“This is not cute. This is not fanciful,” Mr. Newsom said, standing in front of the pink-on-pink facade of a closed massage parlor in the Tenderloin district. “This is a big mistake.”

Supporters of the measure say it is a long-overdue correction of a criminal approach toward prostitutes, which neither rehabilitates nor helps them, and often ignores their complaints of abuse.

The language in Proposition K is far-reaching. It would forbid the city police from using any resources to investigate or prosecute people who engage in prostitution. It would also bar financing for a “first offender” program for prostitutes and their clients or for mandatory “re-education programs.”

But opponents dismiss the notion of legions of prostitutes happily romping through the city’s neighborhoods. “This isn’t ‘Pretty Woman,’ ” was how one put it.