocates argue would present only a modest risk to public health, would weaken cartels and free up funding for other uses, advocates say.

“They’re not saying, ‘Legalize everything today,’ like alcohol and tobacco,” said Ethan Nadelmann, who has advised Latin American leaders and is the director of the New York-based Drug Policy Alliance, an advocacy organization that has criticized U.S. tactics. “What they are saying is we need to give the same consideration to alternative, regulatory and non-prohibitionist drug control policies in the future as we’ve given to the failed drug war strategies of the last 40 years.”

Leaders discussing drug policy at the summit said they do not expect a change soon. Rather, the idea is to plant the seeds of changes in the years ahead.

“We understand perfectly that this is an election year in the United States,” said Perez, Guatemala’s leader, noting that no major policy shift could occur without a regionwide consensus. “There is not a decision that has to be made in this moment, or in six months. This is a process of discussion.”

Santos, who said he wants talks to take place “without a specific proposal” in mind, said that if there are changes in the future, they should be based strictly on serious studies.

“There are good arguments for legalizing, but I would prefer to reach that conclusion after an objective discussion,” he said. “The U.S. says, ‘We don’t support legalization, because the cost of legalization is higher than no legalization.’ But I want to see a discussion where both approaches are analyzed by experts to say, really, the cost is lower or not.”