When Proposition K was added to Tuesday's ballot, many people likely snickered at the possibility that San Francisco might take its place alongside such prostitute-friendly havens as Amsterdam and a few rural counties in nearby Nevada.

But city officials are not laughing anymore about the measure, which would effectively decriminalize the world's oldest profession in the city.

"This is not cute," Mayor Gavin 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."

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.

Supporters say it is a long-overdue correction of a criminal approach, which neither rehabilitates nor helps prostitutes, and often ignores their complaints of abuse by clients. They accuse the city of profiting from prostitution through fines.

Opponents dismiss the notion of legions of prostitutes happily romping through neighborhoods. "This isn't 'Pretty Woman,'" one said.

A CBS poll released Thursday found that 35 percent of likely voters supported the measure, while 39 percent were opposed. But 26 percent were still undecided.