SAN FRANCISCO  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 this week, it became readily apparent that city officials are not laughing anymore about the measure, which would effectively decriminalize the world’s oldest profession in San Francisco. At a news conference on Wednesday, Mayor Gavin Newsom and other opponents seemed genuinely worried that Proposition K might pass.

“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.