MEXICO CITY—As legal marijuana use spreads rapidly across the U.S., Mexican legislators are taking small steps to decriminalize pot in a country where the war on drugs has killed more than 100,000 people over the past decade.
The rising disparity in drug legislation is stoking a debate in Mexico over the effectiveness of its government’s protracted battle against powerful drug cartels when an ever-spreading sweep of the U.S. is giving up the fight.
Voters in California, Massachusetts, Nevada and Maine approved legal recreational marijuana on Nov. 8. Medicinal use of cannabis was also approved in Arkansas, Florida and North Dakota. In all, some 21% of the U.S. population can now have legal access to either recreational or medical marijuana.
Mexico’s Senate on Dec. 13 legalized the cultivation of marijuana for pharmaceutical and scientific purposes, but it didn’t address calls to allow recreational use.
“We are going backwards compared to California and other places,” said Armando Santacruz, a Mexico City businessman who was one of four plaintiffs to win a judgment late last year from the Mexican Supreme Court for permission to grow and consume marijuana for personal use.
The court’s ruling was limited to just Mr. Santacruz and his fellow plaintiffs. Since then, more than 350 people have submitted petitions to cultivate or consume marijuana for personal use to the federal agency that regulates controlled substances. The agency said Dec. 16 that it won’t approve any applications for recreational use.
One problem, experts say, is the resilience of Mexico’s organized crime groups. Even as marijuana cultivation booms in California and elsewhere across the U.S., Mexican criminal gangs continue to be the primary foreign suppliers of the drug to American consumers.
Though less potent than U.S. pot, the Mexican variety maintains its consumer appeal by selling at far lower prices. While gangs have long since diversified into more-lucrative South American cocaine and Mexican-produced heroin and amphetamines, the marijuana trade continues to provide a steady revenue stream, according to U.S. and Mexican law enforcement.
Rather than drive the gangs from the business, the legalization of marijuana cultivation and use could hand them the keys to a lucrative and legitimate market for the drug.
“They will have the growers and the transportation network,” said Mike Vigil, a former head of international enforcement for the Drug Enforcement Administration.
Mexican drug gangs are expected to maintain a dominant influence over the wholesale import and distribution of marijuana and other narcotics in the U.S. in the near term, the DEA said in its annual National Drug Threat Assessment in November.
But legal U.S. cultivation growing apace with legalized use could eventually shut out Mexico’s pot exports. As prices decrease, “what you are earning per kilo is not worth the risk,” said Alejandro Hope, a public-security expert who was previously a senior analyst at Cisen, Mexico’s civilian intelligence agency. “At the moment that it becomes unprofitable, there will be a collapse. We are not far away from that.”
Under such a scenario, Mexico could be vulnerable to collateral damage, said Roberto Gil Zuarth, a senator from the conservative National Action Party who led legislative discussions over marijuana regulations.
“Marijuana legalization creates an invisible wall for Mexico’s drug exporters,” Sen. Gil Zuarth said. “Facing rising stocks, they could dump the product in domestic markets or diversify into other criminal activities.”
A gradual legalization in Mexico would reflect policies elsewhere, he added, and would provide time for institutions to adapt and impacts to be assessed.
Marijuana cultivation, along with fields of opium poppies and laboratories to produce methamphetamine, sustains communities throughout swaths of Mexico. Prime producing areas include northwestern Mexico’s so-called Golden Triangle, rugged mountains home to the powerful Sinaloa Cartel, led by Joaquín “El Chapo” Guzmán until his latest arrest early this year.
Mexican troops and police seized more than 450 tons of pot in the past year and eradicated some 6,000 acres of the crop, according to the government. Further north, the U.S. Border Patrol confiscated more than 750 tons of Mexican cannabis across the border in the fiscal year 2015, according to official statistics.
With more Americans willing to condone marijuana legalization, leading public figures such as former Foreign Minister Jorge Castañeda say Mexican security forces should turn a blind eye to the drug. That might provide Mexico with leverage against a Trump administration aiming to renegotiate free trade and build a wall along the border.
The office of President Enrique Peña Nieto said that Mexico, “as a country with global responsibility,” is required to honor international treaties while discussions over drug policy continue on a hemispheric level.
While medical use of marijuana has gained support among a majority of Mexicans, opinion polls show that decriminalized recreational use remains widely unpopular. Broader legalization proposals are unlikely to advance in Congress any time soon, says Raúl Benitez Manaut, a national security expert at Mexico’s National Autonomous University.
“You might see that debate taking place in two or three years, but it’s likely to be on a state-by-state basis and in areas with a liberal electorate, like Mexico City,” he added.
Write to Dudley Althaus at Dudley.Althaus@wsj.com