On the eve of one of the most dramatic votes in recent Statehouse history, state Senate leaders struggled again to find enough support to pass a bill that would end religion as a reason for children to avoid vaccinations to attend day care and public school in New Jersey.

Democrats who control the 40-member Sendate do not have 21 votes needed to pass the bill, according to legislative sources who spoke on condition of anonymity because they did not have permission to discuss the status of negotiations.

“It’s still a little uncertain,” Senate Majority Leader Loretta Weinberg, D-Bergen, one of the bill’s sponsors, said late Sunday.

Weinberg declined further comment.

The vote is planned to take place Monday afternoon in Trenton.

Hundreds, if not thousands, of sign-waving, drum-beating angry parents are expected to rally outside the Statehouse to call for the bill’s defeat, just as they have three times since mid-December.

They’ve called the push to eliminate the religious exemption a violation of their First Amendment rights, and they’ve jammed lawmakers’ phone lines and email accounts demanding the law remain as is.

It was only Thursday that Senate Democrats had secured the pivotal 21st vote by agreeing to a compromise with Republican state Sen. Declan O’Scanlon of Monmouth County.

The Senate and state Assembly voted that day to amend the bill to allow private schools and day care centers to accept unvaccinated children at O’Scanlon request. He wanted to give parents adamantly opposed to vaccines an option beyond homeschooling their kids.

Thursday’s actions set the scene for a final vote Monday, a day before the two-year legislative session ends Tuesday at noon. Senate President Stephen Sweeney, D-Gloucester, said Thursday he expected the bill would pass.

But the last-minute amendment triggered doubt among other Democrats who only half-heartedly supported the bill (A3818) in the face of fierce opposition, the legislative sources said. Soon after news spread about the private school exemption, first reported by NJ Advance Media, the sponsors were accused of creating a system that favored wealthy families because they could afford to pay private school tuition.

On Saturday, Assemblyman Jamel Holley, D-Union, explained his opposition in a series of tweets that called the amendment “segregation” and “an abomination of what every righteous leader should be standing up against.”

To suggest that we begin to segregate our students is an abomination of what every righteous leader should be standing up against. — Jamel Holley ♍🔌 (@jamelholley) January 11, 2020

Sweeney said as recently as Thursday that if the vote fails, the Senate will try again in the two-year legislative session that begins Tuesday afternoon.

Forty-five states and the District of Columbia allow parents and guardians to use religion as a legitimate reason to avoid the measles, mumps and rubella vaccine and others which have all but eradicated many childhood diseases.

New Jersey’s law is seen as one of the easiest to game because it doesn’t require parents to cite any scripture or tenets, or verification from religious leaders.

Public health experts say people use the religion exemption to mask their distrust of pharmaceutical companies and government in general to spare their children the shots that the vast majority of students receive.

But in recent years, outbreaks of measles and whooping cough have erupted around the country, stoking fears that “herd immunity” — the protection given by vaccinating most to protect the few who are too young or sick — is weakening.

In New Jersey, the number of children in pre-kindergarten through high school who cited religion as an excuse to avoid vaccines grew from 1,641 in the 2005-06 academic year to just under 14,000 in the 2018-19 year. Of these, 8,348 children were enrolled in pre-kindergarten, according to state Department of Education data.

NJ Advance Media staff writer Matt Arco contributed to this report.

Susan K. Livio may be reached at slivio@njadvancemedia.com. Follow her on Twitter @SusanKLivio.