SACRAMENTO, Calif. — California's Assembly on Thursday approved a hotly contested bill requiring that nearly all public schoolchildren be vaccinated, clearing one of its last major legislative obstacles before the measure heads to the desk of Gov. Jerry Brown.

The bill aims to increase immunization rates after a measles outbreak linked to Disneyland in December sickened over 100 people in the U.S. and Mexico.

It would give California one of the nation's strictest vaccine laws by striking the state's personal belief exemption. Only children with serious health issues would be allowed to opt out of mandatory vaccine schedules. Unvaccinated children would need to be homeschooled.

If the bill becomes law, California would join Mississippi and West Virginia as the only states with such strict requirements.

"Do we wait until we have a full-fledged crisis to protect the most vulnerable?" Assemblywoman Lorena Gonzalez, D-San Diego, asked as she presented the bill.

The measure passed on a bipartisan 46-30 vote after weeks of vocal opposition, with thousands of parents placing calls to representatives and donning red shirts to protest at the Capitol.

But proponents have been equally resolute, standing by 7-year-old leukemia survivor Rhett Krawitt Wednesday as he delivered a petition with over 30,000 signatures to the Democratic governor. Krawitt's parents said that because he could not be immunized for a year after receiving chemotherapy, they were nervous to send him to school in the chronically under-vaccinated Marin County.

"We should fight for the liberty not just of those who don't want to vaccine their kids, but for those who cannot," said Republican Assemblywoman Catharine Baker, R-Pleasanton, during Thursday's floor debate.

Two Republican assemblywomen joined the Democratic majority in support. The Senate already approved the bill once, but still must approve amendments before it is sent to Brown. The Senate could take up amendments as early as next week.

Brown has not said whether he would sign the bill authored by Democratic Sens. Richard Pan of Sacramento and Ben Allen of Santa Monica.

"The Governor believes that vaccinations are profoundly important and a major public health benefit and any bill that reaches his desk will be closely considered," spokesman Evan Westrup said in an email that repeated Brown's earlier statement.

Opponents of the vaccine bill gathered on the Capitol steps after the vote, vowing to make their voices heard to the governor.

"Parents are being bullied into having their children vaccinated or not sending their kids to school," said Renate Krogdahl, a Cupertino mother of three who was among the protesters.

Sen. Bill Monning, D-Carmel, said Thursday that he and Pan have been targeted for recall by opponents. Monning said his district, which includes Santa Cruz, has a high number of parents who seek vaccine exemptions for their children and he has had to shut down his district office out of safety concerns.

In the floor debate, some Assembly Republicans argued that the measure trampled parental rights.

"We do not have the right, nor should we have the power, to take away a parent's right to choose," said Assemblyman Devon Mathis, R-Visalia.

There were also divisions among Democrats, with Assemblyman Mike Gatto, D-Glendale, taking the floor to warn of government overreach.

"If anyone can tell me where the line for medical necessity reasonably ends to justify a law, then I will vote for this bill," Gatto said. "Until then, this is a slippery slope."

Associated Press Writer Judy Lin contributed to this story.