It was a scene reminiscent of a middle school dance, with boys on one side of the room and girls on the other, waiting for a brave interloper to get the party started.

A news conference Thursday at the Hilton in San Francisco’s Financial District brought together two groups that disagree on most everything when it comes to marijuana — save their opposition to Proposition 64, which would legalize retail sale and adult use of recreational pot if passed by California voters Nov. 8.

Taking stage right were marijuana foes whose opposition is rooted in calls for morality and public safety. On the left: cannabis advocates who warn that Prop. 64 will allow corporate interests to crush mom-and-pop players in the current medical marijuana industry.

A demilitarized zone was marked by a “No on 64” sign, a nod to the sponsor of the event, Public and Mental Health Advocates Against 64. The two sides hope their coalition will once again defeat legalization, but that doesn’t mean they’re arm in arm.

Kevin Sabet, the founder of the antilegalization group Smart Approaches to Marijuana, broke the uneasy ice, saying he would have “thought you were smoking something” to suggest the two groups would share the same stage as recently as a year ago.

But together, Sabet and the members of the medical marijuana industry he often butts heads with outlined their case that Prop. 64 would allow big business to monopolize profits, while pervading California with reefer billboards and other marketing pitches.

“It would make Colorado look like Saudi Arabia,” Sabet said, referring to changes in Colorado since that state legalized recreational weed in 2012.

Conspicuously absent was Dennis Peron, the driving force behind California’s Proposition 215, which became the nation’s first medical marijuana law in 1996. He had planned to attend, but organizers said he got tied up volunteering at a soup kitchen, and the cannabis activist did not immediately respond to calls and emails seeking comment.

Supporters of Prop. 64, which would implement a 15 percent tax on all marijuana sales, said Thursday that opponents of the measure are getting desperate as they run out of time and polls show strong support for legalization. They said assertions that the law would be a death blow to small growers and dispensary operators were unfounded.

“The scare tactics ... are distortions of potential truths, and there’s a lot we don’t know about commercial cannabis regulation, and making these huge claims is oversimplifying what is a very unknown and nuanced situation,” said Lynne Lyman, state director for the Drug Policy Alliance, which supports Prop. 64.

The legalization push comes six years after a similar ballot measure, Proposition 19, was defeated in 2010. It, too, splintered the medical marijuana industry. Traditionally pot-friendly counties like San Francisco and Marin gave it nearly two-thirds support, but voters in all three counties in the weed-growing Emerald Triangle rejected it.

Jamie Kerr, the owner of a Shasta County medical marijuana dispensary and member of a local planning board, was among those at the San Francisco news conference. She said Prop. 64 removes local control by not having municipal governments license new marijuana businesses.

“Let’s face it, the marijuana business doesn’t have the best reputation,” said Kerr, who conceded that new recreational competition could harm her business. “There are a lot of problems with rushing through recreational before we have answers to a lot of the questions we have now.”

Opponents argued that Prop. 64 will allow one individual to pursue multiple licenses, such as a cultivation site, a production facility and a storefront, and will thus create “Walmarts of weed,” as Sabet put it.

Prop. 64 does include a five-year restriction limiting the size of cultivation sites — an attempt to appease current growers — and leaves a good deal of discretion to state regulators when it comes to issuing licenses. But people on both sides of the debate agree the business landscape would change quickly.

“There’s a lot of people out there who have been making a little money on the side with modest home grows, and that’s going to be a thing of the past,” said Dale Gieringer, the state coordinator of California NORML, which supports Prop. 64.

Michael Bodley is a San Francisco Chronicle staff writer. Email: mbodley@sfchronicle.com Twitter: @michael_bodley