A Santa Clara County judge ruled in May that Christopher Evans Hubbart could be conditionally released after his most recent stint in a state mental hospital.
Los Angeles County Supervisor Mike Antonovich said it's too much of a risk to release Hubbart into the general population.
"A predator of this nature deserves life without parole," he said in a statement Monday.
Hubbart was first arrested in 1974 in Los Angeles for rape charges and was imprisoned until 1979, the Los Angeles Times said. He has since admitted to 25 sexual assaults on women at that time.
After he was released, he moved to the San Francisco Bay Area and was arrested again, having raped an additional 15 women within two years. He was put in jail a second time and released in 1990, the newspaper said.
Shortly after his second release, he attempted to abduct a woman in Santa Clara County. He was convicted of false imprisonment, was sent back to prison and underwent treatment at a state mental hospital.
Santa Clara County Deputy District Attorney Vonda Tracey said Hubbart was found appropriate for conditional release and that he would be under strict supervision, including a regular escort at first and electronic monitoring. Random searches, a curfew and ongoing treatment are also planned, she said.
"With the protections in place, they can keep him on a short leash," Tracey said.
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