SACRAMENTO, Calif. (AP) – Supporters of an effort to repeal California’s new law requiring mandatory vaccines for schoolchildren faced a Monday deadline to turn in enough signatures to qualify a ballot initiative asking voters to repeal the law.

The group had until the end of the day to submit the needed 366,000 signatures to county clerks to ask California voters to repeal SB277, which struck the state’s personal belief exemption for immunizations, a move that requires nearly all public schoolchildren to be vaccinated.

Gov. Jerry Brown signed SB277 into law earlier this year amid fierce opposition from some parents’ rights groups who argue the state should not force their children to be vaccinated, saying the science is clear that vaccines “dramatically protect children against a number of infectious and dangerous diseases.”

Election clerks in Los Angeles and Contra Costa counties reported receiving signature petitions Monday, but officials might not know until next week whether supporters have enough signatures to make it to the ballot. County clerks have eight business days to count and report how many signatures they received, which would likely be followed by a more detailed review.

The leading proponent of the effort, former Republican Assemblyman Tim Donnelly, said in an email Monday that volunteers were coerced and threatened while collecting signatures.

Donnelly did not return repeated messages inquiring about the effort’s chances but said in his email that he was proud of the volunteers who worked on the campaign “whatever the outcome is.”

“The SB277 referendum was sabotaged from without and within by powerful forces from its very inception, but we never gave up and we never gave in,” he wrote.

The bill’s author, Sen. Richard Pan, D-Sacramento, told reporters Monday that he’s confident voters support the vaccination requirement, whether the initiative makes the ballot or not.

“I’m sure the voters of California are not interested in letting a privileged few take away the rights of all Californians to be safe from preventable disease,” Pan said. “If they don’t have the signatures, I think it would be a direct reflection of the fact that Californians wanted to see their communities safe.”

California will join Mississippi and West Virginia as the only states with such strict requirements if the law takes effect as planned next year.

Copyright 2015 The Associated Press.