Marijuana grower Marcelo Vazquez checks the leaves of his plants for fungus on the outskirts of Montevideo, Uruguay, in December 2013. (Matilde Campodonico/AP)

Uruguay, budding world capital of experimental marijuana legislation, is proving that legalized pot may indeed be a gateway drug to something more powerful: voter backlash.

Last year, the South American nation (population 3.5 million) blazed a bold regulatory path when lawmakers approved the production and sale of cannabis through an elaborate state-run system to supervise users' monthly intake. No country had ever gone so far.

But a Nov. 30 presidential runoff will pit ruling party candidate and former president Tabare Vasquez, who backs the marijuana laws, against center-right challenger Luis Lacalle, who wants to roll them back.

Polls show the two are in a tight race, and as many as two-thirds of Uruguayans side with Lacalle in wanting to repeal much of the legalization plan.

"I will keep the law's articles that allow users to grow their own cannabis at home and authorize smoking clubs, and repeal the rest, in particular the state's commercialization of the drug," Lacalle, 41, told Reuters last week.

Nearly a year after legalization was approved, with the backing of outgoing president Jose Mujica, a state-managed marijuana industry still hasn't been set up in the country. Lawmakers have been at odds over how the system would work.

Clouds of marijuana smoke already waft freely along Uruguay's streets as enforcement has gone lax. But popular support for the ambitious experiment was never especially strong, and has eroded further.

Lacalle has acknowledged that, yes, he too enjoyed some high times in his youth, but he argues that the government shouldn't be gardening Uruguay's dope. His opponent, the 74-year-old Vasquez, a physician, has endorsed the state-run model designed by his leftist party, the Broad Front, which maintains a narrow majority in Congress.

It's only one of several differences between the candidates: Lacalle also opposes Uruguay's legalization of abortion, and criticizes labor codes that limit agricultural labor to an eight-hour workday.

Lacalle also opposes giving asylum to a group of six U.S. detainees from the Guantanamo Bay prison camp, as Mujica pledged to do.

In the first round of voting last Sunday, Vasquez won 47 percent, which left him short of the outright majority needed to avoid the runoff. Lacalle received 31 percent, while conservative candidate Pedro Bordaberry finished third with 13 percent. Many of Bordaberry's supporters are expected to back Lacalle in the next round.