Four months after a measles outbreak at Disneyland, state legislators seeking to tighten immunisation laws across the country are running the gauntlet of anti-vaccination activists who have bombarded them with emails and phone calls, heckled them at public meetings, harassed their staff, organized noisy marches and vilified them on social media.

Three states blindsided by the activists’ sheer energy – Oregon, Washington and North Carolina – have either pulled back or killed bills that would have ended a non-specific “personal belief” exemption for parents who don’t want to vaccinate their children.

Now the battleground is California, which bore the brunt of the measles outbreak at the beginning of the year and saw school closures, extraordinary quarantine measures and a vigorous public debate lamenting the fact that a disease declared eradicated 15 years ago is once again a public health threat.

A health committee meeting in Sacramento, the state capital, on Wednesday turned into a tense showdown between lawmakers seeking to argue that the science is unequivocally on the side of universal vaccination, and activists accusing them of being in the pocket of unscrupulous big pharmaceutical companies.

One activist, Terry Roark, told the state senate committee her child had died from a vaccine and feared others could be next if parents lost the right to decide what was in their best interests.

“Innocent people will die,” she said tearfully. “Innocent children will be killed.”

The meeting degenerated at points into yelling and screaming, and two activists were removed.

Lawmakers promoting the new law were tenacious in their own way, challenging the claim that the bill would force vaccinations even on children with legitimate medical reasons not to have them. A doctor sympathetic to the anti-vaccination movement was ultimately forced to concede the bill contained no such language.

“The danger I feel as a policymaker is that when assertions are made in public comment that aren’t fact-based, that’s irresponsible,” state senator Holly Mitchell said.

She and the co-sponsors of the bill, a doctor from northern California and the son of a polio survivor from southern California, have become hate figures to the movement and they and their staff have been chased and shouted at.

The southern California co-sponsor, Ben Allen, told the Guardian that while many of his detractors were respectful he’d also been bewildered by “Facebook memes of me as a Nazi doctor”. He added: “Some of them have definitely crossed a line.”

The activists were boosted by the participation of a Kennedy: the environmentalist and civil rights activist Robert F Kennedy Jr, son of the murdered attorney general and nephew of the murdered president, who has written a book denouncing the use of mercury traces in a vaccine ingredient, which repeated peer-reviewed studies have found to be safe and which has now largely been phased out.

Kennedy showed a documentary based on his book, spoke at a rally and likened vaccinations to the Holocaust.

Medical experts and legislators supporting the bill say vaccinating as many people as possible is vital to provide so-called herd immunity – a degree of protection strong enough to cover infants too young for vaccinations or those too sick to receive them.

The more alarmist, contrary story of an out-of-control medical establishment covering up the “truth” – that vaccinations are responsible for an alarming spike in children diagnosed with autism – is the view of a tiny minority, perhaps 5% of the population.

But the minority is a strikingly vocal one.

In North Carolina, state senator Terry Van Duynsa described the backlash to a bill she sponsored as “very swift and very furious”.

“It created an environment that made it difficult to just even talk about it,” she told the NPR radio affiliate in Charlotte.

The California committee approved the bill 6-2 and, for now, its champions are confident they won’t suffer the same fate as the other states.

“We have an enormous number of sponsors,” Senator Allen said. “We’re ground zero for the latest outbreak and we are a global crossroads … When there are global diseases out there, places like California are particularly susceptible, and people understand that.”

California has some advantages over the other states: a much bigger population where small pockets of activists tend to make less of an impact, a full-time legislature and a cutting-edge medical research establishment.

It also dealt with 134 of the 159 cases of measles ultimately diagnosed across 18 US states and Mexico.

Still, the bill will have to pass through at least three more committees before going to the full Senate floor. Thereafter, it would go through the state Assembly before reaching the governor’s desk for signature – providing the anti-vaccination activists with plenty more opportunities to make themselves heard.