SACRAMENTO — Two state senators said Wednesday they will introduce legislation to eliminate a controversial “personal belief exemption” that allows California parents to refuse to vaccinate their children.

“We shouldn’t wait for more children to sicken or die before we act,” Sen. Richard Pan, a Sacramento Democrat who is also a pediatrician, said at a Wednesday news conference. “Parents are letting us know our current laws are insufficient to protect their kids.”

Pan is sponsoring the legislation with Sen. Ben Allen, D-Redondo Beach.

In Washington, D.C., California’s two Democratic senators, Dianne Feinstein and Barbara Boxer, on Wednesday asked state health officials to go further and consider eliminating the “religious exemption.”

The clamor to repeal the state’s permissive “opt out” vaccination policy appears to be overwhelming the state’s anti-vaccine movement as the number of measles cases climbs.

Gov. Jerry Brown — who pushed for more parental choice just two years ago — quickly signaled that he was open to reform. “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,” said Evan Westrup, Brown’s spokesman.

“This is an important tipping point … when state governments consider badly needed reforms where exemptions have been far too easy to obtain,” said bioethicist Jason Schwartz, who studies vaccine policy at the Princeton University Center for Human Values.

The political push comes at a time when the state is experiencing its worst measles outbreak in two decades. There are now at least 99 Californians infected with the extremely contagious measles virus. Of these, 39 were exposed while visiting Disneyland in December and another 23 contracted the virus from members of their household or close contacts. Three infections were acquired in the community, such as simply sharing space in a hospital emergency room.

Vaccine exemptions have been available since 1961, when California first required all public school teachers and students to be inoculated against polio. But there has been a surge in their popularity in recent years. From 2000 to 2014, the rate of parents seeking exemptions tripled, from 0.77 percent to 2.5 percent — or one in every 40 kids. California is one of 19 states that allow exemptions based purely on parents’ personal beliefs.

“An exemption is something we can only allow under the condition where it very rarely is exercised,” said Stanford University’s David Magnus, a professor of pediatrics who directs the Center for Biomedical Ethics. “The fact that there has been so much misuse means it is time to tighten things.”

Politically, the well-to-do “all-natural” parents who have come to symbolize the anti-vaccination movement in California simply don’t have many political supporters in Sacramento, said Sherry Bebitch Jeffe, a veteran political analyst at the University of Southern California.

Two potential Republican presidential candidates who have questioned forcing parents to get their children vaccinated — such as New Jersey Gov. Chris Christie and Sen. Rand Paul, R-Kentucky — are pandering to the sorts of anti-government primary voters who could torpedo their chances of reaching a general election, Jeffe said.

But in California, so far no state lawmakers have publicly opposed the proposed legislation — and there’s little reason to believe opposition will succeed in a solidly blue state like California, she said.

Still, state Republicans are treading carefully for now.

Bob Huff, the state Senate’s Republican leader, said, “We look forward to reviewing the proposed legislation in detail and weighing what appropriate actions should be taken in the coming months.

“Many Californians are rightfully asking if this is because of California’s ‘exemption’ clause, which has prompted a larger debate of our state’s youth vaccination policies,” Huff added. “While medical experts are overwhelmingly in agreement that vaccines are safe, preserving the freedom of choice has also emerged as part of this important discussion.”

While both “personal belief” and “religious” exemptions are available, most parents use the “personal belief” option.

Vaccine experts cautioned that repeal of just that option could drive parents to choose the “religious” option instead.

Governments are reluctant to police religious objections because “they do not want to become guardians of people’s conscience” — and it’s unclear whether the Constitution allows them to, said Dorit Rubinstein Reiss, a law professor at UC Hastings College of the Law.

California’s vaccine policy was tightened in 2012 by then-Assemblyman Pan, who introduced a law that required parents seeking exemptions to receive counseling and signatures from health care professionals.

The move led to a 20 percent decrease in parents opting out of vaccinating their kindergartners. But despite this effort, many children remain unvaccinated.

One problem has been that vaccine conversations can be initiated by naturopaths, who practice alternative medicine and typically oppose vaccination.

Santa Cruz parent Amy Kelchner, who practices naturopathy and is authorized to write vaccine exemptions, supports the continued availabilty of exemptions because she believes vaccines should be given only to healthy children over age 3.

“Public health refuses to consider the possibility that the astronomical rates of allergies, autism, chronic disease — all legitimate public health concerns — might have something to do with the overuse of suppressive medication, including vaccines,” she wrote in an email.

But many Bay Area parents and children traveled to Sacramento to urge a change in policy.

“My husband and I worry our son will get sick if we put him in day care,” said Palo Alto mother Leah Russin, holding her 16-month-old son. “Many day cares in our area have vaccination rates well below what is necessary to protect the community.”

Mountain View’s Gina Lunde, also holding her toddler son, said, “The studies I’ve read show that it’s impossible to change the minds” of opponents.

“We need this legislation to keep our kids safe,” she said.