A Republican senator has proposed an amendment to an appropriations bill that would prohibit the Justice Department from enforcing key provisions of the Americans with Disabilities Act at private schools that receive public funds, a measure he said was necessary to protect voucher programs from politically motivated attacks.

The bill — the Commerce, Justice, Science and Related Agencies Appropriations Act — has not yet passed. But disability-rights advocates are concerned that the amendment submitted last week by Sen. Ron Johnson (R-Wis.) could make it far more difficult for Justice Department officials to ensure that voucher programs give students with disabilities fair access to private schools.

“That would be a serious concern,” said Karyn L. Rotker, a senior staff attorney at the American Civil Liberties Union of Wisconsin. Wisconsin is home to the nation’s oldest voucher program; in Milwaukee alone, more than 20,000 low-income children receive state-funded vouchers to attend private and religious schools.

“We’re talking about schools that serve tens of thousands of children, and that would not have obligations under the ADA to reasonably accommodate children with disabilities,” Rotker said.

The senator supports equal opportunities for students with disabilities, a spokeswoman said, and he does not believe that his amendment would diminish efforts to hold private schools accountable for meeting those students’ needs.

“In a voucher program, the parent chooses to send her child to that participating school.  Accountability is maintained, and possibly even enhanced, by the fact that a parent is selecting that school as being in the best interest of her child,” said Paige Alwood, a spokeswoman for Johnson. “If the school did not have the resources or the available funding to address what a child’s parents judged to be the child’s educational needs, those parents would not apply for that school.”

A DOJ spokesman declined to comment.

The amendment is the latest turn in a long-running fight over whether private schools receiving voucher funds should be held to the same non-discrimination standards under ADA as public schools. More than half the states in the nation now have some kind of publicly funded program to pay for students’ private-school tuition, programs that proponents hail as offering an escape from failing public schools, and that critics say are starving public schools of needed resources.

The ACLU and Disability Rights Wisconsin filed a federal civil rights complaint in 2011, alleging that Milwaukee’s voucher program was systematically discriminating against children with disabilities. The complaint alleged that private and religious schools were denying admission to children with disabilities, leaving those children no other option than the public school system.

The complaint resulted in a DOJ investigation that provoked sharp, relentless criticism from Johnson, who argued that the long-running probe was motivated by the Obama administration’s opposition to vouchers rather than by any evidence of civil rights violations.

The investigation of Milwaukee’s voucher program was based on the administration’s interpretation that private schools that take public money should be subject to the same ADA provision that outlines what public schools must do to protect individuals with disabilities. That provision, known as Title II, covers fair access both to buildings (by requiring wheelchair ramps or elevators, for example) and to programs (by requiring interpretation for deaf students).

But in Johnson’s view, private schools that take public money do not have the same legal obligations to children with disabilities as public schools. If approved by the Senate, his amendment would prohibit Justice from using federal dollars to enforce Title II at private schools that receive public money.

“School voucher programs are a lifeline for thousands of low-income families seeking a quality education for their children and schools should not be punished for opening their doors to these students,” said Alwood, Johnson’s spokeswoman. “The purpose of the senator’s amendment is to stop the administration’s efforts to shut down school voucher programs by inappropriately expanding Title II over schools that would otherwise not be subject to the law.”

Private schools are subject to the sometimes less-stringent requirements that apply to restaurants and other public accommodations. And importantly, schools run by religious institutions — as many voucher schools are — are exempt from even those rules.

The investigation into Wisconsin’s voucher program continued for longer than four years. In 2013, DOJ officials told Wisconsin’s state education department that the state’s decision to pay for children’s private-school education does not place those children “beyond the reach of the federal laws” governing non-discrimination. DOJ also told state officials to make several changes to the program, including by improving outreach to families of students with disabilities and establishing a new procedure for filing complaints about disability-based discrimination.

Among the directives were several that could not be enforced under state law, according to the Milwaukee Journal-Sentinel, such as requiring private schools to hand over data on enrollment and suspensions and expulsions of students with disabilities.

The Obama administration quietly closed the probe in late 2015 without taking further action. At the time, Johnson said that was evidence that the investigation was “meritless.”

Correction: An earlier version of this story incorrectly implied that Johnson’s amendment has already been accepted as part of the appropriations bill. Johnson has proposed the amendment, but it has not yet been approved by the Senate. The story has been updated.