Sen. Jorge Larranaga had been a favorite to represent the National Party this year and strongly opposes the new law, which puts the ruling Broad Front government at the center of a regulated marijuana industry. The government's aim is to defeat organized crime by producing cheaper, better, legal weed and selling it in pharmacies to registered adults.
"We are going to overturn this law that legalized marijuana growing. Nobody plant anything! Don't plant anything because we're going to knock it down!" Larranaga had said ahead of Sunday's primary.
Other leading candidates to succeed President Jose Mujica in October are on the record favoring the legal cultivation and purchase of marijuana for personal use.
The right-wing National Party's surprise winner by a wide margin was congressman Luis Lacalle Pou, who supports home-grown marijuana and says he would keep much of the law as is.
But political analyst Daniel Chasquetti cautioned against seeing Sunday's results as a sign that sentiment is shifting in favor of Uruguay marijuana law, which Congress approved in December despite overwhelming opposition in opinion polls. He said Lacalle Pou's victory has more to do with the feeling in his party that he stands a better chance of defeating the Broad Front in October.
Lacalle Pou, 40, is the son of former President Luis Alberto Lacalle, who governed Uruguay from 1990 to 1995.
The National Party is backed by about a third of Uruguay's electorate, so Lacalle Pou would need to persuade many centrist voters to abandon the governing bloc. The more likely winner in October is former President Tabare Vazquez, who won the center-left Broad Front's primary Sunday and supports the new law.
The center-right Colorado Party chose Sen. Pedro Bordaberry, who is against legalizing marijuana, but apparently lacks the votes to win.
Lacalle Pou was an early supporter of legalizing marijuana, offering a proposed law to do so even before Mujica's government came on board. But the congressman would let private enterprise take over the industry, reducing the government role and keeping the pot business out of pharmacies.
"The idea of pot-growing clubs doesn't really bother me," Lacalle Pou told the Uruguayan newspaper El Pais in an interview. But "I don't believe that the state should produce marijuana or sell it or register the people who smoke it."