It's not universal health care, and it's not gay marriage, but the bill that may get the biggest rise from Californians this year — mandatory sterilization of dogs and cats — took center stage Saturday at a public forum in Palo Alto.

State Sen. Joe Simitian, D-Palo Alto, who convened the meeting, said he wanted to hear from people on both sides of the debate before the bill reaches a committee on which he serves, possibly next month. He realized how impassioned his constituents were, he said, when they kept approaching him with their thoughts during his "sidewalk office hours," held at local farmers' markets.

"This is not an issue that I've had a lot of experience with," Simitian said to the crowd of about 300 people before listening to speakers for more than four hours in the Palo Alto City Council chambers.

Assembly Bill 1634, by Assemblyman Lloyd Levine, D-Van Nuys, would prohibit possessing dogs or cats older than 4 months that have not been spayed or neutered. The bill offers exceptions for licensed breeders, show animals, working dogs and animals for whom a veterinarian has determined sterilization would be harmful.

Scores of animal adoption organizations have lined up in support of the bill, hoping to reduce the number of cats and dogs euthanized in shelters every year, while a like number of dog owners and breedergroups are fighting the measure.

"We kill a half-million animals every year and spend a quarter-billion dollars doing it," said Sarah Eryavec, adoption supervisor at the Santa Cruz County SPCA shelter, referring to statewide estimates.

Santa Cruz County has enforced a mandatory spay/neuter law since 1995, and shelter workers and animal control officers say they have euthanized fewer animals as a result.

But opponents say euthanasia rates have already been decreasing statewide. They say improved education on the benefits of having pets spayed or neutered — combined with low- or no-cost clinics to perform the procedure for low-income people — will continue that trend.

"AB1634 will do for dogs what Prohibition did for alcohol," said Andrea Sanfilippo, a dog groomer in Campbell, arguing that elimination of "hobby breeders" — those who occasionally breed their dogs at home but not as a business — will lead to more purchases from abusive "puppy mills."

Several speakers at Saturday's forum argued that the only people who would comply with the law were responsible animal owners who wouldn't let their pets produce unwanted litters to begin with. Irresponsible owners won't comply and will continue to be a problem.

But Todd Stosuy, Santa Cruz County's animal control manager, said the ordinance he enforces "does work." He says it gives him the means to shut down bad operators while leaving responsible breeders alone.

The Assembly narrowly approved the bill earlier this month, 41-38. The first hearing on the bill in a Senate committee has not yet been scheduled.