SACRAMENTO, Calif. — Some bills are just more trouble than they’re worth for nervous legislators who must worry about voter support. Like a California measure to tighten the requirement that children be vaccinated against contagious diseases like measles before being admitted to school for the first time.

Kids can be excused from that edict for medical reasons, such as having a weak immune system caused by leukemia. But a few unscrupulous doctors are peddling medical exemptions for phony reasons to parents who obsessively fear vaccinations. Many of these parents get very hostile when lobbying lawmakers and attacking vaccine advocates.

The anti-vaccine people tend to be single-issue voters who care about keeping their kids from being inoculated and little else.

They’re like single-issue voters tied to other controversial topics. Typically, one side — often in the minority — is tenacious and aggressive. Gun advocates are a prime example. The majority side is ho-hum on the issue. They’ve got other things they’re far more concerned about. So it’s the minority that causes the legislators grief.

That’s certainly true with vaccines. Legislators who vote for a bill to increase vaccinations against contagious diseases risk harsh attacks from opponents. But they’ll get little thanks from vaccination supporters.

State Sen. Richard Pan (D-Sacramento) is carrying the bill to stiffen California’s immunization law. As a practicing pediatrician, Pan is a legislative rarity: He’s actually a certified expert on the subject he’s legislating about.

The bill, SB 276, would require the state health department to review all applications for medical exemptions. The state would approve or reject them. It would create a standard application form. And exemptions would go into a state database.

After a 2014 measles outbreak that began at Disneyland, Pan wrote legislation to eliminate all non-medical exemptions. The main exemption he scrubbed was based on religious beliefs.

The bill was passed and signed by then-Gov. Jerry Brown in 2015. It was one of the strongest vaccination laws in the country. And it is credited with being the main reason why the current measles outbreak is much less severe in California than in New York, which has a religious exemption.

Pan vividly remembers the political obstacles he faced in winning passage. Practically all the “yes” votes were cast by Democrats, virtually all the “noes” by Republicans.

“I’d talk to legislators and tell them the problem with not having more children vaccinated,” Pan says, “and they’d tell me, ‘This is my problem ... The people who oppose vaccines, this is their most important issue. They’re very passionate about it. It’s the issue they will remember when they go to the ballot box. The vast majority who support this, it’s not their most important issue. They care about something else. I know I’ll get a bunch of people upset voting for the bill. There’s definitely a downside and not much upside.’”

“My counter,” Pan says, “was when we have outbreaks, all the people worrying about their children getting measles will remember who had the opportunity to protect their families, but decided to put them at risk for political gain. You’re telling your constituents, ‘I’m OK with disease spreading in a community.’”

After Pan’s 2015 bill became law, he says, anti-vaccine parents began doctor-shopping for physicians who would sign medical exemptions. He has heard that exemptions sold for from $200 to $600.

Many parents are afraid that vaccinations can cause other ailments. They mistakenly believe, for example, that a measles shot can trigger autism — a discredited theory promoted in 1998 by a lying researcher. His study was later retracted by the journal that published it.

Fortunately, some smart people invented vaccines to all but eliminate viral scourges in this country.

Now some not-so-smart people are preventing their kids from being vaccinated and endangering whole communities.

“They’re not only passionate, they’re aggressive,” Pan says. “They’re almost like cults. They harass and try to intimidate. They’ve threatened my life and threatened to harm my family. They engage in a lot of hate speech. Compared me to Hitler, Pol Pot and Mao Tse-tung. ‘Stop the vaccine guy. Send him back to his own country.’”

Pan’s parents migrated from Taiwan.

His bill has passed one committee: Senate Health, which Pan chairs. There’s a May 17 deadline for it clear the Senate Appropriations Committee.

It probably will pass both houses and be signed by Gov. Gavin Newsom.

But there’ll be a lot of shameful nastiness for a bill aimed at protecting children from terrible diseases.

George Skelton writes for the Los Angeles Times.