California will require teachers and school staff to be vaccinated against the coronavirus or submit to weekly testing — the first state to impose such a rule — Gov. Gavin Newsom (D) said Wednesday.

Newsom cited the surging delta variant, which has challenged plans for the opening of school this fall across the country. The move echoes decisions by the federal government and many private employers, who are increasingly requiring their workers to prove they have been vaccinated.

“We think this is the right thing to do and we think this is a sustainable way to keeping our schools open,” Newsom told reporters. “It’s science based. It’s based upon argument, evidence. It’s based upon data. And it’s based upon the vexing challenge that we all face now.”

The vast majority of teachers have been vaccinated already, he said; national teachers unions put the figure at about 90 percent across the country.

Newsom said it was urgent to vaccinate school support staff, whose inoculation rates are lower. The new rules apply to paraprofessionals, bus drivers, janitors and all other school staff, as well as teachers. Newsom said the rules would be enforced the same way other school policies and requirements are handled.

The decision underscores how political divides continue to split the nation’s pandemic response. Many Democrats are embracing public health mandates, but some Republican officials are pushing back on them. GOP governors in several states have barred local districts from requiring masks in schools and in Florida, Gov. Ron DeSantis (R) is threatening pay of superintendents who impose them anyway.

On Wednesday, Florida Education Commissioner Richard Corcoran sent letters to school leaders in three counties — Broward, Leon and Alachua — announcing investigations into their mask mandates, saying they run counter to the governor’s executive order. Corcoran said he may recommend withholding state funds equal to the salaries of the superintendent and members of the school board.

“There is no room for error or leniency when it comes to ensuring compliance with policies that allow parents and guardians to make health and educational choices for their children,” Corcoran wrote.

In Texas, superintendents in Houston and Dallas have pushed back against Gov. Greg Abbott’s orders, with both saying they will require masks in schools.

California has already imposed a statewide mask mandate for schools. And Newsom’s vaccine mandate announcement on Wednesday came with backing from labor unions representing teachers and staff, helping to ease controversy about the step.

“Educators want to be in classrooms with their students, and the best way to make sure that happens is for everyone who is medically eligible to be vaccinated,” California Teachers Association President E. Toby Boyd said in a statement.

In recent days, national teachers unions also have signaled support for vaccine requirements. Randi Weingarten, president of the American Federation of Teachers, said in interviews that she was open to the requirement and then amplified those comments Sunday on NBC’s “Meet the Press.”

“As a matter of personal conscience, I think that we need to be working with our employers, not opposing them, on vaccine mandates,” she said on NBC.

Weingarten’s comments stood out following a year in which teachers unions were largely opposed to political figures in battles over opening schools to in-person learning, and had voiced some opposition to vaccine mandates.

Vaccination requirements are becoming increasingly common among both public and private employers. Newsom had already imposed a similar vaccination requirement on state workers, as have several other states, and the federal government has done the same.

On Tuesday, the District of Columbia announced that all city employees, including school staff, would be required to submit to weekly coronavirus tests if they are not vaccinated. Some individual school districts have done the same, including several in California. But California is the first state to require vaccination against the coronavirus for school personnel statewide.

It was not clear how many other states would follow suit. U.S. Education Secretary Miguel Cardona said Wednesday that he supports the mandates but also that he understands the hesitancy, particularly since the vaccine has been approved by the Food and Drug Administration under an emergency authorization.

Asked whether he supports teacher vaccine mandates on Wednesday, Cardona, speaking at a National Press Foundation seminar, initially said, “If they’re safe, I do, but that’s not my decision to make.”

Asked to clarify, he said that the vaccines are safe and that his own children had been vaccinated. “I always say ‘if it’s safe’ because it’s really important for me to signal that we’re not doing anything haphazardly.”

“I would favor the vaccine being required but … having the FDA do the final approval on it would make some who don’t feel comfortable feel comfortable,” he said.

California’s new vaccination policy takes effect Thursday, and schools must be in compliance by Oct. 15.

Responding to questions, Newsom also said he would consider vaccine mandates for students in the future “if necessary.”

“We believe this is a meaningful first step,” he said.