Voters in Denver, a city at the forefront of the widening national debate over legalizing marijuana, will decide on Tuesday whether to be the first in the nation to effectively decriminalize another recreational drug: hallucinogenic mushrooms.

The local ballot measure would not quite legalize “magic” mushrooms, the ones that contain psilocybin, a naturally occurring psychedelic compound. State and federal regulations would have to change to accomplish that.

But if it passes, the measure would make the possession, use or cultivation of the mushrooms by people aged 21 or older the lowest-priority crime for law enforcement in the city of Denver and Denver County. Arrests and prosecutions, already fairly rare, could all but disappear.

Adoption of the measure could signal fledgling public acceptance of a mind-altering drug, outlawed nationally for nearly 50 years, that recent research suggests could have beneficial medical uses. A similar effort failed to get on the ballot in California last year, but it could come up again in 2020; Oregon voters may also vote on a comparable measure next year.