AUGUSTA — The Maine Senate voted Thursday to pass a bill that would end philosophical exemptions to childhood vaccinations but would continue to allow parents to cite religious objections to vaccines

The razor-close, 18-17 vote to preserve religious exemptions for vaccinations puts the bill at odds with a version passed last week by the Maine House, further complicating the path ahead for one of the most contentious and closely watched issues of this year’s legislative session.

The bill now heads back to the House, where lawmakers will debate whether to accept the religious exemption or force a potential show-down with the Senate on the change.

Under current Maine law, parents are able to opt-out of vaccinating children for a host of preventable disease — including measles, mumps, pertussis and polio — for medical reasons but also for philosophical or religious objections. That has resulted in Maine registering one of the nation’s lowest vaccination rates for children entering kindergarten and the country’s highest rate of pertussis or whooping cough.

The Senate initially voted 20-15 to give initial approval to a bill, L.D. 798, that would end both philosophical and religious exemptions, effectively mandating that almost all children be vaccinated in order to attend schools.

But four Democrats — Sen. David Miramant of Camden, Sen. Erin Herbig of Belfast, Sen. Louis Luchini of Ellsworth and Sen. James Dill of Old Town — then joined the chamber’s 14 Republicans to change the bill to preserve religious exemptions.

Miramant, who was the only Democrat to oppose the original bill, warned that ending both exemptions would mean an estimated 9,000 children could be prohibited from attending school because they lacked all mandated vaccinations.

“(Their parents) are not going to rush to get this done so their kids can go to school,” Miramant said. “I’ve had many contacts with people who would leave this state because there are plenty of places you can go that will honor your philosophical or religious exemptions.”

But supporters warned that allowing religious or philosophical exemptions puts at risk children who are immuno-compromised as well as infants susceptible to disease because they are too young for vaccinations.

“At the end of the day, this bill is about making sure our public schools are safe, healthy environments for our young people to learn and grow,” said Sen. Rebecca Millett, D-Cape Elizabeth.

Like many states around the country, Maine has seen an increase in the number of parents who are opting not to vaccinate against a host of infectious diseases because of concerns about side-effects from vaccinations or for religious reasons. But public health officials warn that the growing number of unvaccinated children is compromising the “herd immunity” that prevents spread of infectious diseases.

The bill faces additional votes — and likely debate — in both the House and Senate. Under both of the competing versions approved by the two chambers, parents concerned that a vaccine could harm the health of their child could still seek a medical exemption from their doctor or could seek authorization to change the vaccination schedule.

Three other states – California, Mississippi and West Virginia – have banned all non-medical exemptions that allow parents to forgo school-required vaccines for their children.